root/openpgpsdk/trunk/ref/rfc4880.txt

Network Working Group                                          J. Callas
Request for Comments: 4880                               PGP Corporation
Obsoletes: 1991, 2440                                     L. Donnerhacke
Category: Standards Track                                       IKS GmbH
                                                               H. Finney
                                                         PGP Corporation
                                                                 D. Shaw
                                                               R. Thayer
                                                           November 2007


                         OpenPGP Message Format

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document is maintained in order to publish all necessary
   information needed to develop interoperable applications based on the
   OpenPGP format.  It is not a step-by-step cookbook for writing an
   application.  It describes only the format and methods needed to
   read, check, generate, and write conforming packets crossing any
   network.  It does not deal with storage and implementation questions.
   It does, however, discuss implementation issues necessary to avoid
   security flaws.

   OpenPGP software uses a combination of strong public-key and
   symmetric cryptography to provide security services for electronic
   communications and data storage.  These services include
   confidentiality, key management, authentication, and digital
   signatures.  This document specifies the message formats used in
   OpenPGP.













Callas, et al               Standards Track                     [Page 1]

RFC 4880                 OpenPGP Message Format            November 2007


Table of Contents

   1. Introduction ....................................................5
      1.1. Terms ......................................................5
   2. General functions ...............................................6
      2.1. Confidentiality via Encryption .............................6
      2.2. Authentication via Digital Signature .......................7
      2.3. Compression ................................................7
      2.4. Conversion to Radix-64 .....................................8
      2.5. Signature-Only Applications ................................8
   3. Data Element Formats ............................................8
      3.1. Scalar Numbers .............................................8
      3.2. Multiprecision Integers ....................................9
      3.3. Key IDs ....................................................9
      3.4. Text .......................................................9
      3.5. Time Fields ...............................................10
      3.6. Keyrings ..................................................10
      3.7. String-to-Key (S2K) Specifiers ............................10
           3.7.1. String-to-Key (S2K) Specifier Types ................10
                  3.7.1.1. Simple S2K ................................10
                  3.7.1.2. Salted S2K ................................11
                  3.7.1.3. Iterated and Salted S2K ...................11
           3.7.2. String-to-Key Usage ................................12
                  3.7.2.1. Secret-Key Encryption .....................12
                  3.7.2.2. Symmetric-Key Message Encryption ..........13
   4. Packet Syntax ..................................................13
      4.1. Overview ..................................................13
      4.2. Packet Headers ............................................13
           4.2.1. Old Format Packet Lengths ..........................14
           4.2.2. New Format Packet Lengths ..........................15
                  4.2.2.1. One-Octet Lengths .........................15
                  4.2.2.2. Two-Octet Lengths .........................15
                  4.2.2.3. Five-Octet Lengths ........................15
                  4.2.2.4. Partial Body Lengths ......................16
           4.2.3. Packet Length Examples .............................16
      4.3. Packet Tags ...............................................17
   5. Packet Types ...................................................17
      5.1. Public-Key Encrypted Session Key Packets (Tag 1) ..........17
      5.2. Signature Packet (Tag 2) ..................................19
           5.2.1. Signature Types ....................................19
           5.2.2. Version 3 Signature Packet Format ..................21
           5.2.3. Version 4 Signature Packet Format ..................24
                  5.2.3.1. Signature Subpacket Specification .........25
                  5.2.3.2. Signature Subpacket Types .................27
                  5.2.3.3. Notes on Self-Signatures ..................27
                  5.2.3.4. Signature Creation Time ...................28
                  5.2.3.5. Issuer ....................................28
                  5.2.3.6. Key Expiration Time .......................28



Callas, et al               Standards Track                     [Page 2]

RFC 4880                 OpenPGP Message Format            November 2007


                  5.2.3.7. Preferred Symmetric Algorithms ............28
                  5.2.3.8. Preferred Hash Algorithms .................29
                  5.2.3.9. Preferred Compression Algorithms ..........29
                  5.2.3.10. Signature Expiration Time ................29
                  5.2.3.11. Exportable Certification .................29
                  5.2.3.12. Revocable ................................30
                  5.2.3.13. Trust Signature ..........................30
                  5.2.3.14. Regular Expression .......................31
                  5.2.3.15. Revocation Key ...........................31
                  5.2.3.16. Notation Data ............................31
                  5.2.3.17. Key Server Preferences ...................32
                  5.2.3.18. Preferred Key Server .....................33
                  5.2.3.19. Primary User ID ..........................33
                  5.2.3.20. Policy URI ...............................33
                  5.2.3.21. Key Flags ................................33
                  5.2.3.22. Signer's User ID .........................34
                  5.2.3.23. Reason for Revocation ....................35
                  5.2.3.24. Features .................................36
                  5.2.3.25. Signature Target .........................36
                  5.2.3.26. Embedded Signature .......................37
           5.2.4. Computing Signatures ...............................37
                  5.2.4.1. Subpacket Hints ...........................38
      5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) .......38
      5.4. One-Pass Signature Packets (Tag 4) ........................39
      5.5. Key Material Packet .......................................40
           5.5.1. Key Packet Variants ................................40
                  5.5.1.1. Public-Key Packet (Tag 6) .................40
                  5.5.1.2. Public-Subkey Packet (Tag 14) .............40
                  5.5.1.3. Secret-Key Packet (Tag 5) .................41
                  5.5.1.4. Secret-Subkey Packet (Tag 7) ..............41
           5.5.2. Public-Key Packet Formats ..........................41
           5.5.3. Secret-Key Packet Formats ..........................43
      5.6. Compressed Data Packet (Tag 8) ............................45
      5.7. Symmetrically Encrypted Data Packet (Tag 9) ...............45
      5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) ..........46
      5.9. Literal Data Packet (Tag 11) ..............................46
      5.10. Trust Packet (Tag 12) ....................................47
      5.11. User ID Packet (Tag 13) ..................................48
      5.12. User Attribute Packet (Tag 17) ...........................48
           5.12.1. The Image Attribute Subpacket .....................48
      5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) ..49
      5.14. Modification Detection Code Packet (Tag 19) ..............52
   6. Radix-64 Conversions ...........................................53
      6.1. An Implementation of the CRC-24 in "C" ....................54
      6.2. Forming ASCII Armor .......................................54
      6.3. Encoding Binary in Radix-64 ...............................57
      6.4. Decoding Radix-64 .........................................58
      6.5. Examples of Radix-64 ......................................59



Callas, et al               Standards Track                     [Page 3]

RFC 4880                 OpenPGP Message Format            November 2007


      6.6. Example of an ASCII Armored Message .......................59
   7. Cleartext Signature Framework ..................................59
      7.1. Dash-Escaped Text .........................................60
   8. Regular Expressions ............................................61
   9. Constants ......................................................61
      9.1. Public-Key Algorithms .....................................62
      9.2. Symmetric-Key Algorithms ..................................62
      9.3. Compression Algorithms ....................................63
      9.4. Hash Algorithms ...........................................63
   10. IANA Considerations ...........................................63
      10.1. New String-to-Key Specifier Types ........................64
      10.2. New Packets ..............................................64
           10.2.1. User Attribute Types ..............................64
                  10.2.1.1. Image Format Subpacket Types .............64
           10.2.2. New Signature Subpackets ..........................64
                  10.2.2.1. Signature Notation Data Subpackets .......65
                  10.2.2.2. Key Server Preference Extensions .........65
                  10.2.2.3. Key Flags Extensions .....................65
                  10.2.2.4. Reason For Revocation Extensions .........65
                  10.2.2.5. Implementation Features ..................66
           10.2.3. New Packet Versions ...............................66
      10.3. New Algorithms ...........................................66
           10.3.1. Public-Key Algorithms .............................66
           10.3.2. Symmetric-Key Algorithms ..........................67
           10.3.3. Hash Algorithms ...................................67
           10.3.4. Compression Algorithms ............................67
   11. Packet Composition ............................................67
      11.1. Transferable Public Keys .................................67
      11.2. Transferable Secret Keys .................................69
      11.3. OpenPGP Messages .........................................69
      11.4. Detached Signatures ......................................70
   12. Enhanced Key Formats ..........................................70
      12.1. Key Structures ...........................................70
      12.2. Key IDs and Fingerprints .................................71
   13. Notes on Algorithms ...........................................72
      13.1. PKCS#1 Encoding in OpenPGP ...............................72
           13.1.1. EME-PKCS1-v1_5-ENCODE .............................73
           13.1.2. EME-PKCS1-v1_5-DECODE .............................73
           13.1.3. EMSA-PKCS1-v1_5 ...................................74
      13.2. Symmetric Algorithm Preferences ..........................75
      13.3. Other Algorithm Preferences ..............................76
           13.3.1. Compression Preferences ...........................76
           13.3.2. Hash Algorithm Preferences ........................76
      13.4. Plaintext ................................................77
      13.5. RSA ......................................................77
      13.6. DSA ......................................................77
      13.7. Elgamal ..................................................78
      13.8. Reserved Algorithm Numbers ...............................78



Callas, et al               Standards Track                     [Page 4]

RFC 4880                 OpenPGP Message Format            November 2007


      13.9. OpenPGP CFB Mode .........................................78
      13.10. Private or Experimental Parameters ......................79
      13.11. Extension of the MDC System .............................80
      13.12. Meta-Considerations for Expansion .......................80
   14. Security Considerations .......................................81
   15. Implementation Nits ...........................................84
   16. References ....................................................86
      16.1. Normative References .....................................86
      16.2. Informative References ...................................88

1.  Introduction

   This document provides information on the message-exchange packet
   formats used by OpenPGP to provide encryption, decryption, signing,
   and key management functions.  It is a revision of RFC 2440, "OpenPGP
   Message Format", which itself replaces RFC 1991, "PGP Message
   Exchange Formats" [RFC1991] [RFC2440].

1.1.  Terms

     * OpenPGP - This is a term for security software that uses PGP 5.x
       as a basis, formalized in RFC 2440 and this document.

     * PGP - Pretty Good Privacy.  PGP is a family of software systems
       developed by Philip R. Zimmermann from which OpenPGP is based.

     * PGP 2.6.x - This version of PGP has many variants, hence the term
       PGP 2.6.x.  It used only RSA, MD5, and IDEA for its cryptographic
       transforms.  An informational RFC, RFC 1991, was written
       describing this version of PGP.

     * PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
       community and also in the predecessor of this document, RFC 1991.
       It has new formats and corrects a number of problems in the PGP
       2.6.x design.  It is referred to here as PGP 5.x because that
       software was the first release of the "PGP 3" code base.

     * GnuPG - GNU Privacy Guard, also called GPG.  GnuPG is an OpenPGP
       implementation that avoids all encumbered algorithms.
       Consequently, early versions of GnuPG did not include RSA public
       keys.  GnuPG may or may not have (depending on version) support
       for IDEA or other encumbered algorithms.

   "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP
   Corporation and are used with permission.  The term "OpenPGP" refers
   to the protocol described in this and related documents.





Callas, et al               Standards Track                     [Page 5]

RFC 4880                 OpenPGP Message Format            November 2007


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
   FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
   APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
   this document when used to describe namespace allocation are to be
   interpreted as described in [RFC2434].

2.  General functions

   OpenPGP provides data integrity services for messages and data files
   by using these core technologies:

     - digital signatures

     - encryption

     - compression

     - Radix-64 conversion

   In addition, OpenPGP provides key management and certificate
   services, but many of these are beyond the scope of this document.

2.1.  Confidentiality via Encryption

   OpenPGP combines symmetric-key encryption and public-key encryption
   to provide confidentiality.  When made confidential, first the object
   is encrypted using a symmetric encryption algorithm.  Each symmetric
   key is used only once, for a single object.  A new "session key" is
   generated as a random number for each object (sometimes referred to
   as a session).  Since it is used only once, the session key is bound
   to the message and transmitted with it.  To protect the key, it is
   encrypted with the receiver's public key.  The sequence is as
   follows:

   1.  The sender creates a message.

   2.  The sending OpenPGP generates a random number to be used as a
       session key for this message only.

   3.  The session key is encrypted using each recipient's public key.
       These "encrypted session keys" start the message.






Callas, et al               Standards Track                     [Page 6]

RFC 4880                 OpenPGP Message Format            November 2007


   4.  The sending OpenPGP encrypts the message using the session key,
       which forms the remainder of the message.  Note that the message
       is also usually compressed.

   5.  The receiving OpenPGP decrypts the session key using the
       recipient's private key.

   6.  The receiving OpenPGP decrypts the message using the session key.
       If the message was compressed, it will be decompressed.

   With symmetric-key encryption, an object may be encrypted with a
   symmetric key derived from a passphrase (or other shared secret), or
   a two-stage mechanism similar to the public-key method described
   above in which a session key is itself encrypted with a symmetric
   algorithm keyed from a shared secret.

   Both digital signature and confidentiality services may be applied to
   the same message.  First, a signature is generated for the message
   and attached to the message.  Then the message plus signature is
   encrypted using a symmetric session key.  Finally, the session key is
   encrypted using public-key encryption and prefixed to the encrypted
   block.

2.2.  Authentication via Digital Signature

   The digital signature uses a hash code or message digest algorithm,
   and a public-key signature algorithm.  The sequence is as follows:

   1.  The sender creates a message.

   2.  The sending software generates a hash code of the message.

   3.  The sending software generates a signature from the hash code
       using the sender's private key.

   4.  The binary signature is attached to the message.

   5.  The receiving software keeps a copy of the message signature.

   6.  The receiving software generates a new hash code for the received
       message and verifies it using the message's signature.  If the
       verification is successful, the message is accepted as authentic.

2.3.  Compression

   OpenPGP implementations SHOULD compress the message after applying
   the signature but before encryption.




Callas, et al               Standards Track                     [Page 7]

RFC 4880                 OpenPGP Message Format            November 2007


   If an implementation does not implement compression, its authors
   should be aware that most OpenPGP messages in the world are
   compressed.  Thus, it may even be wise for a space-constrained
   implementation to implement decompression, but not compression.

   Furthermore, compression has the added side effect that some types of
   attacks can be thwarted by the fact that slightly altered, compressed
   data rarely uncompresses without severe errors.  This is hardly
   rigorous, but it is operationally useful.  These attacks can be
   rigorously prevented by implementing and using Modification Detection
   Codes as described in sections following.

2.4.  Conversion to Radix-64

   OpenPGP's underlying native representation for encrypted messages,
   signature certificates, and keys is a stream of arbitrary octets.
   Some systems only permit the use of blocks consisting of seven-bit,
   printable text.  For transporting OpenPGP's native raw binary octets
   through channels that are not safe to raw binary data, a printable
   encoding of these binary octets is needed.  OpenPGP provides the
   service of converting the raw 8-bit binary octet stream to a stream
   of printable ASCII characters, called Radix-64 encoding or ASCII
   Armor.

   Implementations SHOULD provide Radix-64 conversions.

2.5.  Signature-Only Applications

   OpenPGP is designed for applications that use both encryption and
   signatures, but there are a number of problems that are solved by a
   signature-only implementation.  Although this specification requires
   both encryption and signatures, it is reasonable for there to be
   subset implementations that are non-conformant only in that they omit
   encryption.

3.  Data Element Formats

   This section describes the data elements used by OpenPGP.

3.1.  Scalar Numbers

   Scalar numbers are unsigned and are always stored in big-endian
   format.  Using n[k] to refer to the kth octet being interpreted, the
   value of a two-octet scalar is ((n[0] << 8) + n[1]).  The value of a
   four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
   n[3]).





Callas, et al               Standards Track                     [Page 8]

RFC 4880                 OpenPGP Message Format            November 2007


3.2.  Multiprecision Integers

   Multiprecision integers (also called MPIs) are unsigned integers used
   to hold large integers such as the ones used in cryptographic
   calculations.

   An MPI consists of two pieces: a two-octet scalar that is the length
   of the MPI in bits followed by a string of octets that contain the
   actual integer.

   These octets form a big-endian number; a big-endian number can be
   made into an MPI by prefixing it with the appropriate length.

   Examples:

   (all numbers are in hexadecimal)

   The string of octets [00 01 01] forms an MPI with the value 1.  The
   string [00 09 01 FF] forms an MPI with the value of 511.

   Additional rules:

   The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.

   The length field of an MPI describes the length starting from its
   most significant non-zero bit.  Thus, the MPI [00 02 01] is not
   formed correctly.  It should be [00 01 01].

   Unused bits of an MPI MUST be zero.

   Also note that when an MPI is encrypted, the length refers to the
   plaintext MPI.  It may be ill-formed in its ciphertext.

3.3.  Key IDs

   A Key ID is an eight-octet scalar that identifies a key.
   Implementations SHOULD NOT assume that Key IDs are unique.  The
   section "Enhanced Key Formats" below describes how Key IDs are
   formed.

3.4.  Text

   Unless otherwise specified, the character set for text is the UTF-8
   [RFC3629] encoding of Unicode [ISO10646].







Callas, et al               Standards Track                     [Page 9]

RFC 4880                 OpenPGP Message Format            November 2007


3.5.  Time Fields

   A time field is an unsigned four-octet number containing the number
   of seconds elapsed since midnight, 1 January 1970 UTC.

3.6.  Keyrings

   A keyring is a collection of one or more keys in a file or database.
   Traditionally, a keyring is simply a sequential list of keys, but may
   be any suitable database.  It is beyond the scope of this standard to
   discuss the details of keyrings or other databases.

3.7.  String-to-Key (S2K) Specifiers

   String-to-key (S2K) specifiers are used to convert passphrase strings
   into symmetric-key encryption/decryption keys.  They are used in two
   places, currently: to encrypt the secret part of private keys in the
   private keyring, and to convert passphrases to encryption keys for
   symmetrically encrypted messages.

3.7.1.  String-to-Key (S2K) Specifier Types

   There are three types of S2K specifiers currently supported, and
   some reserved values:

       ID          S2K Type
       --          --------
       0           Simple S2K
       1           Salted S2K
       2           Reserved value
       3           Iterated and Salted S2K
       100 to 110  Private/Experimental S2K

   These are described in Sections 3.7.1.1 - 3.7.1.3.

3.7.1.1.  Simple S2K

   This directly hashes the string to produce the key data.  See below
   for how this hashing is done.

       Octet 0:        0x00
       Octet 1:        hash algorithm

   Simple S2K hashes the passphrase to produce the session key.  The
   manner in which this is done depends on the size of the session key
   (which will depend on the cipher used) and the size of the hash





Callas, et al               Standards Track                    [Page 10]

RFC 4880                 OpenPGP Message Format            November 2007


   algorithm's output.  If the hash size is greater than the session key
   size, the high-order (leftmost) octets of the hash are used as the
   key.

   If the hash size is less than the key size, multiple instances of the
   hash context are created -- enough to produce the required key data.
   These instances are preloaded with 0, 1, 2, ... octets of zeros (that
   is to say, the first instance has no preloading, the second gets
   preloaded with 1 octet of zero, the third is preloaded with two
   octets of zeros, and so forth).

   As the data is hashed, it is given independently to each hash
   context.  Since the contexts have been initialized differently, they
   will each produce different hash output.  Once the passphrase is
   hashed, the output data from the multiple hashes is concatenated,
   first hash leftmost, to produce the key data, with any excess octets
   on the right discarded.

3.7.1.2.  Salted S2K

   This includes a "salt" value in the S2K specifier -- some arbitrary
   data -- that gets hashed along with the passphrase string, to help
   prevent dictionary attacks.

       Octet 0:        0x01
       Octet 1:        hash algorithm
       Octets 2-9:     8-octet salt value

   Salted S2K is exactly like Simple S2K, except that the input to the
   hash function(s) consists of the 8 octets of salt from the S2K
   specifier, followed by the passphrase.

3.7.1.3.  Iterated and Salted S2K

   This includes both a salt and an octet count.  The salt is combined
   with the passphrase and the resulting value is hashed repeatedly.
   This further increases the amount of work an attacker must do to try
   dictionary attacks.

