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Versions: (RFC 2440) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 RFC 4880

Network Working Group                                        Jon Callas
Category: INTERNET-DRAFT                       Wave Systems Corporation
draft-ietf-openpgp-rfc2440bis-05.txt
Expires Oct 2002                                       Lutz Donnerhacke
April 2002                           IN-Root-CA Individual Network e.V.

                                                             Hal Finney
                                                     Network Associates

                                                          Rodney Thayer


                         OpenPGP Message Format
                 draft-ietf-openpgp-rfc2440bis-05.txt


   Copyright 2002 by The Internet Society. All Rights Reserved.

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

IESG Note

   This document defines many tag values, yet it doesn't describe a
   mechanism for adding new tags (for new features). Traditionally the
   Internet Assigned Numbers Authority (IANA) handles the allocation of
   new values for future expansion and RFCs usually define the
   procedure to be used by the IANA.  However there are subtle (and not
   so subtle) interactions that may occur in this protocol between new
   features and existing features which result in a significant
   reduction in over all security. Therefore this document does not
   define an extension procedure. Instead requests to define new tag
   values (say for new encryption algorithms for example) should be
   forwarded to the IESG Security Area Directors for consideration or
   forwarding to the appropriate IETF Working Group for consideration.


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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.




































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Table of Contents

            Status of this Memo                                       1
            IESG Note                                                 1
            Abstract                                                  2
            Table of Contents                                         3
   1.       Introduction                                              6
   1.1.     Terms                                                     6
   2.       General functions                                         6
   2.1.     Confidentiality via Encryption                            7
   2.2.     Authentication via Digital signature                      7
   2.3.     Compression                                               8
   2.4.     Conversion to Radix-64                                    8
   2.5.     Signature-Only Applications                               8
   3.       Data Element Formats                                      9
   3.1.     Scalar numbers                                            9
   3.2.     Multi-Precision Integers                                  9
   3.3.     Key IDs                                                   9
   3.4.     Text                                                     10
   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                         12
   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                                14
   4.2.2.1. One-Octet Lengths                                        14
   4.2.2.2. Two-Octet Lengths                                        15
   4.2.2.3. Five-Octet Lengths                                       15
   4.2.2.4. Partial Body Lengths                                     15
   4.2.3.   Packet Length Examples                                   15
   4.3.     Packet Tags                                              16
   5.       Packet Types                                             16
   5.1.     Public-Key Encrypted Session Key Packets (Tag 1)         16
   5.2.     Signature Packet (Tag 2)                                 18
   5.2.1.   Signature Types                                          18
   5.2.2.   Version 3 Signature Packet Format                        20
   5.2.3.   Version 4 Signature Packet Format                        22
   5.2.3.1. Signature Subpacket Specification                        23
   5.2.3.2. Signature Subpacket Types                                25
   5.2.3.3. Notes on Self-Signatures                                 25
   5.2.3.4. Signature creation time                                  26
   5.2.3.5. Issuer                                                   26
   5.2.3.6. Key expiration time                                      26

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   5.2.3.7. Preferred symmetric algorithms                           26
   5.2.3.8. Preferred hash algorithms                                26
   5.2.3.9. Preferred compression algorithms                         27
   5.2.3.10.Signature expiration time                                27
   5.2.3.11.Exportable Certification                                 27
   5.2.3.12.Revocable                                                28
   5.2.3.13.Trust signature                                          28
   5.2.3.14.Regular expression                                       28
   5.2.3.15.Revocation key                                           28
   5.2.3.16.Notation Data                                            29
   5.2.3.17.Key server preferences                                   30
   5.2.3.18.Preferred key server                                     30
   5.2.3.19.Primary user id                                          30
   5.2.3.20.Policy URL                                               30
   5.2.3.21.Key Flags                                                31
   5.2.3.22.Signer's User ID                                         31
   5.2.3.23.Reason for Revocation                                    32
   5.2.3.24.Features                                                 32
   5.2.3.25.Revocation Target                                        33
   5.2.4.   Computing Signatures                                     33
   5.2.4.1. Subpacket Hints                                          34
   5.3.     Symmetric-Key Encrypted Session-Key Packets (Tag 3)      35
   5.4.     One-Pass Signature Packets (Tag 4)                       35
   5.5.     Key Material Packet                                      36
   5.5.1.   Key Packet Variants                                      36
   5.5.1.1. Public Key Packet (Tag 6)                                36
   5.5.1.2. Public Subkey Packet (Tag 14)                            36
   5.5.1.3. Secret Key Packet (Tag 5)                                37
   5.5.1.4. Secret Subkey Packet (Tag 7)                             37
   5.5.2.   Public Key Packet Formats                                37
   5.5.3.   Secret Key Packet Formats                                39
   5.6.     Compressed Data Packet (Tag 8)                           40
   5.7.     Symmetrically Encrypted Data Packet (Tag 9)              41
   5.8.     Marker Packet (Obsolete Literal Packet) (Tag 10)         42
   5.9.     Literal Data Packet (Tag 11)                             42
   5.10.    Trust Packet (Tag 12)                                    43
   5.11.    User ID Packet (Tag 13)                                  43
   5.12.    User Attribute Packet (Tag 17)                           43
   5.12.1.  The Image Attribute Subpacket                            44
   5.13.    Sym. Encrypted Integrity Protected Data Packet (Tag 18)  44
   5.14.    Modification Detection Code Packet (Tag 19)              46
   6.       Radix-64 Conversions                                     47
   6.1.     An Implementation of the CRC-24 in "C"                   47
   6.2.     Forming ASCII Armor                                      48
   6.3.     Encoding Binary in Radix-64                              50
   6.4.     Decoding Radix-64                                        51
   6.5.     Examples of Radix-64                                     51
   6.6.     Example of an ASCII Armored Message                      52
   7.       Cleartext signature framework                            52
   7.1.     Dash-Escaped Text                                        53
   8.       Regular Expressions                                      53
   9.       Constants                                                54

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   9.1.     Public Key Algorithms                                    54
   9.2.     Symmetric Key Algorithms                                 55
   9.3.     Compression Algorithms                                   55
   9.4.     Hash Algorithms                                          55
   10.      Packet Composition                                       56
   10.1.    Transferable Public Keys                                 56
   10.2.    OpenPGP Messages                                         57
   10.3.    Detached Signatures                                      58
   11.      Enhanced Key Formats                                     58
   11.1.    Key Structures                                           58
   11.2.    Key IDs and Fingerprints                                 59
   12.      Notes on Algorithms                                      60
   12.1.    Symmetric Algorithm Preferences                          60
   12.2.    Other Algorithm Preferences                              61
   12.2.1.  Compression Preferences                                  61
   12.2.2.  Hash Algorithm