       Octet  0:        0x03
       Octet  1:        hash algorithm
       Octets 2-9:      8-octet salt value
       Octet  10:       count, a one-octet, coded value








Callas, et al               Standards Track                    [Page 11]

RFC 4880                 OpenPGP Message Format            November 2007


   The count is coded into a one-octet number using the following
   formula:

       #define EXPBIAS 6
           count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);

   The above formula is in C, where "Int32" is a type for a 32-bit
   integer, and the variable "c" is the coded count, Octet 10.

   Iterated-Salted S2K hashes the passphrase and salt data multiple
   times.  The total number of octets to be hashed is specified in the
   encoded count in the S2K specifier.  Note that the resulting count
   value is an octet count of how many octets will be hashed, not an
   iteration count.

   Initially, one or more hash contexts are set up as with the other S2K
   algorithms, depending on how many octets of key data are needed.
   Then the salt, followed by the passphrase data, is repeatedly hashed
   until the number of octets specified by the octet count has been
   hashed.  The one exception is that if the octet count is less than
   the size of the salt plus passphrase, the full salt plus passphrase
   will be hashed even though that is greater than the octet count.
   After the hashing is done, the data is unloaded from the hash
   context(s) as with the other S2K algorithms.

3.7.2.  String-to-Key Usage

   Implementations SHOULD use salted or iterated-and-salted S2K
   specifiers, as simple S2K specifiers are more vulnerable to
   dictionary attacks.

3.7.2.1.  Secret-Key Encryption

   An S2K specifier can be stored in the secret keyring to specify how
   to convert the passphrase to a key that unlocks the secret data.
   Older versions of PGP just stored a cipher algorithm octet preceding
   the secret data or a zero to indicate that the secret data was
   unencrypted.  The MD5 hash function was always used to convert the
   passphrase to a key for the specified cipher algorithm.

   For compatibility, when an S2K specifier is used, the special value
   254 or 255 is stored in the position where the hash algorithm octet
   would have been in the old data structure.  This is then followed
   immediately by a one-octet algorithm identifier, and then by the S2K
   specifier as encoded above.






Callas, et al               Standards Track                    [Page 12]

RFC 4880                 OpenPGP Message Format            November 2007


   Therefore, preceding the secret data there will be one of these
   possibilities:

       0:           secret data is unencrypted (no passphrase)
       255 or 254:  followed by algorithm octet and S2K specifier
       Cipher alg:  use Simple S2K algorithm using MD5 hash

   This last possibility, the cipher algorithm number with an implicit
   use of MD5 and IDEA, is provided for backward compatibility; it MAY
   be understood, but SHOULD NOT be generated, and is deprecated.

   These are followed by an Initial Vector of the same length as the
   block size of the cipher for the decryption of the secret values, if
   they are encrypted, and then the secret-key values themselves.

3.7.2.2.  Symmetric-Key Message Encryption

   OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
   at the front of a message.  This is used to allow S2K specifiers to
   be used for the passphrase conversion or to create messages with a
   mix of symmetric-key ESKs and public-key ESKs.  This allows a message
   to be decrypted either with a passphrase or a public-key pair.

   PGP 2.X always used IDEA with Simple string-to-key conversion when
   encrypting a message with a symmetric algorithm.  This is deprecated,
   but MAY be used for backward-compatibility.

4.  Packet Syntax

   This section describes the packets used by OpenPGP.

4.1.  Overview

   An OpenPGP message is constructed from a number of records that are
   traditionally called packets.  A packet is a chunk of data that has a
   tag specifying its meaning.  An OpenPGP message, keyring,
   certificate, and so forth consists of a number of packets.  Some of
   those packets may contain other OpenPGP packets (for example, a
   compressed data packet, when uncompressed, contains OpenPGP packets).

   Each packet consists of a packet header, followed by the packet body.
   The packet header is of variable length.

4.2.  Packet Headers

   The first octet of the packet header is called the "Packet Tag".  It
   determines the format of the header and denotes the packet contents.
   The remainder of the packet header is the length of the packet.



Callas, et al               Standards Track                    [Page 13]

RFC 4880                 OpenPGP Message Format            November 2007


   Note that the most significant bit is the leftmost bit, called bit 7.
   A mask for this bit is 0x80 in hexadecimal.

              +---------------+
         PTag |7 6 5 4 3 2 1 0|
              +---------------+
         Bit 7 -- Always one
         Bit 6 -- New packet format if set

   PGP 2.6.x only uses old format packets.  Thus, software that
   interoperates with those versions of PGP must only use old format
   packets.  If interoperability is not an issue, the new packet format
   is RECOMMENDED.  Note that old format packets have four bits of
   packet tags, and new format packets have six; some features cannot be
   used and still be backward-compatible.

   Also note that packets with a tag greater than or equal to 16 MUST
   use new format packets.  The old format packets can only express tags
   less than or equal to 15.

   Old format packets contain:

         Bits 5-2 -- packet tag
         Bits 1-0 -- length-type

   New format packets contain:

         Bits 5-0 -- packet tag

4.2.1.  Old Format Packet Lengths

   The meaning of the length-type in old format packets is:

   0 - The packet has a one-octet length.  The header is 2 octets long.

   1 - The packet has a two-octet length.  The header is 3 octets long.

   2 - The packet has a four-octet length.  The header is 5 octets long.

   3 - The packet is of indeterminate length.  The header is 1 octet
       long, and the implementation must determine how long the packet
       is.  If the packet is in a file, this means that the packet
       extends until the end of the file.  In general, an implementation
       SHOULD NOT use indeterminate-length packets except where the end
       of the data will be clear from the context, and even then it is
       better to use a definite length, or a new format header.  The new
       format headers described below have a mechanism for precisely
       encoding data of indeterminate length.



Callas, et al               Standards Track                    [Page 14]

RFC 4880                 OpenPGP Message Format            November 2007


4.2.2.  New Format Packet Lengths

   New format packets have four possible ways of encoding length:

   1. A one-octet Body Length header encodes packet lengths of up to 191
      octets.

   2. A two-octet Body Length header encodes packet lengths of 192 to
      8383 octets.

   3. A five-octet Body Length header encodes packet lengths of up to
      4,294,967,295 (0xFFFFFFFF) octets in length.  (This actually
      encodes a four-octet scalar number.)

   4. When the length of the packet body is not known in advance by the
      issuer, Partial Body Length headers encode a packet of
      indeterminate length, effectively making it a stream.

4.2.2.1.  One-Octet Lengths

   A one-octet Body Length header encodes a length of 0 to 191 octets.
   This type of length header is recognized because the one octet value
   is less than 192.  The body length is equal to:

       bodyLen = 1st_octet;

4.2.2.2.  Two-Octet Lengths

   A two-octet Body Length header encodes a length of 192 to 8383
   octets.  It is recognized because its first octet is in the range 192
   to 223.  The body length is equal to:

       bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

4.2.2.3.  Five-Octet Lengths

   A five-octet Body Length header consists of a single octet holding
   the value 255, followed by a four-octet scalar.  The body length is
   equal to:

       bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
                 (4th_octet << 8)  | 5th_octet

   This basic set of one, two, and five-octet lengths is also used
   internally to some packets.






Callas, et al               Standards Track                    [Page 15]

RFC 4880                 OpenPGP Message Format            November 2007


4.2.2.4.  Partial Body Lengths

   A Partial Body Length header is one octet long and encodes the length
   of only part of the data packet.  This length is a power of 2, from 1
   to 1,073,741,824 (2 to the 30th power).  It is recognized by its one
   octet value that is greater than or equal to 224, and less than 255.
   The Partial Body Length is equal to:

       partialBodyLen = 1 << (1st_octet & 0x1F);

   Each Partial Body Length header is followed by a portion of the
   packet body data.  The Partial Body Length header specifies this
   portion's length.  Another length header (one octet, two-octet,
   five-octet, or partial) follows that portion.  The last length header
   in the packet MUST NOT be a Partial Body Length header.  Partial Body
   Length headers may only be used for the non-final parts of the
   packet.

   Note also that the last Body Length header can be a zero-length
   header.

   An implementation MAY use Partial Body Lengths for data packets, be
   they literal, compressed, or encrypted.  The first partial length
   MUST be at least 512 octets long.  Partial Body Lengths MUST NOT be
   used for any other packet types.

4.2.3.  Packet Length Examples

   These examples show ways that new format packets might encode the
   packet lengths.

   A packet with length 100 may have its length encoded in one octet:
   0x64.  This is followed by 100 octets of data.

   A packet with length 1723 may have its length encoded in two octets:
   0xC5, 0xFB.  This header is followed by the 1723 octets of data.

   A packet with length 100000 may have its length encoded in five
   octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.

   It might also be encoded in the following octet stream: 0xEF, first
   32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
   octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
   octets of data.  This is just one possible encoding, and many
   variations are possible on the size of the Partial Body Length
   headers, as long as a regular Body Length header encodes the last
   portion of the data.




Callas, et al               Standards Track                    [Page 16]

RFC 4880                 OpenPGP Message Format            November 2007


   Please note that in all of these explanations, the total length of
   the packet is the length of the header(s) plus the length of the
   body.

4.3.  Packet Tags

   The packet tag denotes what type of packet the body holds.  Note that
   old format headers can only have tags less than 16, whereas new
   format headers can have tags as great as 63.  The defined tags (in
   decimal) are as follows:

       0        -- Reserved - a packet tag MUST NOT have this value
       1        -- Public-Key Encrypted Session Key Packet
       2        -- Signature Packet
       3        -- Symmetric-Key Encrypted Session Key Packet
       4        -- One-Pass Signature Packet
       5        -- Secret-Key Packet
       6        -- Public-Key Packet
       7        -- Secret-Subkey Packet
       8        -- Compressed Data Packet
       9        -- Symmetrically Encrypted Data Packet
       10       -- Marker Packet
       11       -- Literal Data Packet
       12       -- Trust Packet
       13       -- User ID Packet
       14       -- Public-Subkey Packet
       17       -- User Attribute Packet
       18       -- Sym. Encrypted and Integrity Protected Data Packet
       19       -- Modification Detection Code Packet
       60 to 63 -- Private or Experimental Values

5.  Packet Types

5.1.  Public-Key Encrypted Session Key Packets (Tag 1)

   A Public-Key Encrypted Session Key packet holds the session key used
   to encrypt a message.  Zero or more Public-Key Encrypted Session Key
   packets and/or Symmetric-Key Encrypted Session Key packets may
   precede a Symmetrically Encrypted Data Packet, which holds an
   encrypted message.  The message is encrypted with the session key,
   and the session key is itself encrypted and stored in the Encrypted
   Session Key packet(s).  The Symmetrically Encrypted Data Packet is
   preceded by one Public-Key Encrypted Session Key packet for each
   OpenPGP key to which the message is encrypted.  The recipient of the
   message finds a session key that is encrypted to their public key,
   decrypts the session key, and then uses the session key to decrypt
   the message.




Callas, et al               Standards Track                    [Page 17]

RFC 4880                 OpenPGP Message Format            November 2007


   The body of this packet consists of:

     - A one-octet number giving the version number of the packet type.
       The currently defined value for packet version is 3.

     - An eight-octet number that gives the Key ID of the public key to
       which the session key is encrypted.  If the session key is
       encrypted to a subkey, then the Key ID of this subkey is used
       here instead of the Key ID of the primary key.

     - A one-octet number giving the public-key algorithm used.

     - A string of octets that is the encrypted session key.  This
       string takes up the remainder of the packet, and its contents are
       dependent on the public-key algorithm used.

   Algorithm Specific Fields for RSA encryption

     - multiprecision integer (MPI) of RSA encrypted value m**e mod n.

   Algorithm Specific Fields for Elgamal encryption:

     - MPI of Elgamal (Diffie-Hellman) value g**k mod p.

     - MPI of Elgamal (Diffie-Hellman) value m * y**k mod p.

   The value "m" in the above formulas is derived from the session key
   as follows.  First, the session key is prefixed with a one-octet
   algorithm identifier that specifies the symmetric encryption
   algorithm used to encrypt the following Symmetrically Encrypted Data
   Packet.  Then a two-octet checksum is appended, which is equal to the
   sum of the preceding session key octets, not including the algorithm
   identifier, modulo 65536.  This value is then encoded as described in
   PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
   form the "m" value used in the formulas above.  See Section 13.1 of
   this document for notes on OpenPGP's use of PKCS#1.

   Note that when an implementation forms several PKESKs with one
   session key, forming a message that can be decrypted by several keys,
   the implementation MUST make a new PKCS#1 encoding for each key.

   An implementation MAY accept or use a Key ID of zero as a "wild card"
   or "speculative" Key ID.  In this case, the receiving implementation
   would try all available private keys, checking for a valid decrypted
   session key.  This format helps reduce traffic analysis of messages.






Callas, et al               Standards Track                    [Page 18]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.  Signature Packet (Tag 2)

   A Signature packet describes a binding between some public key and
   some data.  The most common signatures are a signature of a file or a
   block of text, and a signature that is a certification of a User ID.

   Two versions of Signature packets are defined.  Version 3 provides
   basic signature information, while version 4 provides an expandable
   format with subpackets that can specify more information about the
   signature.  PGP 2.6.x only accepts version 3 signatures.

   Implementations SHOULD accept V3 signatures.  Implementations SHOULD
   generate V4 signatures.

   Note that if an implementation is creating an encrypted and signed
   message that is encrypted to a V3 key, it is reasonable to create a
   V3 signature.

5.2.1.  Signature Types

   There are a number of possible meanings for a signature, which are
   indicated in a signature type octet in any given signature.  Please
   note that the vagueness of these meanings is not a flaw, but a
   feature of the system.  Because OpenPGP places final authority for
   validity upon the receiver of a signature, it may be that one
   signer's casual act might be more rigorous than some other
   authority's positive act.  See Section 5.2.4, "Computing Signatures",
   for detailed information on how to compute and verify signatures of
   each type.

   These meanings are as follows:

   0x00: Signature of a binary document.
       This means the signer owns it, created it, or certifies that it
       has not been modified.

   0x01: Signature of a canonical text document.
       This means the signer owns it, created it, or certifies that it
       has not been modified.  The signature is calculated over the text
       data with its line endings converted to <CR><LF>.

   0x02: Standalone signature.
       This signature is a signature of only its own subpacket contents.
       It is calculated identically to a signature over a zero-length
       binary document.  Note that it doesn't make sense to have a V3
       standalone signature.





Callas, et al               Standards Track                    [Page 19]

RFC 4880                 OpenPGP Message Format            November 2007


   0x10: Generic certification of a User ID and Public-Key packet.
       The issuer of this certification does not make any particular
       assertion as to how well the certifier has checked that the owner
       of the key is in fact the person described by the User ID.

   0x11: Persona certification of a User ID and Public-Key packet.
       The issuer of this certification has not done any verification of
       the claim that the owner of this key is the User ID specified.

   0x12: Casual certification of a User ID and Public-Key packet.
       The issuer of this certification has done some casual
       verification of the claim of identity.

   0x13: Positive certification of a User ID and Public-Key packet.
       The issuer of this certification has done substantial
       verification of the claim of identity.

       Most OpenPGP implementations make their "key signatures" as 0x10
       certifications.  Some implementations can issue 0x11-0x13
       certifications, but few differentiate between the types.

   0x18: Subkey Binding Signature
       This signature is a statement by the top-level signing key that
       indicates that it owns the subkey.  This signature is calculated
       directly on the primary key and subkey, and not on any User ID or
       other packets.  A signature that binds a signing subkey MUST have
       an Embedded Signature subpacket in this binding signature that
       contains a 0x19 signature made by the signing subkey on the
       primary key and subkey.

   0x19: Primary Key Binding Signature
       This signature is a statement by a signing subkey, indicating
       that it is owned by the primary key and subkey.  This signature
       is calculated the same way as a 0x18 signature: directly on the
       primary key and subkey, and not on any User ID or other packets.

   0x1F: Signature directly on a key
       This signature is calculated directly on a key.  It binds the
       information in the Signature subpackets to the key, and is
       appropriate to be used for subpackets that provide information
       about the key, such as the Revocation Key subpacket.  It is also
       appropriate for statements that non-self certifiers want to make
       about the key itself, rather than the binding between a key and a
       name.







Callas, et al               Standards Track                    [Page 20]

RFC 4880                 OpenPGP Message Format            November 2007


   0x20: Key revocation signature
       The signature is calculated directly on the key being revoked.  A
       revoked key is not to be used.  Only revocation signatures by the
       key being revoked, or by an authorized revocation key, should be
       considered valid revocation signatures.

   0x28: Subkey revocation signature
       The signature is calculated directly on the subkey being revoked.
       A revoked subkey is not to be used.  Only revocation signatures
       by the top-level signature key that is bound to this subkey, or
       by an authorized revocation key, should be considered valid
       revocation signatures.

   0x30: Certification revocation signature
       This signature revokes an earlier User ID certification signature
       (signature class 0x10 through 0x13) or direct-key signature
       (0x1F).  It should be issued by the same key that issued the
       revoked signature or an authorized revocation key.  The signature
       is computed over the same data as the certificate that it
       revokes, and should have a later creation date than that
       certificate.

   0x40: Timestamp signature.
       This signature is only meaningful for the timestamp contained in
       it.

   0x50: Third-Party Confirmation signature.
       This signature is a signature over some other OpenPGP Signature
       packet(s).  It is analogous to a notary seal on the signed data.
       A third-party signature SHOULD include Signature Target
       subpacket(s) to give easy identification.  Note that we really do
       mean SHOULD.  There are plausible uses for this (such as a blind
       party that only sees the signature, not the key or source
       document) that cannot include a target subpacket.

5.2.2.  Version 3 Signature Packet Format

   The body of a version 3 Signature Packet contains:

     - One-octet version number (3).

     - One-octet length of following hashed material.  MUST be 5.

         - One-octet signature type.

         - Four-octet creation time.

     - Eight-octet Key ID of signer.



Callas, et al               Standards Track                    [Page 21]

RFC 4880                 OpenPGP Message Format            November 2007


     - One-octet public-key algorithm.

     - One-octet hash algorithm.

     - Two-octet field holding left 16 bits of signed hash value.

     - One or more multiprecision integers comprising the signature.
       This portion is algorithm specific, as described below.

   The concatenation of the data to be signed, the signature type, and
   creation time from the Signature packet (5 additional octets) is
   hashed.  The resulting hash value is used in the signature algorithm.
   The high 16 bits (first two octets) of the hash are included in the
   Signature packet to provide a quick test to reject some invalid
   signatures.

   Algorithm-Specific Fields for RSA signatures:

     - multiprecision integer (MPI) of RSA signature value m**d mod n.

   Algorithm-Specific Fields for DSA signatures:

     - MPI of DSA value r.

     - MPI of DSA value s.

   The signature calculation is based on a hash of the signed data, as
   described above.  The details of the calculation are different for
   DSA signatures than for RSA signatures.

   With RSA signatures, the hash value is encoded using PKCS#1 encoding
   type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447.  This
   requires inserting the hash value as an octet string into an ASN.1
   structure.  The object identifier for the type of hash being used is
   included in the structure.  The hexadecimal representations for the
   currently defined hash algorithms are as follows:

     - MD5:        0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05

     - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01

     - SHA-1:      0x2B, 0x0E, 0x03, 0x02, 0x1A

     - SHA224:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04

     - SHA256:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01

     - SHA384:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02



Callas, et al               Standards Track                    [Page 22]

RFC 4880                 OpenPGP Message Format            November 2007


     - SHA512:     0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03

   The ASN.1 Object Identifiers (OIDs) are as follows:

     - MD5:        1.2.840.113549.2.5

     - RIPEMD-160: 1.3.36.3.2.1

     - SHA-1:      1.3.14.3.2.26

     - SHA224:     2.16.840.1.101.3.4.2.4

     - SHA256:     2.16.840.1.101.3.4.2.1

     - SHA384:     2.16.840.1.101.3.4.2.2

     - SHA512:     2.16.840.1.101.3.4.2.3

   The full hash prefixes for these are as follows:

       MD5:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
                   0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
                   0x04, 0x10

       RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
                   0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14

       SHA-1:      0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
                   0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14

       SHA224:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
                   0x00, 0x04, 0x1C

       SHA256:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
                   0x00, 0x04, 0x20

       SHA384:     0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
                   0x00, 0x04, 0x30

       SHA512:     0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                   0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
                   0x00, 0x04, 0x40

   DSA signatures MUST use hashes that are equal in size to the number
   of bits of q, the group generated by the DSA key's generator value.



Callas, et al               Standards Track                    [Page 23]

RFC 4880                 OpenPGP Message Format            November 2007


   If the output size of the chosen hash is larger than the number of
   bits of q, the hash result is truncated to fit by taking the number
   of leftmost bits equal to the number of bits of q.  This (possibly
   truncated) hash function result is treated as a number and used
   directly in the DSA signature algorithm.

5.2.3.  Version 4 Signature Packet Format

   The body of a version 4 Signature packet contains:

     - One-octet version number (4).

     - One-octet signature type.

     - One-octet public-key algorithm.

     - One-octet hash algorithm.

     - Two-octet scalar octet count for following hashed subpacket data.
       Note that this is the length in octets of all of the hashed
       subpackets; a pointer incremented by this number will skip over
       the hashed subpackets.

     - Hashed subpacket data set (zero or more subpackets).

     - Two-octet scalar octet count for the following unhashed subpacket
       data.  Note that this is the length in octets of all of the
       unhashed subpackets; a pointer incremented by this number will
       skip over the unhashed subpackets.

     - Unhashed subpacket data set (zero or more subpackets).

     - Two-octet field holding the left 16 bits of the signed hash
       value.

     - One or more multiprecision integers comprising the signature.
       This portion is algorithm specific, as described above.

   The concatenation of the data being signed and the signature data
   from the version number through the hashed subpacket data (inclusive)
   is hashed.  The resulting hash value is what is signed.  The left 16
   bits of the hash are included in the Signature packet to provide a
   quick test to reject some invalid signatures.

   There are two fields consisting of Signature subpackets.  The first
   field is hashed with the rest of the signature data, while the second
   is unhashed.  The second set of subpackets is not cryptographically




Callas, et al               Standards Track                    [Page 24]

RFC 4880                 OpenPGP Message Format            November 2007


   protected by the signature and should include only advisory
   information.

   The algorithms for converting the hash function result to a signature
   are described in a section below.

5.2.3.1.  Signature Subpacket Specification

   A subpacket data set consists of zero or more Signature subpackets.
   In Signature packets, the subpacket data set is preceded by a two-
   octet scalar count of the length in octets of all the subpackets.  A
   pointer incremented by this number will skip over the subpacket data
   set.

   Each subpacket consists of a subpacket header and a body.  The header
   consists of:

     - the subpacket length (1, 2, or 5 octets),

     - the subpacket type (1 octet),

   and is followed by the subpacket-specific data.

   The length includes the type octet but not this length.  Its format
   is similar to the "new" format packet header lengths, but cannot have
   Partial Body Lengths.  That is:

       if the 1st octet <  192, then
           lengthOfLength = 1
           subpacketLen = 1st_octet

       if the 1st octet >= 192 and < 255, then
           lengthOfLength = 2
           subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

       if the 1st octet = 255, then
           lengthOfLength = 5
           subpacket length = [four-octet scalar starting at 2nd_octet]

   The value of the subpacket type octet may be:

            0 = Reserved
            1 = Reserved
            2 = Signature Creation Time
            3 = Signature Expiration Time
            4 = Exportable Certification
            5 = Trust Signature
            6 = Regular Expression



Callas, et al               Standards Track                    [Page 25]

RFC 4880                 OpenPGP Message Format            November 2007


            7 = Revocable
            8 = Reserved
            9 = Key Expiration Time
           10 = Placeholder for backward compatibility
           11 = Preferred Symmetric Algorithms
           12 = Revocation Key
           13 = Reserved
           14 = Reserved
           15 = Reserved
           16 = Issuer
           17 = Reserved
           18 = Reserved
           19 = Reserved
           20 = Notation Data
           21 = Preferred Hash Algorithms
           22 = Preferred Compression Algorithms
           23 = Key Server Preferences
           24 = Preferred Key Server
           25 = Primary User ID
           26 = Policy URI
           27 = Key Flags
           28 = Signer's User ID
           29 = Reason for Revocation
           30 = Features
           31 = Signature Target
           32 = Embedded Signature
   100 To 110 = Private or experimental

   An implementation SHOULD ignore any subpacket of a type that it does
   not recognize.

   Bit 7 of the subpacket type is the "critical" bit.  If set, it
   denotes that the subpacket is one that is critical for the evaluator
   of the signature to recognize.  If a subpacket is encountered that is
   marked critical but is unknown to the evaluating software, the
   evaluator SHOULD consider the signature to be in error.

   An evaluator may "recognize" a subpacket, but not implement it.  The
   purpose of the critical bit is to allow the signer to tell an
   evaluator that it would prefer a new, unknown feature to generate an
   error than be ignored.

   Implementations SHOULD implement the three preferred algorithm
   subpackets (11, 21, and 22), as well as the "Reason for Revocation"
   subpacket.  Note, however, that if an implementation chooses not to
   implement some of the preferences, it is required to behave in a
   polite manner to respect the wishes of those users who do implement
   these preferences.



Callas, et al               Standards Track                    [Page 26]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.3.2.  Signature Subpacket Types

   A number of subpackets are currently defined.  Some subpackets apply
   to the signature itself and some are attributes of the key.
   Subpackets that are found on a self-signature are placed on a
   certification made by the key itself.  Note that a key may have more
   than one User ID, and thus may have more than one self-signature, and
   differing subpackets.

   A subpacket may be found either in the hashed or unhashed subpacket
   sections of a signature.  If a subpacket is not hashed, then the
   information in it cannot be considered definitive because it is not
   part of the signature proper.

5.2.3.3.  Notes on Self-Signatures

   A self-signature is a binding signature made by the key to which the
   signature refers.  There are three types of self-signatures, the
   certification signatures (types 0x10-0x13), the direct-key signature
   (type 0x1F), and the subkey binding signature (type 0x18).  For
   certification self-signatures, each User ID may have a self-
   signature, and thus different subpackets in those self-signatures.
   For subkey binding signatures, each subkey in fact has a self-
   signature.  Subpackets that appear in a certification self-signature
   apply to the user name, and subpackets that appear in the subkey
   self-signature apply to the subkey.  Lastly, subpackets on the
   direct-key signature apply to the entire key.

   Implementing software should interpret a self-signature's preference
   subpackets as narrowly as possible.  For example, suppose a key has
   two user names, Alice and Bob.  Suppose that Alice prefers the
   symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES.  If the
   software locates this key via Alice's name, then the preferred
   algorithm is CAST5; if software locates the key via Bob's name, then
   the preferred algorithm is IDEA.  If the key is located by Key ID,
   the algorithm of the primary User ID of the key provides the
   preferred symmetric algorithm.

   Revoking a self-signature or allowing it to expire has a semantic
   meaning that varies with the signature type.  Revoking the self-
   signature on a User ID effectively retires that user name.  The
   self-signature is a statement, "My name X is tied to my signing key
   K" and is corroborated by other users' certifications.  If another
   user revokes their certification, they are effectively saying that
   they no longer believe that name and that key are tied together.
   Similarly, if the users themselves revoke their self-signature, then
   the users no longer go by that name, no longer have that email
   address, etc.  Revoking a binding signature effectively retires that



Callas, et al               Standards Track                    [Page 27]

RFC 4880                 OpenPGP Message Format            November 2007


   subkey.  Revoking a direct-key signature cancels that signature.
   Please see the "Reason for Revocation" subpacket (Section 5.2.3.23)
   for more relevant detail.

   Since a self-signature contains important information about the key's
   use, an implementation SHOULD allow the user to rewrite the self-
   signature, and important information in it, such as preferences and
   key expiration.

   It is good practice to verify that a self-signature imported into an
   implementation doesn't advertise features that the implementation
   doesn't support, rewriting the signature as appropriate.

   An implementation that encounters multiple self-signatures on the
   same object may resolve the ambiguity in any way it sees fit, but it
   is RECOMMENDED that priority be given to the most recent self-
   signature.

5.2.3.4.  Signature Creation Time

   (4-octet time field)

   The time the signature was made.

   MUST be present in the hashed area.

5.2.3.5.  Issuer

   (8-octet Key ID)

   The OpenPGP Key ID of the key issuing the signature.

5.2.3.6.  Key Expiration Time

   (4-octet time field)

   The validity period of the key.  This is the number of seconds after
   the key creation time that the key expires.  If this is not present
   or has a value of zero, the key never expires.  This is found only on
   a self-signature.

5.2.3.7.  Preferred Symmetric Algorithms

   (array of one-octet values)

   Symmetric algorithm numbers that indicate which algorithms the key
   holder prefers to use.  The subpacket body is an ordered list of
   octets with the most preferred listed first.  It is assumed that only



Callas, et al               Standards Track                    [Page 28]

RFC 4880                 OpenPGP Message Format            November 2007


   algorithms listed are supported by the recipient's software.
   Algorithm numbers are in Section 9.  This is only found on a self-
   signature.

5.2.3.8.  Preferred Hash Algorithms

   (array of one-octet values)

   Message digest algorithm numbers that indicate which algorithms the
   key holder prefers to receive.  Like the preferred symmetric
   algorithms, the list is ordered.  Algorithm numbers are in Section 9.
   This is only found on a self-signature.

5.2.3.9.  Preferred Compression Algorithms

   (array of one-octet values)

   Compression algorithm numbers that indicate which algorithms the key
   holder prefers to use.  Like the preferred symmetric algorithms, the
   list is ordered.  Algorithm numbers are in Section 9.  If this
   subpacket is not included, ZIP is preferred.  A zero denotes that
   uncompressed data is preferred; the key holder's software might have
   no compression software in that implementation.  This is only found
   on a self-signature.

5.2.3.10.  Signature Expiration Time

   (4-octet time field)

   The validity period of the signature.  This is the number of seconds
   after the signature creation time that the signature expires.  If
   this is not present or has a value of zero, it never expires.

5.2.3.11.  Exportable Certification

   (1 octet of exportability, 0 for not, 1 for exportable)

   This subpacket denotes whether a certification signature is
   "exportable", to be used by other users than the signature's issuer.
   The packet body contains a Boolean flag indicating whether the
   signature is exportable.  If this packet is not present, the
   certification is exportable; it is equivalent to a flag containing a
   1.

   Non-exportable, or "local", certifications are signatures made by a
   user to mark a key as valid within that user's implementation only.





Callas, et al               Standards Track                    [Page 29]

RFC 4880                 OpenPGP Message Format            November 2007


   Thus, when an implementation prepares a user's copy of a key for
   transport to another user (this is the process of "exporting" the
   key), any local certification signatures are deleted from the key.

   The receiver of a transported key "imports" it, and likewise trims
   any local certifications.  In normal operation, there won't be any,
   assuming the import is performed on an exported key.  However, there
   are instances where this can reasonably happen.  For example, if an
   implementation allows keys to be imported from a key database in
   addition to an exported key, then this situation can arise.

   Some implementations do not represent the interest of a single user
   (for example, a key server).  Such implementations always trim local
   certifications from any key they handle.

5.2.3.12.  Revocable

   (1 octet of revocability, 0 for not, 1 for revocable)

   Signature's revocability status.  The packet body contains a Boolean
   flag indicating whether the signature is revocable.  Signatures that
   are not revocable have any later revocation signatures ignored.  They
   represent a commitment by the signer that he cannot revoke his
   signature for the life of his key.  If this packet is not present,
   the signature is revocable.

5.2.3.13.  Trust Signature

   (1 octet "level" (depth), 1 octet of trust amount)

   Signer asserts that the key is not only valid but also trustworthy at
   the specified level.  Level 0 has the same meaning as an ordinary
   validity signature.  Level 1 means that the signed key is asserted to
   be a valid trusted introducer, with the 2nd octet of the body
   specifying the degree of trust.  Level 2 means that the signed key is
   asserted to be trusted to issue level 1 trust signatures, i.e., that
   it is a "meta introducer".  Generally, a level n trust signature
   asserts that a key is trusted to issue level n-1 trust signatures.
   The trust amount is in a range from 0-255, interpreted such that
   values less than 120 indicate partial trust and values of 120 or
   greater indicate complete trust.  Implementations SHOULD emit values
   of 60 for partial trust and 120 for complete trust.









Callas, et al               Standards Track                    [Page 30]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.3.14.  Regular Expression

   (null-terminated regular expression)

   Used in conjunction with trust Signature packets (of level > 0) to
   limit the scope of trust that is extended.  Only signatures by the
   target key on User IDs that match the regular expression in the body
   of this packet have trust extended by the trust Signature subpacket.
   The regular expression uses the same syntax as the Henry Spencer's
   "almost public domain" regular expression [REGEX] package.  A
   description of the syntax is found in Section 8 below.

5.2.3.15.  Revocation Key

   (1 octet of class, 1 octet of public-key algorithm ID, 20 octets of
   fingerprint)

   Authorizes the specified key to issue revocation signatures for this
   key.  Class octet must have bit 0x80 set.  If the bit 0x40 is set,
   then this means that the revocation information is sensitive.  Other
   bits are for future expansion to other kinds of authorizations.  This
   is found on a self-signature.

   If the "sensitive" flag is set, the keyholder feels this subpacket
   contains private trust information that describes a real-world
   sensitive relationship.  If this flag is set, implementations SHOULD
   NOT export this signature to other users except in cases where the
   data needs to be available: when the signature is being sent to the
   designated revoker, or when it is accompanied by a revocation
   signature from that revoker.  Note that it may be appropriate to
   isolate this subpacket within a separate signature so that it is not
   combined with other subpackets that need to be exported.

5.2.3.16.  Notation Data

       (4 octets of flags, 2 octets of name length (M),
                           2 octets of value length (N),
                           M octets of name data,
                           N octets of value data)

   This subpacket describes a "notation" on the signature that the
   issuer wishes to make.  The notation has a name and a value, each of
   which are strings of octets.  There may be more than one notation in
   a signature.  Notations can be used for any extension the issuer of
   the signature cares to make.  The "flags" field holds four octets of
   flags.





Callas, et al               Standards Track                    [Page 31]

RFC 4880                 OpenPGP Message Format            November 2007


   All undefined flags MUST be zero.  Defined flags are as follows:

       First octet: 0x80 = human-readable.  This note value is text.
       Other octets: none.

   Notation names are arbitrary strings encoded in UTF-8.  They reside
   in two namespaces: The IETF namespace and the user namespace.

   The IETF namespace is registered with IANA.  These names MUST NOT
   contain the "@" character (0x40).  This is a tag for the user
   namespace.

   Names in the user namespace consist of a UTF-8 string tag followed by
   "@" followed by a DNS domain name.  Note that the tag MUST NOT
   contain an "@" character.  For example, the "sample" tag used by
   Example Corporation could be "sample@example.com".

   Names in a user space are owned and controlled by the owners of that
   domain.  Obviously, it's bad form to create a new name in a DNS space
   that you don't own.

   Since the user namespace is in the form of an email address,
   implementers MAY wish to arrange for that address to reach a person
   who can be consulted about the use of the named tag.  Note that due
   to UTF-8 encoding, not all valid user space name tags are valid email
   addresses.

   If there is a critical notation, the criticality applies to that
   specific notation and not to notations in general.

5.2.3.17.  Key Server Preferences

   (N octets of flags)

   This is a list of one-bit flags that indicate preferences that the
   key holder has about how the key is handled on a key server.  All
   undefined flags MUST be zero.

   First octet: 0x80 = No-modify
       the key holder requests that this key only be modified or updated
       by the key holder or an administrator of the key server.

   This is found only on a self-signature.








Callas, et al               Standards Track                    [Page 32]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.3.18.  Preferred Key Server

   (String)

   This is a URI of a key server that the key holder prefers be used for
   updates.  Note that keys with multiple User IDs can have a preferred
   key server for each User ID.  Note also that since this is a URI, the
   key server can actually be a copy of the key retrieved by ftp, http,
   finger, etc.

5.2.3.19.  Primary User ID

   (1 octet, Boolean)

   This is a flag in a User ID's self-signature that states whether this
   User ID is the main User ID for this key.  It is reasonable for an
   implementation to resolve ambiguities in preferences, etc. by
   referring to the primary User ID.  If this flag is absent, its value
   is zero.  If more than one User ID in a key is marked as primary, the
   implementation may resolve the ambiguity in any way it sees fit, but
   it is RECOMMENDED that priority be given to the User ID with the most
   recent self-signature.

   When appearing on a self-signature on a User ID packet, this
   subpacket applies only to User ID packets.  When appearing on a
   self-signature on a User Attribute packet, this subpacket applies
   only to User Attribute packets.  That is to say, there are two
   different and independent "primaries" -- one for User IDs, and one
   for User Attributes.

5.2.3.20.  Policy URI

   (String)

   This subpacket contains a URI of a document that describes the policy
   under which the signature was issued.

5.2.3.21.  Key Flags

   (N octets of flags)

   This subpacket contains a list of binary flags that hold information
   about a key.  It is a string of octets, and an implementation MUST
   NOT assume a fixed size.  This is so it can grow over time.  If a
   list is shorter than an implementation expects, the unstated flags
   are considered to be zero.  The defined flags are as follows:





Callas, et al               Standards Track                    [Page 33]

RFC 4880                 OpenPGP Message Format            November 2007


       First octet:

       0x01 - This key may be used to certify other keys.

       0x02 - This key may be used to sign data.

       0x04 - This key may be used to encrypt communications.

       0x08 - This key may be used to encrypt storage.

       0x10 - The private component of this key may have been split
              by a secret-sharing mechanism.

       0x20 - This key may be used for authentication.

       0x80 - The private component of this key may be in the
              possession of more than one person.

   Usage notes:

   The flags in this packet may appear in self-signatures or in
   certification signatures.  They mean different things depending on
   who is making the statement -- for example, a certification signature
   that has the "sign data" flag is stating that the certification is
   for that use.  On the other hand, the "communications encryption"
   flag in a self-signature is stating a preference that a given key be
   used for communications.  Note however, that it is a thorny issue to
   determine what is "communications" and what is "storage".  This
   decision is left wholly up to the implementation; the authors of this
   document do not claim any special wisdom on the issue and realize
   that accepted opinion may change.

   The "split key" (0x10) and "group key" (0x80) flags are placed on a
   self-signature only; they are meaningless on a certification
   signature.  They SHOULD be placed only on a direct-key signature
   (type 0x1F) or a subkey signature (type 0x18), one that refers to the
   key the flag applies to.

5.2.3.22.  Signer's User ID

   (String)

   This subpacket allows a keyholder to state which User ID is
   responsible for the signing.  Many keyholders use a single key for
   different purposes, such as business communications as well as
   personal communications.  This subpacket allows such a keyholder to
   state which of their roles is making a signature.




Callas, et al               Standards Track                    [Page 34]

RFC 4880                 OpenPGP Message Format            November 2007


   This subpacket is not appropriate to use to refer to a User Attribute
   packet.

5.2.3.23.  Reason for Revocation

   (1 octet of revocation code, N octets of reason string)

   This subpacket is used only in key revocation and certification
   revocation signatures.  It describes the reason why the key or
   certificate was revoked.

   The first octet contains a machine-readable code that denotes the
   reason for the revocation:

        0  - No reason specified (key revocations or cert revocations)
        1  - Key is superseded (key revocations)
        2  - Key material has been compromised (key revocations)
        3  - Key is retired and no longer used (key revocations)
        32 - User ID information is no longer valid (cert revocations)
   100-110 - Private Use

   Following the revocation code is a string of octets that gives
   information about the Reason for Revocation in human-readable form
   (UTF-8).  The string may be null, that is, of zero length.  The
   length of the subpacket is the length of the reason string plus one.
   An implementation SHOULD implement this subpacket, include it in all
   revocation signatures, and interpret revocations appropriately.
   There are important semantic differences between the reasons, and
   there are thus important reasons for revoking signatures.

   If a key has been revoked because of a compromise, all signatures
   created by that key are suspect.  However, if it was merely
   superseded or retired, old signatures are still valid.  If the
   revoked signature is the self-signature for certifying a User ID, a
   revocation denotes that that user name is no longer in use.  Such a
   revocation SHOULD include a 0x20 code.

   Note that any signature may be revoked, including a certification on
   some other person's key.  There are many good reasons for revoking a
   certification signature, such as the case where the keyholder leaves
   the employ of a business with an email address.  A revoked
   certification is no longer a part of validity calculations.









Callas, et al               Standards Track                    [Page 35]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.3.24.  Features

   (N octets of flags)

   The Features subpacket denotes which advanced OpenPGP features a
   user's implementation supports.  This is so that as features are
   added to OpenPGP that cannot be backwards-compatible, a user can
   state that they can use that feature.  The flags are single bits that
   indicate that a given feature is supported.

   This subpacket is similar to a preferences subpacket, and only
   appears in a self-signature.

   An implementation SHOULD NOT use a feature listed when sending to a
   user who does not state that they can use it.

   Defined features are as follows:

       First octet:

       0x01 - Modification Detection (packets 18 and 19)

   If an implementation implements any of the defined features, it
   SHOULD implement the Features subpacket, too.

   An implementation may freely infer features from other suitable
   implementation-dependent mechanisms.

5.2.3.25.  Signature Target

   (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)

   This subpacket identifies a specific target signature to which a
   signature refers.  For revocation signatures, this subpacket
   provides explicit designation of which signature is being revoked.
   For a third-party or timestamp signature, this designates what
   signature is signed.  All arguments are an identifier of that target
   signature.

   The N octets of hash data MUST be the size of the hash of the
   signature.  For example, a target signature with a SHA-1 hash MUST
   have 20 octets of hash data.









Callas, et al               Standards Track                    [Page 36]

RFC 4880                 OpenPGP Message Format            November 2007


5.2.3.26.  Embedded Signature

   (1 signature packet body)

   This subpacket contains a complete Signature packet body as
   specified in Section 5.2 above.  It is useful when one signature
   needs to refer to, or be incorporated in, another signature.

5.2.4.  Computing Signatures

   All signatures are formed by producing a hash over the signature
   data, and then using the resulting hash in the signature algorithm.

   For binary document signatures (type 0x00), the document data is
   hashed directly.  For text document signatures (type 0x01), the
   document is canonicalized by converting line endings to <CR><LF>,
   and the resulting data is hashed.

   When a signature is made over a key, the hash data starts with the
   octet 0x99, followed by a two-octet length of the key, and then body
   of the key packet.  (Note that this is an old-style packet header for
   a key packet with two-octet length.)  A subkey binding signature
   (type 0x18) or primary key binding signature (type 0x19) then hashes
   the subkey using the same format as the main key (also using 0x99 as
   the first octet).  Key revocation signatures (types 0x20 and 0x28)
   hash only the key being revoked.

   A certification signature (type 0x10 through 0x13) hashes the User
   ID being bound to the key into the hash context after the above
   data.  A V3 certification hashes the contents of the User ID or
   attribute packet packet, without any header.  A V4 certification
   hashes the constant 0xB4 for User ID certifications or the constant
   0xD1 for User Attribute certifications, followed by a four-octet
   number giving the length of the User ID or User Attribute data, and
   then the User ID or User Attribute data.

   When a signature is made over a Signature packet (type 0x50), the
   hash data starts with the octet 0x88, followed by the four-octet
   length of the signature, and then the body of the Signature packet.
   (Note that this is an old-style packet header for a Signature packet
   with the length-of-length set to zero.)  The unhashed subpacket data
   of the Signature packet being hashed is not included in the hash, and
   the unhashed subpacket data length value is set to zero.

   Once the data body is hashed, then a trailer is hashed.  A V3
   signature hashes five octets of the packet body, starting from the
   signature type field.  This data is the signature type, followed by
   the four-octet signature time.  A V4 signature hashes the packet body



Callas, et al               Standards Track                    [Page 37]

RFC 4880                 OpenPGP Message Format            November 2007


   starting from its first field, the version number, through the end
   of the hashed subpacket data.  Thus, the fields hashed are the
   signature version, the signature type, the public-key algorithm, the
   hash algorithm, the hashed subpacket length, and the hashed
   subpacket body.

   V4 signatures also hash in a final trailer of six octets: the
   version of the Signature packet, i.e., 0x04; 0xFF; and a four-octet,
   big-endian number that is the length of the hashed data from the
   Signature packet (note that this number does not include these final
   six octets).

   After all this has been hashed in a single hash context, the
   resulting hash field is used in the signature algorithm and placed
   at the end of the Signature packet.

5.2.4.1.  Subpacket Hints

   It is certainly possible for a signature to contain conflicting
   information in subpackets.  For example, a signature may contain
   multiple copies of a preference or multiple expiration times.  In
   most cases, an implementation SHOULD use the last subpacket in the
   signature, but MAY use any conflict resolution scheme that makes
   more sense.  Please note that we are intentionally leaving conflict
   resolution to the implementer; most conflicts are simply syntax
   errors, and the wishy-washy language here allows a receiver to be
   generous in what they accept, while putting pressure on a creator to
   be stingy in what they generate.

   Some apparent conflicts may actually make sense -- for example,
   suppose a keyholder has a V3 key and a V4 key that share the same
   RSA key material.  Either of these keys can verify a signature
   created by the other, and it may be reasonable for a signature to
   contain an issuer subpacket for each key, as a way of explicitly
   tying those keys to the signature.

5.3.  Symmetric-Key Encrypted Session Key Packets (Tag 3)

   The Symmetric-Key Encrypted Session Key packet holds the
   symmetric-key encryption of a session key used to encrypt a message.
   Zero or more Public-Key Encrypted Session Key packets and/or
   Symmetric-Key Encrypted Session Key packets may precede a
   Symmetrically Encrypted Data packet that holds an encrypted message.
   The message is encrypted with a session key, and the session key is
   itself encrypted and stored in the Encrypted Session Key packet or
   the Symmetric-Key Encrypted Session Key packet.





Callas, et al               Standards Track                    [Page 38]

RFC 4880                 OpenPGP Message Format            November 2007


   If the Symmetrically Encrypted Data packet is preceded by one or
   more Symmetric-Key Encrypted Session Key packets, each specifies a
   passphrase that may be used to decrypt the message.  This allows a
   message to be encrypted to a number of public keys, and also to one
   or more passphrases.  This packet type is new and is not generated
   by PGP 2.x or PGP 5.0.

   The body of this packet consists of:

     - A one-octet version number.  The only currently defined version
       is 4.

     - A one-octet number describing the symmetric algorithm used.

     - A string-to-key (S2K) specifier, length as defined above.

     - Optionally, the encrypted session key itself, which is decrypted
       with the string-to-key object.

   If the encrypted session key is not present (which can be detected
   on the basis of packet length and S2K specifier size), then the S2K
   algorithm applied to the passphrase produces the session key for
   decrypting the file, using the symmetric cipher algorithm from the
   Symmetric-Key Encrypted Session Key packet.

   If the encrypted session key is present, the result of applying the
   S2K algorithm to the passphrase is used to decrypt just that
   encrypted session key field, using CFB mode with an IV of all zeros.
   The decryption result consists of a one-octet algorithm identifier
   that specifies the symmetric-key encryption algorithm used to
   encrypt the following Symmetrically Encrypted Data packet, followed
   by the session key octets themselves.

   Note: because an all-zero IV is used for this decryption, the S2K
   specifier MUST use a salt value, either a Salted S2K or an
   Iterated-Salted S2K.  The salt value will ensure that the decryption
   key is not repeated even if the passphrase is reused.

5.4.  One-Pass Signature Packets (Tag 4)

   The One-Pass Signature packet precedes the signed data and contains
   enough information to allow the receiver to begin calculating any
   hashes needed to verify the signature.  It allows the Signature
   packet to be placed at the end of the message, so that the signer
   can compute the entire signed message in one pass.

   A One-Pass Signature does not interoperate with PGP 2.6.x or
   earlier.



Callas, et al               Standards Track                    [Page 39]

RFC 4880                 OpenPGP Message Format            November 2007


   The body of this packet consists of:

     - A one-octet version number.  The current version is 3.

     - A one-octet signature type.  Signature types are described in
       Section 5.2.1.

     - A one-octet number describing the hash algorithm used.

     - A one-octet number describing the public-key algorithm used.

     - An eight-octet number holding the Key ID of the signing key.

     - A one-octet number holding a flag showing whether the signature
       is nested.  A zero value indicates that the next packet is
       another One-Pass Signature packet that describes another
       signature to be applied to the same message data.

   Note that if a message contains more than one one-pass signature,
   then the Signature packets bracket the message; that is, the first
   Signature packet after the message corresponds to the last one-pass
   packet and the final Signature packet corresponds to the first
   one-pass packet.

5.5.  Key Material Packet

   A key material packet contains all the information about a public or
   private key.  There are four variants of this packet type, and two
   major versions.  Consequently, this section is complex.

5.5.1.  Key Packet Variants

5.5.1.1.  Public-Key Packet (Tag 6)

   A Public-Key packet starts a series of packets that forms an OpenPGP
   key (sometimes called an OpenPGP certificate).

5.5.1.2.  Public-Subkey Packet (Tag 14)

   A Public-Subkey packet (tag 14) has exactly the same format as a
   Public-Key packet, but denotes a subkey.  One or more subkeys may be
   associated with a top-level key.  By convention, the top-level key
   provides signature services, and the subkeys provide encryption
   services.

   Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
   packet.  This tag was selected for reuse because no previous version
   of PGP ever emitted comment packets but they did properly ignore



Callas, et al               Standards Track                    [Page 40]

RFC 4880                 OpenPGP Message Format            November 2007


   them.  Public-Subkey packets are ignored by PGP 2.6.x and do not
   cause it to fail, providing a limited degree of backward
   compatibility.

5.5.1.3.  Secret-Key Packet (Tag 5)

   A Secret-Key packet contains all the information that is found in a
   Public-Key packet, including the public-key material, but also
   includes the secret-key material after all the public-key fields.

5.5.1.4.  Secret-Subkey Packet (Tag 7)

   A Secret-Subkey packet (tag 7) is the subkey analog of the Secret
   Key packet and has exactly the same format.

5.5.2.  Public-Key Packet Formats

   There are two versions of key-material packets.  Version 3 packets
   were first generated by PGP 2.6.  Version 4 keys first appeared in
   PGP 5.0 and are the preferred key version for OpenPGP.

   OpenPGP implementations MUST create keys with version 4 format.  V3
   keys are deprecated; an implementation MUST NOT generate a V3 key,
   but MAY accept it.

   A version 3 public key or public-subkey packet contains:

     - A one-octet version number (3).

     - A four-octet number denoting the time that the key was created.

     - A two-octet number denoting the time in days that this key is
       valid.  If this number is zero, then it does not expire.

     - A one-octet number denoting the public-key algorithm of this key.

     - A series of multiprecision integers comprising the key material:

           - a multiprecision integer (MPI) of RSA public modulus n;

           - an MPI of RSA public encryption exponent e.

   V3 keys are deprecated.  They contain three weaknesses.  First, it is
   relatively easy to construct a V3 key that has the same Key ID as any
   other key because the Key ID is simply the low 64 bits of the public
   modulus.  Secondly, because the fingerprint of a V3 key hashes the
   key material, but not its length, there is an increased opportunity
   for fingerprint collisions.  Third, there are weaknesses in the MD5



Callas, et al               Standards Track                    [Page 41]

RFC 4880                 OpenPGP Message Format            November 2007


   hash algorithm that make developers prefer other algorithms.  See
   below for a fuller discussion of Key IDs and fingerprints.

   V2 keys are identical to the deprecated V3 keys except for the
   version number.  An implementation MUST NOT generate them and MAY
   accept or reject them as it sees fit.

   The version 4 format is similar to the version 3 format except for
   the absence of a validity period.  This has been moved to the
   Signature packet.  In addition, fingerprints of version 4 keys are
   calculated differently from version 3 keys, as described in the
   section "Enhanced Key Formats".

   A version 4 packet contains:

     - A one-octet version number (4).

     - A four-octet number denoting the time that the key was created.

     - A one-octet number denoting the public-key algorithm of this key.

     - A series of multiprecision integers comprising the key material.
       This algorithm-specific portion is:

       Algorithm-Specific Fields for RSA public keys:

         - multiprecision integer (MPI) of RSA public modulus n;

         - MPI of RSA public encryption exponent e.

       Algorithm-Specific Fields for DSA public keys:

         - MPI of DSA prime p;

         - MPI of DSA group order q (q is a prime divisor of p-1);

         - MPI of DSA group generator g;

         - MPI of DSA public-key value y (= g**x mod p where x
           is secret).

       Algorithm-Specific Fields for Elgamal public keys:

         - MPI of Elgamal prime p;

         - MPI of Elgamal group generator g;





Callas, et al               Standards Track                    [Page 42]

RFC 4880                 OpenPGP Message Format            November 2007


         - MPI of Elgamal public key value y (= g**x mod p where x
           is secret).

5.5.3.  Secret-Key Packet Formats

   The Secret-Key and Secret-Subkey packets contain all the data of the
   Public-Key and Public-Subkey packets, with additional algorithm-
   specific secret-key data appended, usually in encrypted form.

   The packet contains:

     - A Public-Key or Public-Subkey packet, as described above.

     - One octet indicating string-to-key usage conventions.  Zero
       indicates that the secret-key data is not encrypted.  255 or 254
       indicates that a string-to-key specifier is being given.  Any
       other value is a symmetric-key encryption algorithm identifier.

     - [Optional] If string-to-key usage octet was 255 or 254, a one-
       octet symmetric encryption algorithm.

     - [Optional] If string-to-key usage octet was 255 or 254, a
       string-to-key specifier.  The length of the string-to-key
       specifier is implied by its type, as described above.

     - [Optional] If secret data is encrypted (string-to-key usage octet
       not zero), an Initial Vector (IV) of the same length as the
       cipher's block size.

     - Plain or encrypted multiprecision integers comprising the secret
       key data.  These algorithm-specific fields are as described
       below.

     - If the string-to-key usage octet is zero or 255, then a two-octet
       checksum of the plaintext of the algorithm-specific portion (sum
       of all octets, mod 65536).  If the string-to-key usage octet was
       254, then a 20-octet SHA-1 hash of the plaintext of the
       algorithm-specific portion.  This checksum or hash is encrypted
       together with the algorithm-specific fields (if string-to-key
       usage octet is not zero).  Note that for all other values, a
       two-octet checksum is required.

       Algorithm-Specific Fields for RSA secret keys:

       - multiprecision integer (MPI) of RSA secret exponent d.

       - MPI of RSA secret prime value p.




Callas, et al               Standards Track                    [Page 43]

RFC 4880                 OpenPGP Message Format            November 2007


       - MPI of RSA secret prime value q (p < q).

       - MPI of u, the multiplicative inverse of p, mod q.

       Algorithm-Specific Fields for DSA secret keys:

       - MPI of DSA secret exponent x.

       Algorithm-Specific Fields for Elgamal secret keys:

       - MPI of Elgamal secret exponent x.

   Secret MPI values can be encrypted using a passphrase.  If a string-
   to-key specifier is given, that describes the algorithm for
   converting the passphrase to a key, else a simple MD5 hash of the
   passphrase is used.  Implementations MUST use a string-to-key
   specifier; the simple hash is for backward compatibility and is
   deprecated, though implementations MAY continue to use existing
   private keys in the old format.  The cipher for encrypting the MPIs
   is specified in the Secret-Key packet.

   Encryption/decryption of the secret data is done in CFB mode using
   the key created from the passphrase and the Initial Vector from the
   packet.  A different mode is used with V3 keys (which are only RSA)
   than with other key formats.  With V3 keys, the MPI bit count prefix
   (i.e., the first two octets) is not encrypted.  Only the MPI non-
   prefix data is encrypted.  Furthermore, the CFB state is
   resynchronized at the beginning of each new MPI value, so that the
   CFB block boundary is aligned with the start of the MPI data.

   With V4 keys, a simpler method is used.  All secret MPI values are
   encrypted in CFB mode, including the MPI bitcount prefix.

   The two-octet checksum that follows the algorithm-specific portion is
   the algebraic sum, mod 65536, of the plaintext of all the algorithm-
   specific octets (including MPI prefix and data).  With V3 keys, the
   checksum is stored in the clear.  With V4 keys, the checksum is
   encrypted like the algorithm-specific data.  This value is used to
   check that the passphrase was correct.  However, this checksum is
   deprecated; an implementation SHOULD NOT use it, but should rather
   use the SHA-1 hash denoted with a usage octet of 254.  The reason for
   this is that there are some attacks that involve undetectably
   modifying the secret key.








Callas, et al               Standards Track                    [Page 44]

RFC 4880                 OpenPGP Message Format            November 2007


5.6.  Compressed Data Packet (Tag 8)

   The Compressed Data packet contains compressed data.  Typically, this
   packet is found as the contents of an encrypted packet, or following
   a Signature or One-Pass Signature packet, and contains a literal data
   packet.

   The body of this packet consists of:

     - One octet that gives the algorithm used to compress the packet.

     - Compressed data, which makes up the remainder of the packet.

   A Compressed Data Packet's body contains an block that compresses
   some set of packets.  See section "Packet Composition" for details on
   how messages are formed.

   ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
   DEFLATE blocks.  Note that PGP V2.6 uses 13 bits of compression.  If
   an implementation uses more bits of compression, PGP V2.6 cannot
   decompress it.

   ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
   style blocks.

   BZip2-compressed packets are compressed using the BZip2 [BZ2]
   algorithm.

5.7.  Symmetrically Encrypted Data Packet (Tag 9)

   The Symmetrically Encrypted Data packet contains data encrypted with
   a symmetric-key algorithm.  When it has been decrypted, it contains
   other packets (usually a literal data packet or compressed data
   packet, but in theory other Symmetrically Encrypted Data packets or
   sequences of packets that form whole OpenPGP messages).

   The body of this packet consists of:

     - Encrypted data, the output of the selected symmetric-key cipher
       operating in OpenPGP's variant of Cipher Feedback (CFB) mode.

   The symmetric cipher used may be specified in a Public-Key or
   Symmetric-Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted Data packet.  In that case, the cipher
   algorithm octet is prefixed to the session key before it is
   encrypted.  If no packets of these types precede the encrypted data,
   the IDEA algorithm is used with the session key calculated as the MD5
   hash of the passphrase, though this use is deprecated.



Callas, et al               Standards Track                    [Page 45]

RFC 4880                 OpenPGP Message Format            November 2007


   The data is encrypted in CFB mode, with a CFB shift size equal to the
   cipher's block size.  The Initial Vector (IV) is specified as all
   zeros.  Instead of using an IV, OpenPGP prefixes a string of length
   equal to the block size of the cipher plus two to the data before it
   is encrypted.  The first block-size octets (for example, 8 octets for
   a 64-bit block length) are random, and the following two octets are
   copies of the last two octets of the IV.  For example, in an 8-octet
   block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
   octet 8.  In a cipher of length 16, octet 17 is a repeat of octet 15
   and octet 18 is a repeat of octet 16.  As a pedantic clarification,
   in both these examples, we consider the first octet to be numbered 1.

   After encrypting the first block-size-plus-two octets, the CFB state
   is resynchronized.  The last block-size octets of ciphertext are
   passed through the cipher and the block boundary is reset.

   The repetition of 16 bits in the random data prefixed to the message
   allows the receiver to immediately check whether the session key is
   incorrect.  See the "Security Considerations" section for hints on
   the proper use of this "quick check".

5.8.  Marker Packet (Obsolete Literal Packet) (Tag 10)

   An experimental version of PGP used this packet as the Literal
   packet, but no released version of PGP generated Literal packets with
   this tag.  With PGP 5.x, this packet has been reassigned and is
   reserved for use as the Marker packet.

   The body of this packet consists of:

     - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).

   Such a packet MUST be ignored when received.  It may be placed at the
   beginning of a message that uses features not available in PGP 2.6.x
   in order to cause that version to report that newer software is
   necessary to process the message.

5.9.  Literal Data Packet (Tag 11)

   A Literal Data packet contains the body of a message; data that is
   not to be further interpreted.

   The body of this packet consists of:

     - A one-octet field that describes how the data is formatted.






Callas, et al               Standards Track                    [Page 46]

RFC 4880                 OpenPGP Message Format            November 2007


   If it is a 'b' (0x62), then the Literal packet contains binary data.
   If it is a 't' (0x74), then it contains text data, and thus may need
   line ends converted to local form, or other text-mode changes.  The
   tag 'u' (0x75) means the same as 't', but also indicates that
   implementation believes that the literal data contains UTF-8 text.

   Early versions of PGP also defined a value of 'l' as a 'local' mode
   for machine-local conversions.  RFC 1991 [RFC1991] incorrectly stated
   this local mode flag as '1' (ASCII numeral one).  Both of these local
   modes are deprecated.

     - File name as a string (one-octet length, followed by a file
       name).  This may be a zero-length string.  Commonly, if the
       source of the encrypted data is a file, this will be the name of
       the encrypted file.  An implementation MAY consider the file name
       in the Literal packet to be a more authoritative name than the
       actual file name.

   If the special name "_CONSOLE" is used, the message is considered to
   be "for your eyes only".  This advises that the message data is
   unusually sensitive, and the receiving program should process it more
   carefully, perhaps avoiding storing the received data to disk, for
   example.

     - A four-octet number that indicates a date associated with the
       literal data.  Commonly, the date might be the modification date
       of a file, or the time the packet was created, or a zero that
       indicates no specific time.

     - The remainder of the packet is literal data.

       Text data is stored with <CR><LF> text endings (i.e., network-
       normal line endings).  These should be converted to native line
       endings by the receiving software.

5.10.  Trust Packet (Tag 12)

   The Trust packet is used only within keyrings and is not normally
   exported.  Trust packets contain data that record the user's
   specifications of which key holders are trustworthy introducers,
   along with other information that implementing software uses for
   trust information.  The format of Trust packets is defined by a given
   implementation.

   Trust packets SHOULD NOT be emitted to output streams that are
   transferred to other users, and they SHOULD be ignored on any input
   other than local keyring files.




Callas, et al               Standards Track                    [Page 47]

RFC 4880                 OpenPGP Message Format            November 2007


5.11.  User ID Packet (Tag 13)

   A User ID packet consists of UTF-8 text that is intended to represent
   the name and email address of the key holder.  By convention, it
   includes an RFC 2822 [RFC2822] mail name-addr, but there are no
   restrictions on its content.  The packet length in the header
   specifies the length of the User ID.

5.12.  User Attribute Packet (Tag 17)

   The User Attribute packet is a variation of the User ID packet.  It
   is capable of storing more types of data than the User ID packet,
   which is limited to text.  Like the User ID packet, a User Attribute
   packet may be certified by the key owner ("self-signed") or any other
   key owner who cares to certify it.  Except as noted, a User Attribute
   packet may be used anywhere that a User ID packet may be used.

   While User Attribute packets are not a required part of the OpenPGP
   standard, implementations SHOULD provide at least enough
   compatibility to properly handle a certification signature on the
   User Attribute packet.  A simple way to do this is by treating the
   User Attribute packet as a User ID packet with opaque contents, but
   an implementation may use any method desired.

   The User Attribute packet is made up of one or more attribute
   subpackets.  Each subpacket consists of a subpacket header and a
   body.  The header consists of:

     - the subpacket length (1, 2, or 5 octets)

     - the subpacket type (1 octet)

   and is followed by the subpacket specific data.

   The only currently defined subpacket type is 1, signifying an image.
   An implementation SHOULD ignore any subpacket of a type that it does
   not recognize.  Subpacket types 100 through 110 are reserved for
   private or experimental use.

5.12.1.  The Image Attribute Subpacket

   The Image Attribute subpacket is used to encode an image, presumably
   (but not required to be) that of the key owner.

   The Image Attribute subpacket begins with an image header.  The first
   two octets of the image header contain the length of the image
   header.  Note that unlike other multi-octet numerical values in this
   document, due to a historical accident this value is encoded as a



Callas, et al               Standards Track                    [Page 48]

RFC 4880                 OpenPGP Message Format            November 2007


   little-endian number.  The image header length is followed by a
   single octet for the image header version.  The only currently
   defined version of the image header is 1, which is a 16-octet image
   header.  The first three octets of a version 1 image header are thus
   0x10, 0x00, 0x01.

   The fourth octet of a version 1 image header designates the encoding
   format of the image.  The only currently defined encoding format is
   the value 1 to indicate JPEG.  Image format types 100 through 110 are
   reserved for private or experimental use.  The rest of the version 1
   image header is made up of 12 reserved octets, all of which MUST be
   set to 0.

   The rest of the image subpacket contains the image itself.  As the
   only currently defined image type is JPEG, the image is encoded in
   the JPEG File Interchange Format (JFIF), a standard file format for
   JPEG images [JFIF].

   An implementation MAY try to determine the type of an image by
   examination of the image data if it is unable to handle a particular
   version of the image header or if a specified encoding format value
   is not recognized.

5.13.  Sym. Encrypted Integrity Protected Data Packet (Tag 18)

   The Symmetrically Encrypted Integrity Protected Data packet is a
   variant of the Symmetrically Encrypted Data packet.  It is a new
   feature created for OpenPGP that addresses the problem of detecting a
   modification to encrypted data.  It is used in combination with a
   Modification Detection Code packet.

   There is a corresponding feature in the features Signature subpacket
   that denotes that an implementation can properly use this packet
   type.  An implementation MUST support decrypting these packets and
   SHOULD prefer generating them to the older Symmetrically Encrypted
   Data packet when possible.  Since this data packet protects against
   modification attacks, this standard encourages its proliferation.
   While blanket adoption of this data packet would create
   interoperability problems, rapid adoption is nevertheless important.
   An implementation SHOULD specifically denote support for this packet,
   but it MAY infer it from other mechanisms.

   For example, an implementation might infer from the use of a cipher
   such as Advanced Encryption Standard (AES) or Twofish that a user
   supports this feature.  It might place in the unhashed portion of
   another user's key signature a Features subpacket.  It might also
   present a user with an opportunity to regenerate their own self-
   signature with a Features subpacket.



Callas, et al               Standards Track                    [Page 49]

RFC 4880                 OpenPGP Message Format            November 2007


   This packet contains data encrypted with a symmetric-key algorithm
   and protected against modification by the SHA-1 hash algorithm.  When
   it has been decrypted, it will typically contain other packets (often
   a Literal Data packet or Compressed Data packet).  The last decrypted
   packet in this packet's payload MUST be a Modification Detection Code
   packet.

   The body of this packet consists of:

     - A one-octet version number.  The only currently defined value is
       1.

     - Encrypted data, the output of the selected symmetric-key cipher
       operating in Cipher Feedback mode with shift amount equal to the
       block size of the cipher (CFB-n where n is the block size).

   The symmetric cipher used MUST be specified in a Public-Key or
   Symmetric-Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted Data packet.  In either case, the cipher
   algorithm octet is prefixed to the session key before it is
   encrypted.

   The data is encrypted in CFB mode, with a CFB shift size equal to the
   cipher's block size.  The Initial Vector (IV) is specified as all
   zeros.  Instead of using an IV, OpenPGP prefixes an octet string to
   the data before it is encrypted.  The length of the octet string
   equals the block size of the cipher in octets, plus two.  The first
   octets in the group, of length equal to the block size of the cipher,
   are random; the last two octets are each copies of their 2nd
   preceding octet.  For example, with a cipher whose block size is 128
   bits or 16 octets, the prefix data will contain 16 random octets,
   then two more octets, which are copies of the 15th and 16th octets,
   respectively.  Unlike the Symmetrically Encrypted Data Packet, no
   special CFB resynchronization is done after encrypting this prefix
   data.  See "OpenPGP CFB Mode" below for more details.

   The repetition of 16 bits in the random data prefixed to the message
   allows the receiver to immediately check whether the session key is
   incorrect.

   The plaintext of the data to be encrypted is passed through the SHA-1
   hash function, and the result of the hash is appended to the
   plaintext in a Modification Detection Code packet.  The input to the
   hash function includes the prefix data described above; it includes
   all of the plaintext, and then also includes two octets of values
   0xD3, 0x14.  These represent the encoding of a Modification Detection
   Code packet tag and length field of 20 octets.




Callas, et al               Standards Track                    [Page 50]

RFC 4880                 OpenPGP Message Format            November 2007


   The resulting hash value is stored in a Modification Detection Code
   (MDC) packet, which MUST use the two octet encoding just given to
   represent its tag and length field.  The body of the MDC packet is
   the 20-octet output of the SHA-1 hash.

   The Modification Detection Code packet is appended to the plaintext
   and encrypted along with the plaintext using the same CFB context.

   During decryption, the plaintext data should be hashed with SHA-1,
   including the prefix data as well as the packet tag and length field
   of the Modification Detection Code packet.  The body of the MDC
   packet, upon decryption, is compared with the result of the SHA-1
   hash.

   Any failure of the MDC indicates that the message has been modified
   and MUST be treated as a security problem.  Failures include a
   difference in the hash values, but also the absence of an MDC packet,
   or an MDC packet in any position other than the end of the plaintext.
   Any failure SHOULD be reported to the user.

   Note: future designs of new versions of this packet should consider
   rollback attacks since it will be possible for an attacker to change
   the version back to 1.

      NON-NORMATIVE EXPLANATION

      The MDC system, as packets 18 and 19 are called, were created to
      provide an integrity mechanism that is less strong than a
      signature, yet stronger than bare CFB encryption.

      It is a limitation of CFB encryption that damage to the ciphertext
      will corrupt the affected cipher blocks and the block following.
      Additionally, if data is removed from the end of a CFB-encrypted
      block, that removal is undetectable.  (Note also that CBC mode has
      a similar limitation, but data removed from the front of the block
      is undetectable.)

      The obvious way to protect or authenticate an encrypted block is
      to digitally sign it.  However, many people do not wish to
      habitually sign data, for a large number of reasons beyond the
      scope of this document.  Suffice it to say that many people
      consider properties such as deniability to be as valuable as
      integrity.

      OpenPGP addresses this desire to have more security than raw
      encryption and yet preserve deniability with the MDC system.  An
      MDC is intentionally not a MAC.  Its name was not selected by
      accident.  It is analogous to a checksum.



Callas, et al               Standards Track                    [Page 51]

RFC 4880                 OpenPGP Message Format            November 2007


      Despite the fact that it is a relatively modest system, it has
      proved itself in the real world.  It is an effective defense to
      several attacks that have surfaced since it has been created.  It
      has met its modest goals admirably.

      Consequently, because it is a modest security system, it has
      modest requirements on the hash function(s) it employs.  It does
      not rely on a hash function being collision-free, it relies on a
      hash function being one-way.  If a forger, Frank, wishes to send
      Alice a (digitally) unsigned message that says, "I've always
      secretly loved you, signed Bob", it is far easier for him to
      construct a new message than it is to modify anything intercepted
      from Bob.  (Note also that if Bob wishes to communicate secretly
      with Alice, but without authentication or identification and with
      a threat model that includes forgers, he has a problem that
      transcends mere cryptography.)

      Note also that unlike nearly every other OpenPGP subsystem, there
      are no parameters in the MDC system.  It hard-defines SHA-1 as its
      hash function.  This is not an accident.  It is an intentional
      choice to avoid downgrade and cross-grade attacks while making a
      simple, fast system.  (A downgrade attack would be an attack that
      replaced SHA-256 with SHA-1, for example.  A cross-grade attack
      would replace SHA-1 with another 160-bit hash, such as RIPE-
      MD/160, for example.)

      However, given the present state of hash function cryptanalysis
      and cryptography, it may be desirable to upgrade the MDC system to
      a new hash function.  See Section 13.11 in the "IANA
      Considerations" for guidance.

5.14.  Modification Detection Code Packet (Tag 19)

   The Modification Detection Code packet contains a SHA-1 hash of
   plaintext data, which is used to detect message modification.  It is
   only used with a Symmetrically Encrypted Integrity Protected Data
   packet.  The Modification Detection Code packet MUST be the last
   packet in the plaintext data that is encrypted in the Symmetrically
   Encrypted Integrity Protected Data packet, and MUST appear in no
   other place.

   A Modification Detection Code packet MUST have a length of 20 octets.









Callas, et al               Standards Track                    [Page 52]

RFC 4880                 OpenPGP Message Format            November 2007


   The body of this packet consists of:

     - A 20-octet SHA-1 hash of the preceding plaintext data of the
       Symmetrically Encrypted Integrity Protected Data packet,
       including prefix data, the tag octet, and length octet of the
       Modification Detection Code packet.

   Note that the Modification Detection Code packet MUST always use a
   new format encoding of the packet tag, and a one-octet encoding of
   the packet length.  The reason for this is that the hashing rules for
   modification detection include a one-octet tag and one-octet length
   in the data hash.  While this is a bit restrictive, it reduces
   complexity.

6.  Radix-64 Conversions

   As stated in the introduction, OpenPGP's underlying native
   representation for objects is a stream of arbitrary octets, and some
   systems desire these objects to be immune to damage caused by
   character set translation, data conversions, etc.

   In principle, any printable encoding scheme that met the requirements
   of the unsafe channel would suffice, since it would not change the
   underlying binary bit streams of the native OpenPGP data structures.
   The OpenPGP standard specifies one such printable encoding scheme to
   ensure interoperability.

   OpenPGP's Radix-64 encoding is composed of two parts: a base64
   encoding of the binary data and a checksum.  The base64 encoding is
   identical to the MIME base64 content-transfer-encoding [RFC2045].

   The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
   four characters of radix-64 encoding by the same MIME base64
   transformation, preceded by an equal sign (=).  The CRC is computed
   by using the generator 0x864CFB and an initialization of 0xB704CE.
   The accumulation is done on the data before it is converted to
   radix-64, rather than on the converted data.  A sample implementation
   of this algorithm is in the next section.

   The checksum with its leading equal sign MAY appear on the first line
   after the base64 encoded data.

   Rationale for CRC-24: The size of 24 bits fits evenly into printable
   base64.  The nonzero initialization can detect more errors than a
   zero initialization.






Callas, et al               Standards Track                    [Page 53]

RFC 4880                 OpenPGP Message Format            November 2007


6.1.  An Implementation of the CRC-24 in "C"

      #define CRC24_INIT 0xB704CEL
      #define CRC24_POLY 0x1864CFBL

      typedef long crc24;
      crc24 crc_octets(unsigned char *octets, size_t len)
      {
          crc24 crc = CRC24_INIT;
          int i;
          while (len--) {
              crc ^= (*octets++) << 16;
              for (i = 0; i < 8; i++) {
                  crc <<= 1;
                  if (crc & 0x1000000)
                      crc ^= CRC24_POLY;
              }
          }
          return crc & 0xFFFFFFL;
      }

6.2.  Forming ASCII Armor

   When OpenPGP encodes data into ASCII Armor, it puts specific headers
   around the Radix-64 encoded data, so OpenPGP can reconstruct the data
   later.  An OpenPGP implementation MAY use ASCII armor to protect raw
   binary data.  OpenPGP informs the user what kind of data is encoded
   in the ASCII armor through the use of the headers.

   Concatenating the following data creates ASCII Armor:

     - An Armor Header Line, appropriate for the type of data

     - Armor Headers

     - A blank (zero-length, or containing only whitespace) line

     - The ASCII-Armored data

     - An Armor Checksum

     - The Armor Tail, which depends on the Armor Header Line

   An Armor Header Line consists of the appropriate header line text
   surrounded by five (5) dashes ('-', 0x2D) on either side of the
   header line text.  The header line text is chosen based upon the type
   of data that is being encoded in Armor, and how it is being encoded.
   Header line texts include the following strings:



Callas, et al               Standards Track                    [Page 54]

RFC 4880                 OpenPGP Message Format            November 2007


   BEGIN PGP MESSAGE
       Used for signed, encrypted, or compressed files.

   BEGIN PGP PUBLIC KEY BLOCK
       Used for armoring public keys.

   BEGIN PGP PRIVATE KEY BLOCK
       Used for armoring private keys.

   BEGIN PGP MESSAGE, PART X/Y
       Used for multi-part messages, where the armor is split amongst Y
       parts, and this is the Xth part out of Y.

   BEGIN PGP MESSAGE, PART X
       Used for multi-part messages, where this is the Xth part of an
       unspecified number of parts.  Requires the MESSAGE-ID Armor
       Header to be used.

   BEGIN PGP SIGNATURE
       Used for detached signatures, OpenPGP/MIME signatures, and
       cleartext signatures.  Note that PGP 2.x uses BEGIN PGP MESSAGE
       for detached signatures.

   Note that all these Armor Header Lines are to consist of a complete
   line.  That is to say, there is always a line ending preceding the
   starting five dashes, and following the ending five dashes.  The
   header lines, therefore, MUST start at the beginning of a line, and
   MUST NOT have text other than whitespace following them on the same
   line.  These line endings are considered a part of the Armor Header
   Line for the purposes of determining the content they delimit.  This
   is particularly important when computing a cleartext signature (see
   below).

   The Armor Headers are pairs of strings that can give the user or the
   receiving OpenPGP implementation some information about how to decode
   or use the message.  The Armor Headers are a part of the armor, not a
   part of the message, and hence are not protected by any signatures
   applied to the message.

   The format of an Armor Header is that of a key-value pair.  A colon
   (':' 0x38) and a single space (0x20) separate the key and value.
   OpenPGP should consider improperly formatted Armor Headers to be
   corruption of the ASCII Armor.  Unknown keys should be reported to
   the user, but OpenPGP should continue to process the message.

   Note that some transport methods are sensitive to line length.  While
   there is a limit of 76 characters for the Radix-64 data (Section
   6.3), there is no limit to the length of Armor Headers.  Care should



Callas, et al               Standards Track                    [Page 55]

RFC 4880                 OpenPGP Message Format            November 2007


   be taken that the Armor Headers are short enough to survive
   transport.  One way to do this is to repeat an Armor Header key
   multiple times with different values for each so that no one line is
   overly long.

   Currently defined Armor Header Keys are as follows:

     - "Version", which states the OpenPGP implementation and version
       used to encode the message.

     - "Comment", a user-defined comment.  OpenPGP defines all text to
       be in UTF-8.  A comment may be any UTF-8 string.  However, the
       whole point of armoring is to provide seven-bit-clean data.
       Consequently, if a comment has characters that are outside the
       US-ASCII range of UTF, they may very well not survive transport.

     - "MessageID", a 32-character string of printable characters.  The
       string must be the same for all parts of a multi-part message
       that uses the "PART X" Armor Header.  MessageID strings should be
       unique enough that the recipient of the mail can associate all
       the parts of a message with each other.  A good checksum or
       cryptographic hash function is sufficient.

       The MessageID SHOULD NOT appear unless it is in a multi-part
       message.  If it appears at all, it MUST be computed from the
       finished (encrypted, signed, etc.) message in a deterministic
       fashion, rather than contain a purely random value.  This is to
       allow the legitimate recipient to determine that the MessageID
       cannot serve as a covert means of leaking cryptographic key
       information.

     - "Hash", a comma-separated list of hash algorithms used in this
       message.  This is used only in cleartext signed messages.

     - "Charset", a description of the character set that the plaintext
       is in.  Please note that OpenPGP defines text to be in UTF-8.  An
       implementation will get best results by translating into and out
       of UTF-8.  However, there are many instances where this is easier
       said than done.  Also, there are communities of users who have no
       need for UTF-8 because they are all happy with a character set
       like ISO Latin-5 or a Japanese character set.  In such instances,
       an implementation MAY override the UTF-8 default by using this
       header key.  An implementation MAY implement this key and any
       translations it cares to; an implementation MAY ignore it and
       assume all text is UTF-8.






Callas, et al               Standards Track                    [Page 56]

RFC 4880                 OpenPGP Message Format            November 2007


       The Armor Tail Line is composed in the same manner as the Armor
       Header Line, except the string "BEGIN" is replaced by the string
       "END".

6.3.  Encoding Binary in Radix-64

   The encoding process represents 24-bit groups of input bits as output
   strings of 4 encoded characters.  Proceeding from left to right, a
   24-bit input group is formed by concatenating three 8-bit input
   groups.  These 24 bits are then treated as four concatenated 6-bit
   groups, each of which is translated into a single digit in the
   Radix-64 alphabet.  When encoding a bit stream with the Radix-64
   encoding, the bit stream must be presumed to be ordered with the most
   significant bit first.  That is, the first bit in the stream will be
   the high-order bit in the first 8-bit octet, and the eighth bit will
   be the low-order bit in the first 8-bit octet, and so on.

         +--first octet--+-second octet--+--third octet--+
         |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
         +-----------+---+-------+-------+---+-----------+
         |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
         +--1.index--+--2.index--+--3.index--+--4.index--+

   Each 6-bit group is used as an index into an array of 64 printable
   characters from the table below.  The character referenced by the
   index is placed in the output string.

     Value Encoding  Value Encoding  Value Encoding  Value Encoding
         0 A            17 R            34 i            51 z
         1 B            18 S            35 j            52 0
         2 C            19 T            36 k            53 1
         3 D            20 U            37 l            54 2
         4 E            21 V            38 m            55 3
         5 F            22 W            39 n            56 4
         6 G            23 X            40 o            57 5
         7 H            24 Y            41 p            58 6
         8 I            25 Z            42 q            59 7
         9 J            26 a            43 r            60 8
        10 K            27 b            44 s            61 9
        11 L            28 c            45 t            62 +
        12 M            29 d            46 u            63 /
        13 N            30 e            47 v
        14 O            31 f            48 w         (pad) =
        15 P            32 g            49 x
        16 Q            33 h            50 y

   The encoded output stream must be represented in lines of no more
   than 76 characters each.



Callas, et al               Standards Track                    [Page 57]

RFC 4880                 OpenPGP Message Format            November 2007


   Special processing is performed if fewer than 24 bits are available
   at the end of the data being encoded.  There are three possibilities:

   1. The last data group has 24 bits (3 octets).  No special processing
      is needed.

   2. The last data group has 16 bits (2 octets).  The first two 6-bit
      groups are processed as above.  The third (incomplete) data group
      has two zero-value bits added to it, and is processed as above.  A
      pad character (=) is added to the output.

   3. The last data group has 8 bits (1 octet).  The first 6-bit group
      is processed as above.  The second (incomplete) data group has
      four zero-value bits added to it, and is processed as above.  Two
      pad characters (=) are added to the output.

6.4.  Decoding Radix-64

   In Radix-64 data, characters other than those in the table, line
   breaks, and other white space probably indicate a transmission error,
   about which a warning message or even a message rejection might be
   appropriate under some circumstances.  Decoding software must ignore
   all white space.

   Because it is used only for padding at the end of the data, the
   occurrence of any "=" characters may be taken as evidence that the
   end of the data has been reached (without truncation in transit).  No
   such assurance is possible, however, when the number of octets
   transmitted was a multiple of three and no "=" characters are
   present.





















Callas, et al               Standards Track                    [Page 58]

RFC 4880                 OpenPGP Message Format            November 2007


6.5.  Examples of Radix-64

   Input data:  0x14FB9C03D97E
   Hex:     1   4    F   B    9   C     | 0   3    D   9    7   E
   8-bit:   00010100 11111011 10011100  | 00000011 11011001 11111110
   6-bit:   000101 001111 101110 011100 | 000000 111101 100111 111110
   Decimal: 5      15     46     28       0      61     37     62
   Output:  F      P      u      c        A      9      l      +
   Input data:  0x14FB9C03D9
   Hex:     1   4    F   B    9   C     | 0   3    D   9
   8-bit:   00010100 11111011 10011100  | 00000011 11011001
                                                   pad with 00
   6-bit:   000101 001111 101110 011100 | 000000 111101 100100
   Decimal: 5      15     46     28       0      61     36
                                                      pad with =
   Output:  F      P      u      c        A      9      k      =
   Input data:  0x14FB9C03
   Hex:     1   4    F   B    9   C     | 0   3
   8-bit:   00010100 11111011 10011100  | 00000011
                                          pad with 0000
   6-bit:   000101 001111 101110 011100 | 000000 110000
   Decimal: 5      15     46     28       0      48
                                               pad with =      =
   Output:  F      P      u      c        A      w      =      =

6.6.  Example of an ASCII Armored Message

   -----BEGIN PGP MESSAGE-----
   Version: OpenPrivacy 0.99

   yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
   vBSFjNSiVHsuAA==
   =njUN
   -----END PGP MESSAGE-----

   Note that this example has extra indenting; an actual armored message
   would have no leading whitespace.

7.  Cleartext Signature Framework

   It is desirable to be able to sign a textual octet stream without
   ASCII armoring the stream itself, so the signed text is still
   readable without special software.  In order to bind a signature to
   such a cleartext, this framework is used.  (Note that this framework
   is not intended to be reversible.  RFC 3156 [RFC3156] defines another
   way to sign cleartext messages for environments that support MIME.)





Callas, et al               Standards Track                    [Page 59]

RFC 4880                 OpenPGP Message Format            November 2007


   The cleartext signed message consists of:

     - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
       single line,

     - One or more "Hash" Armor Headers,

     - Exactly one empty line not included into the message digest,

     - The dash-escaped cleartext that is included into the message
       digest,

     - The ASCII armored signature(s) including the '-----BEGIN PGP
       SIGNATURE-----' Armor Header and Armor Tail Lines.

   If the "Hash" Armor Header is given, the specified message digest
   algorithm(s) are used for the signature.  If there are no such
   headers, MD5 is used.  If MD5 is the only hash used, then an
   implementation MAY omit this header for improved V2.x compatibility.
   If more than one message digest is used in the signature, the "Hash"
   armor header contains a comma-delimited list of used message digests.

   Current message digest names are described below with the algorithm
   IDs.

   An implementation SHOULD add a line break after the cleartext, but
   MAY omit it if the cleartext ends with a line break.  This is for
   visual clarity.

7.1.  Dash-Escaped Text

   The cleartext content of the message must also be dash-escaped.

   Dash-escaped cleartext is the ordinary cleartext where every line
   starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
   (0x2D) and space ' ' (0x20).  This prevents the parser from
   recognizing armor headers of the cleartext itself.  An implementation
   MAY dash-escape any line, SHOULD dash-escape lines commencing "From"
   followed by a space, and MUST dash-escape any line commencing in a
   dash.  The message digest is computed using the cleartext itself, not
   the dash-escaped form.

   As with binary signatures on text documents, a cleartext signature is
   calculated on the text using canonical <CR><LF> line endings.  The
   line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP
   SIGNATURE-----' line that terminates the signed text is not
   considered part of the signed text.




Callas, et al               Standards Track                    [Page 60]

RFC 4880                 OpenPGP Message Format            November 2007


   When reversing dash-escaping, an implementation MUST strip the string
   "- " if it occurs at the beginning of a line, and SHOULD warn on "-"
   and any character other than a space at the beginning of a line.

   Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
   the end of any line is removed when the cleartext signature is
   generated.

8.  Regular Expressions

   A regular expression is zero or more branches, separated by '|'.  It
   matches anything that matches one of the branches.

   A branch is zero or more pieces, concatenated.  It matches a match
   for the first, followed by a match for the second, etc.

   A piece is an atom possibly followed by '*', '+', or '?'.  An atom
   followed by '*' matches a sequence of 0 or more matches of the atom.
   An atom followed by '+' matches a sequence of 1 or more matches of
   the atom.  An atom followed by '?' matches a match of the atom, or
   the null string.

   An atom is a regular expression in parentheses (matching a match for
   the regular expression), a range (see below), '.' (matching any
   single character), '^' (matching the null string at the beginning of
   the input string), '$' (matching the null string at the end of the
   input string), a '\' followed by a single character (matching that
   character), or a single character with no other significance
   (matching that character).

   A range is a sequence of characters enclosed in '[]'.  It normally
   matches any single character from the sequence.  If the sequence
   begins with '^', it matches any single character not from the rest of
   the sequence.  If two characters in the sequence are separated
   by '-', this is shorthand for the full list of ASCII characters
   between them (e.g., '[0-9]' matches any decimal digit).  To include a
   literal ']' in the sequence, make it the first character (following a
   possible '^').  To include a literal '-', make it the first or last
   character.

9.  Constants

   This section describes the constants used in OpenPGP.

   Note that these tables are not exhaustive lists; an implementation
   MAY implement an algorithm not on these lists, so long as the
   algorithm numbers are chosen from the private or experimental
   algorithm range.



Callas, et al               Standards Track                    [Page 61]

RFC 4880                 OpenPGP Message Format            November 2007


   See the section "Notes on Algorithms" below for more discussion of
   the algorithms.

9.1.  Public-Key Algorithms

      ID           Algorithm
      --           ---------
      1          - RSA (Encrypt or Sign) [HAC]
      2          - RSA Encrypt-Only [HAC]
      3          - RSA Sign-Only [HAC]
      16         - Elgamal (Encrypt-Only) [ELGAMAL] [HAC]
      17         - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
      18         - Reserved for Elliptic Curve
      19         - Reserved for ECDSA
      20         - Reserved (formerly Elgamal Encrypt or Sign)
      21         - Reserved for Diffie-Hellman (X9.42,
                   as defined for IETF-S/MIME)
      100 to 110 - Private/Experimental algorithm

   Implementations MUST implement DSA for signatures, and Elgamal for
   encryption.  Implementations SHOULD implement RSA keys (1).  RSA
   Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be
   generated, but may be interpreted.  See Section 13.5.  See Section
   13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt or
   Sign (20), and X9.42 (21).  Implementations MAY implement any other
   algorithm.

9.2.  Symmetric-Key Algorithms

       ID           Algorithm
       --           ---------
       0          - Plaintext or unencrypted data
       1          - IDEA [IDEA]
       2          - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
                    168 bit key derived from 192)
       3          - CAST5 (128 bit key, as per [RFC2144])
       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
       5          - Reserved
       6          - Reserved
       7          - AES with 128-bit key [AES]
       8          - AES with 192-bit key
       9          - AES with 256-bit key
       10         - Twofish with 256-bit key [TWOFISH]
       100 to 110 - Private/Experimental algorithm

   Implementations MUST implement TripleDES.  Implementations SHOULD
   implement AES-128 and CAST5.  Implementations that interoperate with




Callas, et al               Standards Track                    [Page 62]

RFC 4880                 OpenPGP Message Format            November 2007


   PGP 2.6 or earlier need to support IDEA, as that is the only
   symmetric cipher those versions use.  Implementations MAY implement
   any other algorithm.

9.3.  Compression Algorithms

       ID           Algorithm
       --           ---------
       0          - Uncompressed
       1          - ZIP [RFC1951]
       2          - ZLIB [RFC1950]
       3          - BZip2 [BZ2]
       100 to 110 - Private/Experimental algorithm

   Implementations MUST implement uncompressed data.  Implementations
   SHOULD implement ZIP.  Implementations MAY implement any other
   algorithm.

9.4.  Hash Algorithms

      ID           Algorithm                             Text Name
      --           ---------                             ---------
      1          - MD5 [HAC]                             "MD5"
      2          - SHA-1 [FIPS180]                       "SHA1"
      3          - RIPE-MD/160 [HAC]                     "RIPEMD160"
      4          - Reserved
      5          - Reserved
      6          - Reserved
      7          - Reserved
      8          - SHA256 [FIPS180]                      "SHA256"
      9          - SHA384 [FIPS180]                      "SHA384"
      10         - SHA512 [FIPS180]                      "SHA512"
      11         - SHA224 [FIPS180]                      "SHA224"
      100 to 110 - Private/Experimental algorithm

   Implementations MUST implement SHA-1.  Implementations MAY implement
   other algorithms.  MD5 is deprecated.

10.  IANA Considerations

   OpenPGP is highly parameterized, and consequently there are a number
   of considerations for allocating parameters for extensions.  This
   section describes how IANA should look at extensions to the protocol
   as described in this document.







Callas, et al               Standards Track                    [Page 63]

RFC 4880                 OpenPGP Message Format            November 2007


10.1.  New String-to-Key Specifier Types

   OpenPGP S2K specifiers contain a mechanism for new algorithms to turn
   a string into a key.  This specification creates a registry of S2K
   specifier types.  The registry includes the S2K type, the name of the
   S2K, and a reference to the defining specification.  The initial
   values for this registry can be found in Section 3.7.1.  Adding a new
   S2K specifier MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

10.2.  New Packets

   Major new features of OpenPGP are defined through new packet types.
   This specification creates a registry of packet types.  The registry
   includes the packet type, the name of the packet, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 4.3.  Adding a new packet type MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].

10.2.1.  User Attribute Types

   The User Attribute packet permits an extensible mechanism for other
   types of certificate identification.  This specification creates a
   registry of User Attribute types.  The registry includes the User
   Attribute type, the name of the User Attribute, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 5.12.  Adding a new User Attribute type MUST be
   done through the IETF CONSENSUS method, as described in [RFC2434].

10.2.1.1.  Image Format Subpacket Types

   Within User Attribute packets, there is an extensible mechanism for
   other types of image-based user attributes.  This specification
   creates a registry of Image Attribute subpacket types.  The registry
   includes the Image Attribute subpacket type, the name of the Image
   Attribute subpacket, and a reference to the defining specification.
   The initial values for this registry can be found in Section 5.12.1.
   Adding a new Image Attribute subpacket type MUST be done through the
   IETF CONSENSUS method, as described in [RFC2434].

10.2.2.  New Signature Subpackets

   OpenPGP signatures contain a mechanism for signed (or unsigned) data
   to be added to them for a variety of purposes in the Signature
   subpackets as discussed in Section 5.2.3.1.  This specification
   creates a registry of Signature subpacket types.  The registry
   includes the Signature subpacket type, the name of the subpacket, and
   a reference to the defining specification.  The initial values for



Callas, et al               Standards Track                    [Page 64]

RFC 4880                 OpenPGP Message Format            November 2007


   this registry can be found in Section 5.2.3.1.  Adding a new
   Signature subpacket MUST be done through the IETF CONSENSUS method,
   as described in [RFC2434].

10.2.2.1.  Signature Notation Data Subpackets

   OpenPGP signatures further contain a mechanism for extensions in
   signatures.  These are the Notation Data subpackets, which contain a
   key/value pair.  Notations contain a user space that is completely
   unmanaged and an IETF space.

   This specification creates a registry of Signature Notation Data
   types.  The registry includes the Signature Notation Data type, the
   name of the Signature Notation Data, its allowed values, and a
   reference to the defining specification.  The initial values for this
   registry can be found in Section 5.2.3.16.  Adding a new Signature
   Notation Data subpacket MUST be done through the EXPERT REVIEW
   method, as described in [RFC2434].

10.2.2.2.  Key Server Preference Extensions

   OpenPGP signatures contain a mechanism for preferences to be
   specified about key servers.  This specification creates a registry
   of key server preferences.  The registry includes the key server
   preference, the name of the preference, and a reference to the
   defining specification.  The initial values for this registry can be
   found in Section 5.2.3.17.  Adding a new key server preference MUST
   be done through the IETF CONSENSUS method, as described in [RFC2434].

10.2.2.3.  Key Flags Extensions

   OpenPGP signatures contain a mechanism for flags to be specified
   about key usage.  This specification creates a registry of key usage
   flags.  The registry includes the key flags value, the name of the
   flag, and a reference to the defining specification.  The initial
   values for this registry can be found in Section 5.2.3.21.  Adding a
   new key usage flag MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

10.2.2.4.  Reason for Revocation Extensions

   OpenPGP signatures contain a mechanism for flags to be specified
   about why a key was revoked.  This specification creates a registry
   of "Reason for Revocation" flags.  The registry includes the "Reason
   for Revocation" flags value, the name of the flag, and a reference to
   the defining specification.  The initial values for this registry can
   be found in Section 5.2.3.23.  Adding a new feature flag MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].



Callas, et al               Standards Track                    [Page 65]

RFC 4880                 OpenPGP Message Format            November 2007


10.2.2.5.  Implementation Features

   OpenPGP signatures contain a mechanism for flags to be specified
   stating which optional features an implementation supports.  This
   specification creates a registry of feature-implementation flags.
   The registry includes the feature-implementation flags value, the
   name of the flag, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 5.2.3.24.
   Adding a new feature-implementation flag MUST be done through the
   IETF CONSENSUS method, as described in [RFC2434].

   Also see Section 13.12 for more information about when feature flags
   are needed.

10.2.3.  New Packet Versions

   The core OpenPGP packets all have version numbers, and can be revised
   by introducing a new version of an existing packet.  This
   specification creates a registry of packet types.  The registry
   includes the packet type, the number of the version, and a reference
   to the defining specification.  The initial values for this registry
   can be found in Section 5.  Adding a new packet version MUST be done
   through the IETF CONSENSUS method, as described in [RFC2434].

10.3.  New Algorithms

   Section 9 lists the core algorithms that OpenPGP uses.  Adding in a
   new algorithm is usually simple.  For example, adding in a new
   symmetric cipher usually would not need anything more than allocating
   a constant for that cipher.  If that cipher had other than a 64-bit
   or 128-bit block size, there might need to be additional
   documentation describing how OpenPGP-CFB mode would be adjusted.
   Similarly, when DSA was expanded from a maximum of 1024-bit public
   keys to 3072-bit public keys, the revision of FIPS 186 contained
   enough information itself to allow implementation.  Changes to this
   document were made mainly for emphasis.

10.3.1.  Public-Key Algorithms

   OpenPGP specifies a number of public-key algorithms.  This
   specification creates a registry of public-key algorithm identifiers.
   The registry includes the algorithm name, its key sizes and
   parameters, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 9.  Adding a
   new public-key algorithm MUST be done through the IETF CONSENSUS
   method, as described in [RFC2434].





Callas, et al               Standards Track                    [Page 66]

RFC 4880                 OpenPGP Message Format            November 2007


10.3.2.  Symmetric-Key Algorithms

   OpenPGP specifies a number of symmetric-key algorithms.  This
   specification creates a registry of symmetric-key algorithm
   identifiers.  The registry includes the algorithm name, its key sizes
   and block size, and a reference to the defining specification.  The
   initial values for this registry can be found in Section 9.  Adding a
   new symmetric-key algorithm MUST be done through the IETF CONSENSUS
   method, as described in [RFC2434].

10.3.3.  Hash Algorithms

   OpenPGP specifies a number of hash algorithms.  This specification
   creates a registry of hash algorithm identifiers.  The registry
   includes the algorithm name, a text representation of that name, its
   block size, an OID hash prefix, and a reference to the defining
   specification.  The initial values for this registry can be found in
   Section 9 for the algorithm identifiers and text names, and Section
   5.2.2 for the OIDs and expanded signature prefixes.  Adding a new
   hash algorithm MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

10.3.4.  Compression Algorithms

   OpenPGP specifies a number of compression algorithms.  This
   specification creates a registry of compression algorithm
   identifiers.  The registry includes the algorithm name and a
   reference to the defining specification.  The initial values for this
   registry can be found in Section 9.3.  Adding a new compression key
   algorithm MUST be done through the IETF CONSENSUS method, as
   described in [RFC2434].

11.  Packet Composition

   OpenPGP packets are assembled into sequences in order to create
   messages and to transfer keys.  Not all possible packet sequences are
   meaningful and correct.  This section describes the rules for how
   packets should be placed into sequences.

11.1.  Transferable Public Keys

   OpenPGP users may transfer public keys.  The essential elements of a
   transferable public key are as follows:

     - One Public-Key packet

     - Zero or more revocation signatures




Callas, et al               Standards Track                    [Page 67]

RFC 4880                 OpenPGP Message Format            November 2007


     - One or more User ID packets

     - After each User ID packet, zero or more Signature packets
       (certifications)

     - Zero or more User Attribute packets

     - After each User Attribute packet, zero or more Signature packets
       (certifications)

     - Zero or more Subkey packets

     - After each Subkey packet, one Signature packet, plus optionally a
       revocation

   The Public-Key packet occurs first.  Each of the following User ID
   packets provides the identity of the owner of this public key.  If
   there are multiple User ID packets, this corresponds to multiple
   means of identifying the same unique individual user; for example, a
   user may have more than one email address, and construct a User ID
   for each one.

   Immediately following each User ID packet, there are zero or more
   Signature packets.  Each Signature packet is calculated on the
   immediately preceding User ID packet and the initial Public-Key
   packet.  The signature serves to certify the corresponding public key
   and User ID.  In effect, the signer is testifying to his or her
   belief that this public key belongs to the user identified by this
   User ID.

   Within the same section as the User ID packets, there are zero or
   more User Attribute packets.  Like the User ID packets, a User
   Attribute packet is followed by zero or more Signature packets
   calculated on the immediately preceding User Attribute packet and the
   initial Public-Key packet.

   User Attribute packets and User ID packets may be freely intermixed
   in this section, so long as the signatures that follow them are
   maintained on the proper User Attribute or User ID packet.

   After the User ID packet or Attribute packet, there may be zero or
   more Subkey packets.  In general, subkeys are provided in cases where
   the top-level public key is a signature-only key.  However, any V4
   key may have subkeys, and the subkeys may be encryption-only keys,
   signature-only keys, or general-purpose keys.  V3 keys MUST NOT have
   subkeys.





Callas, et al               Standards Track                    [Page 68]

RFC 4880                 OpenPGP Message Format            November 2007


   Each Subkey packet MUST be followed by one Signature packet, which
   should be a subkey binding signature issued by the top-level key.
   For subkeys that can issue signatures, the subkey binding signature
   MUST contain an Embedded Signature subpacket with a primary key
   binding signature (0x19) issued by the subkey on the top-level key.

   Subkey and Key packets may each be followed by a revocation Signature
   packet to indicate that the key is revoked.  Revocation signatures
   are only accepted if they are issued by the key itself, or by a key
   that is authorized to issue revocations via a Revocation Key
   subpacket in a self-signature by the top-level key.

   Transferable public-key packet sequences may be concatenated to allow
   transferring multiple public keys in one operation.

11.2.  Transferable Secret Keys

   OpenPGP users may transfer secret keys.  The format of a transferable
   secret key is the same as a transferable public key except that
   secret-key and secret-subkey packets are used instead of the public
   key and public-subkey packets.  Implementations SHOULD include self-
   signatures on any user IDs and subkeys, as this allows for a complete
   public key to be automatically extracted from the transferable secret
   key.  Implementations MAY choose to omit the self-signatures,
   especially if a transferable public key accompanies the transferable
   secret key.

11.3.  OpenPGP Messages

   An OpenPGP message is a packet or sequence of packets that
   corresponds to the following grammatical rules (comma represents
   sequential composition, and vertical bar separates alternatives):

   OpenPGP Message :- Encrypted Message | Signed Message |
                      Compressed Message | Literal Message.

   Compressed Message :- Compressed Data Packet.

   Literal Message :- Literal Data Packet.

   ESK :- Public-Key Encrypted Session Key Packet |
          Symmetric-Key Encrypted Session Key Packet.

   ESK Sequence :- ESK | ESK Sequence, ESK.

   Encrypted Data :- Symmetrically Encrypted Data Packet |
         Symmetrically Encrypted Integrity Protected Data Packet




Callas, et al               Standards Track                    [Page 69]

RFC 4880                 OpenPGP Message Format            November 2007


   Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.

   One-Pass Signed Message :- One-Pass Signature Packet,
               OpenPGP Message, Corresponding Signature Packet.

   Signed Message :- Signature Packet, OpenPGP Message |
               One-Pass Signed Message.

   In addition, decrypting a Symmetrically Encrypted Data packet or a
   Symmetrically Encrypted Integrity Protected Data packet as well as
   decompressing a Compressed Data packet must yield a valid OpenPGP
   Message.

11.4.  Detached Signatures

   Some OpenPGP applications use so-called "detached signatures".  For
   example, a program bundle may contain a file, and with it a second
   file that is a detached signature of the first file.  These detached
   signatures are simply a Signature packet stored separately from the
   data for which they are a signature.

12.  Enhanced Key Formats

12.1.  Key Structures

   The format of an OpenPGP V3 key is as follows.  Entries in square
   brackets are optional and ellipses indicate repetition.

           RSA Public Key
              [Revocation Self Signature]
               User ID [Signature ...]
              [User ID [Signature ...] ...]

   Each signature certifies the RSA public key and the preceding User
   ID.  The RSA public key can have many User IDs and each User ID can
   have many signatures.  V3 keys are deprecated.  Implementations MUST
   NOT generate new V3 keys, but MAY continue to use existing ones.

   The format of an OpenPGP V4 key that uses multiple public keys is
   similar except that the other keys are added to the end as "subkeys"
   of the primary key.










Callas, et al               Standards Track                    [Page 70]

RFC 4880                 OpenPGP Message Format            November 2007


           Primary-Key
              [Revocation Self Signature]
              [Direct Key Signature...]
               User ID [Signature ...]
              [User ID [Signature ...] ...]
              [User Attribute [Signature ...] ...]
              [[Subkey [Binding-Signature-Revocation]
                      Primary-Key-Binding-Signature] ...]

   A subkey always has a single signature after it that is issued using
   the primary key to tie the two keys together.  This binding signature
   may be in either V3 or V4 format, but SHOULD be V4.  Subkeys that can
   issue signatures MUST have a V4 binding signature due to the REQUIRED
   embedded primary key binding signature.

   In the above diagram, if the binding signature of a subkey has been
   revoked, the revoked key may be removed, leaving only one key.

   In a V4 key, the primary key MUST be a key capable of certification.
   The subkeys may be keys of any other type.  There may be other
   constructions of V4 keys, too.  For example, there may be a single-
   key RSA key in V4 format, a DSA primary key with an RSA encryption
   key, or RSA primary key with an Elgamal subkey, etc.

   It is also possible to have a signature-only subkey.  This permits a
   primary key that collects certifications (key signatures), but is
   used only for certifying subkeys that are used for encryption and
   signatures.

12.2.  Key IDs and Fingerprints

   For a V3 key, the eight-octet Key ID consists of the low 64 bits of
   the public modulus of the RSA key.

   The fingerprint of a V3 key is formed by hashing the body (but not
   the two-octet length) of the MPIs that form the key material (public
   modulus n, followed by exponent e) with MD5.  Note that both V3 keys
   and MD5 are deprecated.

   A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
   followed by the two-octet packet length, followed by the entire
   Public-Key packet starting with the version field.  The Key ID is the
   low-order 64 bits of the fingerprint.  Here are the fields of the
   hash material, with the example of a DSA key:

   a.1) 0x99 (1 octet)

   a.2) high-order length octet of (b)-(e) (1 octet)



Callas, et al               Standards Track                    [Page 71]

RFC 4880                 OpenPGP Message Format            November 2007


   a.3) low-order length octet of (b)-(e) (1 octet)

     b) version number = 4 (1 octet);

     c) timestamp of key creation (4 octets);

     d) algorithm (1 octet): 17 = DSA (example);

     e) Algorithm-specific fields.

   Algorithm-Specific Fields for DSA keys (example):

   e.1) MPI of DSA prime p;

   e.2) MPI of DSA group order q (q is a prime divisor of p-1);

   e.3) MPI of DSA group generator g;

   e.4) MPI of DSA public-key value y (= g**x mod p where x is secret).

   Note that it is possible for there to be collisions of Key IDs -- two
   different keys with the same Key ID.  Note that there is a much
   smaller, but still non-zero, probability that two different keys have
   the same fingerprint.

   Also note that if V3 and V4 format keys share the same RSA key
   material, they will have different Key IDs as well as different
   fingerprints.

   Finally, the Key ID and fingerprint of a subkey are calculated in the
   same way as for a primary key, including the 0x99 as the first octet
   (even though this is not a valid packet ID for a public subkey).

13.  Notes on Algorithms

13.1.  PKCS#1 Encoding in OpenPGP

   This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
   EMSA-PKCS1-v1_5.  However, the calling conventions of these functions
   has changed in the past.  To avoid potential confusion and
   interoperability problems, we are including local copies in this
   document, adapted from those in PKCS#1 v2.1 [RFC3447].  RFC 3447
   should be treated as the ultimate authority on PKCS#1 for OpenPGP.
   Nonetheless, we believe that there is value in having a self-
   contained document that avoids problems in the future with needed
   changes in the conventions.





Callas, et al               Standards Track                    [Page 72]

RFC 4880                 OpenPGP Message Format            November 2007


13.1.1.  EME-PKCS1-v1_5-ENCODE

   Input:

   k  = the length in octets of the key modulus

   M  = message to be encoded, an octet string of length mLen, where
        mLen <= k - 11

   Output:

   EM = encoded message, an octet string of length k

   Error:   "message too long"

     1. Length checking: If mLen > k - 11, output "message too long" and
        stop.

     2. Generate an octet string PS of length k - mLen - 3 consisting of
        pseudo-randomly generated nonzero octets.  The length of PS will
        be at least eight octets.

     3. Concatenate PS, the message M, and other padding to form an
        encoded message EM of length k octets as

        EM = 0x00 || 0x02 || PS || 0x00 || M.

     4. Output EM.

13.1.2.  EME-PKCS1-v1_5-DECODE

   Input:

   EM = encoded message, an octet string

   Output:

   M  = message, an octet string

   Error:   "decryption error"

   To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
   into an octet string PS consisting of nonzero octets and a message M
   as follows

     EM = 0x00 || 0x02 || PS || 0x00 || M.





Callas, et al               Standards Track                    [Page 73]

RFC 4880                 OpenPGP Message Format            November 2007


   If the first octet of EM does not have hexadecimal value 0x00, if the
   second octet of EM does not have hexadecimal value 0x02, if there is
   no octet with hexadecimal value 0x00 to separate PS from M, or if the
   length of PS is less than 8 octets, output "decryption error" and
   stop.  See also the security note in Section 14 regarding differences
   in reporting between a decryption error and a padding error.

13.1.3.  EMSA-PKCS1-v1_5

   This encoding method is deterministic and only has an encoding
   operation.

   Option:

   Hash - a hash function in which hLen denotes the length in octets of
         the hash function output

   Input:

   M  = message to be encoded

   mL = intended length in octets of the encoded message, at least tLen
        + 11, where tLen is the octet length of the DER encoding T of a
        certain value computed during the encoding operation

   Output:

   EM = encoded message, an octet string of length emLen

   Errors: "message too long"; "intended encoded message length too
   short"

   Steps:

     1. Apply the hash function to the message M to produce a hash value
        H:

        H = Hash(M).

        If the hash function outputs "message too long," output "message
        too long" and stop.

     2. Using the list in Section 5.2.2, produce an ASN.1 DER value for
        the hash function used.  Let T be the full hash prefix from
        Section 5.2.2, and let tLen be the length in octets of T.

     3. If emLen < tLen + 11, output "intended encoded message length
        too short" and stop.



Callas, et al               Standards Track                    [Page 74]

RFC 4880                 OpenPGP Message Format            November 2007


     4. Generate an octet string PS consisting of emLen - tLen - 3
        octets with hexadecimal value 0xFF.  The length of PS will be at
        least 8 octets.

     5. Concatenate PS, the hash prefix T, and other padding to form the
        encoded message EM as

        EM = 0x00 || 0x01 || PS || 0x00 || T.

     6. Output EM.

13.2.  Symmetric Algorithm Preferences

   The symmetric algorithm preference is an ordered list of algorithms
   that the keyholder accepts.  Since it is found on a self-signature,
   it is possible that a keyholder may have multiple, different
   preferences.  For example, Alice may have TripleDES only specified
   for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
   "alice@home.org".  Note that it is also possible for preferences to
   be in a subkey's binding signature.

   Since TripleDES is the MUST-implement algorithm, if it is not
   explicitly in the list, it is tacitly at the end.  However, it is
   good form to place it there explicitly.  Note also that if an
   implementation does not implement the preference, then it is
   implicitly a TripleDES-only implementation.

   An implementation MUST NOT use a symmetric algorithm that is not in
   the recipient's preference list.  When encrypting to more than one
   recipient, the implementation finds a suitable algorithm by taking
   the intersection of the preferences of the recipients.  Note that the
   MUST-implement algorithm, TripleDES, ensures that the intersection is
   not null.  The implementation may use any mechanism to pick an
   algorithm in the intersection.

   If an implementation can decrypt a message that a keyholder doesn't
   have in their preferences, the implementation SHOULD decrypt the
   message anyway, but MUST warn the keyholder that the protocol has
   been violated.  For example, suppose that Alice, above, has software
   that implements all algorithms in this specification.  Nonetheless,
   she prefers subsets for work or home.  If she is sent a message
   encrypted with IDEA, which is not in her preferences, the software
   warns her that someone sent her an IDEA-encrypted message, but it
   would ideally decrypt it anyway.







Callas, et al               Standards Track                    [Page 75]

RFC 4880                 OpenPGP Message Format            November 2007


13.3.  Other Algorithm Preferences

   Other algorithm preferences work similarly to the symmetric algorithm
   preference, in that they specify which algorithms the keyholder
   accepts.  There are two interesting cases that other comments need to
   be made about, though, the compression preferences and the hash
   preferences.

13.3.1.  Compression Preferences

   Compression has been an integral part of PGP since its first days.
   OpenPGP and all previous versions of PGP have offered compression.
   In this specification, the default is for messages to be compressed,
   although an implementation is not required to do so.  Consequently,
   the compression preference gives a way for a keyholder to request
   that messages not be compressed, presumably because they are using a
   minimal implementation that does not include compression.
   Additionally, this gives a keyholder a way to state that it can
   support alternate algorithms.

   Like the algorithm preferences, an implementation MUST NOT use an
   algorithm that is not in the preference vector.  If the preferences
   are not present, then they are assumed to be [ZIP(1),
   Uncompressed(0)].

   Additionally, an implementation MUST implement this preference to the
   degree of recognizing when to send an uncompressed message.  A robust
   implementation would satisfy this requirement by looking at the
   recipient's preference and acting accordingly.  A minimal
   implementation can satisfy this requirement by never generating a
   compressed message, since all implementations can handle messages
   that have not been compressed.

13.3.2.  Hash Algorithm Preferences

   Typically, the choice of a hash algorithm is something the signer
   does, rather than the verifier, because a signer rarely knows who is
   going to be verifying the signature.  This preference, though, allows
   a protocol based upon digital signatures ease in negotiation.

   Thus, if Alice is authenticating herself to Bob with a signature, it
   makes sense for her to use a hash algorithm that Bob's software uses.
   This preference allows Bob to state in his key which algorithms Alice
   may use.

   Since SHA1 is the MUST-implement hash algorithm, if it is not
   explicitly in the list, it is tacitly at the end.  However, it is
   good form to place it there explicitly.



Callas, et al               Standards Track                    [Page 76]

RFC 4880                 OpenPGP Message Format            November 2007


13.4.  Plaintext

   Algorithm 0, "plaintext", may only be used to denote secret keys that
   are stored in the clear.  Implementations MUST NOT use plaintext in
   Symmetrically Encrypted Data packets; they must use Literal Data
   packets to encode unencrypted or literal data.

13.5.  RSA

   There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
   keys.  These types are deprecated.  The "key flags" subpacket in a
   signature is a much better way to express the same idea, and
   generalizes it to all algorithms.  An implementation SHOULD NOT
   create such a key, but MAY interpret it.

   An implementation SHOULD NOT implement RSA keys of size less than
   1024 bits.

13.6.  DSA

   An implementation SHOULD NOT implement DSA keys of size less than
   1024 bits.  It MUST NOT implement a DSA key with a q size of less
   than 160 bits.  DSA keys MUST also be a multiple of 64 bits, and the
   q size MUST be a multiple of 8 bits.  The Digital Signature Standard
   (DSS) [FIPS186] specifies that DSA be used in one of the following
   ways:

     * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384, or
       SHA-512 hash

     * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384, or SHA-512
       hash

     * 2048-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash

     * 3072-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash

   The above key and q size pairs were chosen to best balance the
   strength of the key with the strength of the hash.  Implementations
   SHOULD use one of the above key and q size pairs when generating DSA
   keys.  If DSS compliance is desired, one of the specified SHA hashes
   must be used as well.  [FIPS186] is the ultimate authority on DSS,
   and should be consulted for all questions of DSS compliance.

   Note that earlier versions of this standard only allowed a 160-bit q
   with no truncation allowed, so earlier implementations may not be
   able to handle signatures with a different q size or a truncated
   hash.



Callas, et al               Standards Track                    [Page 77]

RFC 4880                 OpenPGP Message Format            November 2007


13.7.  Elgamal

   An implementation SHOULD NOT implement Elgamal keys of size less than
   1024 bits.

13.8.  Reserved Algorithm Numbers

   A number of algorithm IDs have been reserved for algorithms that
   would be useful to use in an OpenPGP implementation, yet there are
   issues that prevent an implementer from actually implementing the
   algorithm.  These are marked in Section 9.1, "Public-Key Algorithms",
   as "reserved for".

   The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19),
   and X9.42 (21), do not have the necessary parameters, parameter
   order, or semantics defined.

   Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
   with a public-key identifier of 20.  These are no longer permitted.
   An implementation MUST NOT generate such keys.  An implementation
   MUST NOT generate Elgamal signatures.  See [BLEICHENBACHER].

13.9.  OpenPGP CFB Mode

   OpenPGP does symmetric encryption using a variant of Cipher Feedback
   mode (CFB mode).  This section describes the procedure it uses in
   detail.  This mode is what is used for Symmetrically Encrypted Data
   Packets; the mechanism used for encrypting secret-key material is
   similar, and is described in the sections above.

   In the description below, the value BS is the block size in octets of
   the cipher.  Most ciphers have a block size of 8 octets.  The AES and
   Twofish have a block size of 16 octets.  Also note that the
   description below assumes that the IV and CFB arrays start with an
   index of 1 (unlike the C language, which assumes arrays start with a
   zero index).

   OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
   prefixes the plaintext with BS+2 octets of random data, such that
   octets BS+1 and BS+2 match octets BS-1 and BS.  It does a CFB
   resynchronization after encrypting those BS+2 octets.

   Thus, for an algorithm that has a block size of 8 octets (64 bits),
   the IV is 10 octets long and octets 7 and 8 of the IV are the same as
   octets 9 and 10.  For an algorithm with a block size of 16 octets
   (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
   octets 15 and 16.  Those extra two octets are an easy check for a
   correct key.



Callas, et al               Standards Track                    [Page 78]

RFC 4880                 OpenPGP Message Format            November 2007


   Step by step, here is the procedure:

   1.  The feedback register (FR) is set to the IV, which is all zeros.

   2.  FR is encrypted to produce FRE (FR Encrypted).  This is the
       encryption of an all-zero value.

   3.  FRE is xored with the first BS octets of random data prefixed to
       the plaintext to produce C[1] through C[BS], the first BS octets
       of ciphertext.

   4.  FR is loaded with C[1] through C[BS].

   5.  FR is encrypted to produce FRE, the encryption of the first BS
       octets of ciphertext.

   6.  The left two octets of FRE get xored with the next two octets of
       data that were prefixed to the plaintext.  This produces C[BS+1]
       and C[BS+2], the next two octets of ciphertext.

   7.  (The resynchronization step) FR is loaded with C[3] through
       C[BS+2].

   8.  FR is encrypted to produce FRE.

   9.  FRE is xored with the first BS octets of the given plaintext, now
       that we have finished encrypting the BS+2 octets of prefixed
       data.  This produces C[BS+3] through C[BS+(BS+2)], the next BS
       octets of ciphertext.

   10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 for
       an 8-octet block).

       11. FR is encrypted to produce FRE.

       12. FRE is xored with the next BS octets of plaintext, to produce
       the next BS octets of ciphertext.  These are loaded into FR, and
       the process is repeated until the plaintext is used up.

13.10.  Private or Experimental Parameters

   S2K specifiers, Signature subpacket types, user attribute types,
   image format types, and algorithms described in Section 9 all reserve
   the range 100 to 110 for private and experimental use.  Packet types
   reserve the range 60 to 63 for private and experimental use.  These
   are intentionally managed with the PRIVATE USE method, as described
   in [RFC2434].




Callas, et al               Standards Track                    [Page 79]

RFC 4880                 OpenPGP Message Format            November 2007


   However, implementations need to be careful with these and promote
   them to full IANA-managed parameters when they grow beyond the
   original, limited system.

13.11.  Extension of the MDC System

   As described in the non-normative explanation in Section 5.13, the
   MDC system is uniquely unparameterized in OpenPGP.  This was an
   intentional decision to avoid cross-grade attacks.  If the MDC system
   is extended to a stronger hash function, care must be taken to avoid
   downgrade and cross-grade attacks.

   One simple way to do this is to create new packets for a new MDC.
   For example, instead of the MDC system using packets 18 and 19, a new
   MDC could use 20 and 21.  This has obvious drawbacks (it uses two
   packet numbers for each new hash function in a space that is limited
   to a maximum of 60).

   Another simple way to extend the MDC system is to create new versions
   of packet 18, and reflect this in packet 19.  For example, suppose
   that V2 of packet 18 implicitly used SHA-256.  This would require
   packet 19 to have a length of 32 octets.  The change in the version
   in packet 18 and the size of packet 19 prevent a downgrade attack.

   There are two drawbacks to this latter approach.  The first is that
   using the version number of a packet to carry algorithm information
   is not tidy from a protocol-design standpoint.  It is possible that
   there might be several versions of the MDC system in common use, but
   this untidiness would reflect untidiness in cryptographic consensus
   about hash function security.  The second is that different versions
   of packet 19 would have to have unique sizes.  If there were two
   versions each with 256-bit hashes, they could not both have 32-octet
   packet 19s without admitting the chance of a cross-grade attack.

   Yet another, complex approach to extend the MDC system would be a
   hybrid of the two above -- create a new pair of MDC packets that are
   fully parameterized, and yet protected from downgrade and cross-
   grade.

   Any change to the MDC system MUST be done through the IETF CONSENSUS
   method, as described in [RFC2434].

13.12.  Meta-Considerations for Expansion

   If OpenPGP is extended in a way that is not backwards-compatible,
   meaning that old implementations will not gracefully handle their





Callas, et al               Standards Track                    [Page 80]

RFC 4880                 OpenPGP Message Format            November 2007


   absence of a new feature, the extension proposal can be declared in
   the key holder's self-signature as part of the Features signature
   subpacket.

   We cannot state definitively what extensions will not be upwards-
   compatible, but typically new algorithms are upwards-compatible,
   whereas new packets are not.

   If an extension proposal does not update the Features system, it
   SHOULD include an explanation of why this is unnecessary.  If the
   proposal contains neither an extension to the Features system nor an
   explanation of why such an extension is unnecessary, the proposal
   SHOULD be rejected.

14.  Security Considerations

   * As with any technology involving cryptography, you should check the
     current literature to determine if any algorithms used here have
     been found to be vulnerable to attack.

   * This specification uses Public-Key Cryptography technologies.  It
     is assumed that the private key portion of a public-private key
     pair is controlled and secured by the proper party or parties.

   * Certain operations in this specification involve the use of random
     numbers.  An appropriate entropy source should be used to generate
     these numbers (see [RFC4086]).

   * The MD5 hash algorithm has been found to have weaknesses, with
     collisions found in a number of cases.  MD5 is deprecated for use
     in OpenPGP.  Implementations MUST NOT generate new signatures using
     MD5 as a hash function.  They MAY continue to consider old
     signatures that used MD5 as valid.

   * SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512,
     respectively.  In general, there are few reasons to use them
     outside of DSS compatibility.  You need a situation where one needs
     more security than smaller hashes, but does not want to have the
     full 256-bit or 512-bit data length.

   * Many security protocol designers think that it is a bad idea to use
     a single key for both privacy (encryption) and integrity
     (signatures).  In fact, this was one of the motivating forces
     behind the V4 key format with separate signature and encryption
     keys.  If you as an implementer promote dual-use keys, you should
     at least be aware of this controversy.





Callas, et al               Standards Track                    [Page 81]

RFC 4880                 OpenPGP Message Format            November 2007


   * The DSA algorithm will work with any hash, but is sensitive to the
     quality of the hash algorithm.  Verifiers should be aware that even
     if the signer used a strong hash, an attacker could have modified
     the signature to use a weak one.  Only signatures using acceptably
     strong hash algorithms should be accepted as valid.

   * As OpenPGP combines many different asymmetric, symmetric, and hash
     algorithms, each with different measures of strength, care should
     be taken that the weakest element of an OpenPGP message is still
     sufficiently strong for the purpose at hand.  While consensus about
     the strength of a given algorithm may evolve, NIST Special
     Publication 800-57 [SP800-57] recommends the following list of
     equivalent strengths:

           Asymmetric  |  Hash  |  Symmetric
            key size   |  size  |   key size
           ------------+--------+-----------
              1024        160         80
              2048        224        112
              3072        256        128
              7680        384        192
             15360        512        256

   * There is a somewhat-related potential security problem in
     signatures.  If an attacker can find a message that hashes to the
     same hash with a different algorithm, a bogus signature structure
     can be constructed that evaluates correctly.

     For example, suppose Alice DSA signs message M using hash algorithm
     H.  Suppose that Mallet finds a message M' that has the same hash
     value as M with H'.  Mallet can then construct a signature block
     that verifies as Alice's signature of M' with H'.  However, this
     would also constitute a weakness in either H or H' or both.  Should
     this ever occur, a revision will have to be made to this document
     to revise the allowed hash algorithms.

   * If you are building an authentication system, the recipient may
     specify a preferred signing algorithm.  However, the signer would
     be foolish to use a weak algorithm simply because the recipient
     requests it.

   * Some of the encryption algorithms mentioned in this document have
     been analyzed less than others.  For example, although CAST5 is
     presently considered strong, it has been analyzed less than
     TripleDES.  Other algorithms may have other controversies
     surrounding them.





Callas, et al               Standards Track                    [Page 82]

RFC 4880                 OpenPGP Message Format            November 2007


   * In late summer 2002, Jallad, Katz, and Schneier published an
     interesting attack on the OpenPGP protocol and some of its
     implementations [JKS02].  In this attack, the attacker modifies a
     message and sends it to a user who then returns the erroneously
     decrypted message to the attacker.  The attacker is thus using the
     user as a random oracle, and can often decrypt the message.

     Compressing data can ameliorate this attack.  The incorrectly
     decrypted data nearly always decompresses in ways that defeat the
     attack.  However, this is not a rigorous fix, and leaves open some
     small vulnerabilities.  For example, if an implementation does not
     compress a message before encryption (perhaps because it knows it
     was already compressed), then that message is vulnerable.  Because
     of this happenstance -- that modification attacks can be thwarted
     by decompression errors -- an implementation SHOULD treat a
     decompression error as a security problem, not merely a data
     problem.

     This attack can be defeated by the use of Modification Detection,
     provided that the implementation does not let the user naively
     return the data to the attacker.  An implementation MUST treat an
     MDC failure as a security problem, not merely a data problem.

     In either case, the implementation MAY allow the user access to the
     erroneous data, but MUST warn the user as to potential security
     problems should that data be returned to the sender.

     While this attack is somewhat obscure, requiring a special set of
     circumstances to create it, it is nonetheless quite serious as it
     permits someone to trick a user to decrypt a message.
     Consequently, it is important that:

      1. Implementers treat MDC errors and decompression failures as
         security problems.

      2. Implementers implement Modification Detection with all due
         speed and encourage its spread.

      3. Users migrate to implementations that support Modification
         Detection with all due speed.

   * PKCS#1 has been found to be vulnerable to attacks in which a system
     that reports errors in padding differently from errors in
     decryption becomes a random oracle that can leak the private key in
     mere millions of queries.  Implementations must be aware of this
     attack and prevent it from happening.  The simplest solution is to
     report a single error code for all variants of decryption errors so
     as not to leak information to an attacker.



Callas, et al               Standards Track                    [Page 83]

RFC 4880                 OpenPGP Message Format            November 2007


   * Some technologies mentioned here may be subject to government
     control in some countries.

   * In winter 2005, Serge Mister and Robert Zuccherato from Entrust
     released a paper describing a way that the "quick check" in OpenPGP
     CFB mode can be used with a random oracle to decrypt two octets of
     every cipher block [MZ05].  They recommend as prevention not using
     the quick check at all.

     Many implementers have taken this advice to heart for any data that
     is symmetrically encrypted and for which the session key is
     public-key encrypted.  In this case, the quick check is not needed
     as the public-key encryption of the session key should guarantee
     that it is the right session key.  In other cases, the
     implementation should use the quick check with care.

     On the one hand, there is a danger to using it if there is a random
     oracle that can leak information to an attacker.  In plainer
     language, there is a danger to using the quick check if timing
     information about the check can be exposed to an attacker,
     particularly via an automated service that allows rapidly repeated
     queries.

     On the other hand, it is inconvenient to the user to be informed
     that they typed in the wrong passphrase only after a petabyte of
     data is decrypted.  There are many cases in cryptographic
     engineering where the implementer must use care and wisdom, and
     this is one.

15.  Implementation Nits

   This section is a collection of comments to help an implementer,
   particularly with an eye to backward compatibility.  Previous
   implementations of PGP are not OpenPGP compliant.  Often the
   differences are small, but small differences are frequently more
   vexing than large differences.  Thus, this is a non-comprehensive
   list of potential problems and gotchas for a developer who is trying
   to be backward-compatible.

     * The IDEA algorithm is patented, and yet it is required for PGP
       2.x interoperability.  It is also the de-facto preferred
       algorithm for a V3 key with a V3 self-signature (or no self-
       signature).

     * When exporting a private key, PGP 2.x generates the header "BEGIN
       PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".
       All previous versions ignore the implied data type, and look
       directly at the packet data type.



Callas, et al               Standards Track                    [Page 84]

RFC 4880                 OpenPGP Message Format            November 2007


     * PGP 2.0 through 2.5 generated V2 Public-Key packets.  These are
       identical to the deprecated V3 keys except for the version
       number.  An implementation MUST NOT generate them and may accept
       or reject them as it sees fit.  Some older PGP versions generated
       V2 PKESK packets (Tag 1) as well.  An implementation may accept
       or reject V2 PKESK packets as it sees fit, and MUST NOT generate
       them.

     * PGP 2.6.x will not accept key-material packets with versions
       greater than 3.

     * There are many ways possible for two keys to have the same key
       material, but different fingerprints (and thus Key IDs).  Perhaps
       the most interesting is an RSA key that has been "upgraded" to V4
       format, but since a V4 fingerprint is constructed by hashing the
       key creation time along with other things, two V4 keys created at
       different times, yet with the same key material will have
       different fingerprints.

     * If an implementation is using zlib to interoperate with PGP 2.x,
       then the "windowBits" parameter should be set to -13.

     * The 0x19 back signatures were not required for signing subkeys
       until relatively recently.  Consequently, there may be keys in
       the wild that do not have these back signatures.  Implementing
       software may handle these keys as it sees fit.

     * OpenPGP does not put limits on the size of public keys.  However,
       larger keys are not necessarily better keys.  Larger keys take
       more computation time to use, and this can quickly become
       impractical.  Different OpenPGP implementations may also use
       different upper bounds for public key sizes, and so care should
       be taken when choosing sizes to maintain interoperability.  As of
       2007 most implementations have an upper bound of 4096 bits.

     * ASCII armor is an optional feature of OpenPGP.  The OpenPGP
       working group strives for a minimal set of mandatory-to-implement
       features, and since there could be useful implementations that
       only use binary object formats, this is not a "MUST" feature for
       an implementation.  For example, an implementation that is using
       OpenPGP as a mechanism for file signatures may find ASCII armor
       unnecessary. OpenPGP permits an implementation to declare what
       features it does and does not support, but ASCII armor is not one
       of these.  Since most implementations allow binary and armored
       objects to be used indiscriminately, an implementation that does
       not implement ASCII armor may find itself with compatibility
       issues with general-purpose implementations.  Moreover,
       implementations of OpenPGP-MIME [RFC3156] already have a



Callas, et al               Standards Track                    [Page 85]

RFC 4880                 OpenPGP Message Format            November 2007


       requirement for ASCII armor so those implementations will
       necessarily have support.

16.  References

16.1.  Normative References

   [AES]            NIST, FIPS PUB 197, "Advanced Encryption Standard
                    (AES)," November 2001.
                    http://csrc.nist.gov/publications/fips/fips197/fips-
                    197.{ps,pdf}

   [BLOWFISH]       Schneier, B. "Description of a New Variable-Length
                    Key, 64-Bit Block Cipher (Blowfish)" Fast Software
                    Encryption, Cambridge Security Workshop Proceedings
                    (December 1993), Springer-Verlag, 1994, pp191-204
                    <http://www.counterpane.com/bfsverlag.html>

   [BZ2]            J. Seward, jseward@acm.org, "The Bzip2 and libbzip2
                    home page" <http://www.bzip.org/>

   [ELGAMAL]        T. Elgamal, "A Public-Key Cryptosystem and a
                    Signature Scheme Based on Discrete Logarithms," IEEE
                    Transactions on Information Theory, v. IT-31, n. 4,
                    1985, pp. 469-472.

   [FIPS180]        Secure Hash Signature Standard (SHS) (FIPS PUB 180-
                    2).
                    <http://csrc.nist.gov/publications/fips/fips180-
                    2/fips180-2withchangenotice.pdf>

   [FIPS186]        Digital Signature Standard (DSS) (FIPS PUB 186-2).
                    <http://csrc.nist.gov/publications/fips/fips186-2/
                     fips186-2-change1.pdf> FIPS 186-3 describes keys
                    greater than 1024 bits.  The latest draft is at:
                    <http://csrc.nist.gov/publications/drafts/
                    fips_186-3/Draft-FIPS-186-3%20_March2006.pdf>

   [HAC]            Alfred Menezes, Paul van Oorschot, and Scott
                    Vanstone, "Handbook of Applied Cryptography," CRC
                    Press, 1996.
                    <http://www.cacr.math.uwaterloo.ca/hac/>

   [IDEA]           Lai, X, "On the design and security of block
                    ciphers", ETH Series in Information Processing, J.L.
                    Massey (editor), Vol. 1, Hartung-Gorre Verlag
                    Knostanz, Technische Hochschule (Zurich), 1992




Callas, et al               Standards Track                    [Page 86]

RFC 4880                 OpenPGP Message Format            November 2007


   [ISO10646]       ISO/IEC 10646-1:1993. International Standard --
                    Information technology -- Universal Multiple-Octet
                    Coded Character Set (UCS) -- Part 1: Architecture
                    and Basic Multilingual Plane.

   [JFIF]           JPEG File Interchange Format (Version 1.02).  Eric
                    Hamilton, C-Cube Microsystems, Milpitas, CA,
                    September 1, 1992.

   [RFC1950]        Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
                    Format Specification version 3.3", RFC 1950, May
                    1996.

   [RFC1951]        Deutsch, P., "DEFLATE Compressed Data Format
                    Specification version 1.3", RFC 1951, May 1996.

   [RFC2045]        Freed, N. and N. Borenstein, "Multipurpose Internet
                    Mail Extensions (MIME) Part One: Format of Internet
                    Message Bodies", RFC 2045, November 1996

   [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                    Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2144]        Adams, C., "The CAST-128 Encryption Algorithm", RFC
                    2144, May 1997.

   [RFC2434]        Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs", BCP
                    26, RFC 2434, October 1998.

   [RFC2822]        Resnick, P., "Internet Message Format", RFC 2822,
                    April 2001.

   [RFC3156]        Elkins, M., Del Torto, D., Levien, R., and T.
                    Roessler, "MIME Security with OpenPGP", RFC 3156,
                    August 2001.

   [RFC3447]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                    Standards (PKCS) #1: RSA Cryptography Specifications
                    Version 2.1", RFC 3447, February 2003.

   [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO
                    10646", STD 63, RFC 3629, November 2003.

   [RFC4086]        Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106, RFC
                    4086, June 2005.




Callas, et al               Standards Track                    [Page 87]

RFC 4880                 OpenPGP Message Format            November 2007


   [SCHNEIER]      Schneier, B., "Applied Cryptography Second Edition:
                    protocols, algorithms, and source code in C", 1996.

   [TWOFISH]        B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.
                    Hall, and N. Ferguson, "The Twofish Encryption
                    Algorithm", John Wiley & Sons, 1999.

16.2.  Informative References

   [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal
                    signatures without knowing the secret key,"
                    Eurocrypt 96. Note that the version in the
                    proceedings has an error. A revised version is
                    available at the time of writing from
                    <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti
                    /isc/ElGamal.ps>

   [JKS02]          Kahil Jallad, Jonathan Katz, Bruce Schneier
                    "Implementation of Chosen-Ciphertext Attacks against
                    PGP and GnuPG" http://www.counterpane.com/pgp-
                    attack.html

   [MAURER]         Ueli Maurer, "Modelling a Public-Key
                    Infrastructure", Proc. 1996 European Symposium on
                    Research in Computer Security (ESORICS' 96), Lecture
                    Notes in Computer Science, Springer-Verlag, vol.
                    1146, pp. 325-350, Sep 1996.

   [MZ05]           Serge Mister, Robert Zuccherato, "An Attack on CFB
                    Mode Encryption As Used By OpenPGP," IACR ePrint
                    Archive: Report 2005/033, 8 Feb 2005
                    http://eprint.iacr.org/2005/033

   [REGEX]          Jeffrey Friedl, "Mastering Regular Expressions,"
                    O'Reilly, ISBN 0-596-00289-0.

   [RFC1423]        Balenson, D., "Privacy Enhancement for Internet
                    Electronic Mail: Part III: Algorithms, Modes, and
                    Identifiers", RFC 1423, February 1993.

   [RFC1991]        Atkins, D., Stallings, W., and P. Zimmermann, "PGP
                    Message Exchange Formats", RFC 1991, August 1996.

   [RFC2440]        Callas, J., Donnerhacke, L., Finney, H., and R.
                    Thayer, "OpenPGP Message Format", RFC 2440, November
                    1998.





Callas, et al               Standards Track                    [Page 88]

RFC 4880                 OpenPGP Message Format            November 2007


   [SP800-57]       NIST Special Publication 800-57, Recommendation on
                    Key Management
                    <http://csrc.nist.gov/publications/nistpubs/ 800-
                    57/SP800-57-Part1.pdf>
                    <http://csrc.nist.gov/publications/nistpubs/ 800-
                    57/SP800-57-Part2.pdf>

Acknowledgements

   This memo also draws on much previous work from a number of other
   authors, including: Derek Atkins, Charles Breed, Dave Del Torto, Marc
   Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben Laurie,
   Raph Levien, Colin Plumb, Will Price, David Shaw, William Stallings,
   Mark Weaver, and Philip R. Zimmermann.

Authors' Addresses

   The working group can be contacted via the current chair:

      Derek Atkins
      IHTFP Consulting, Inc.
      4 Farragut Ave
      Somerville, MA  02144  USA

      EMail: derek@ihtfp.com
      Tel: +1 617 623 3745

   The principal authors of this document are as follows:

      Jon Callas
      EMail: jon@callas.org

      Lutz Donnerhacke
      IKS GmbH
      Wildenbruchstr. 15
      07745 Jena, Germany
      EMail: lutz@iks-jena.de

      Hal Finney
      EMail: hal@finney.org

      David Shaw
      EMail: dshaw@jabberwocky.com

      Rodney Thayer
      EMail: rodney@canola-jones.com





Callas, et al               Standards Track                    [Page 89]

RFC 4880                 OpenPGP Message Format            November 2007


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












Callas, et al               Standards Track                    [Page 90]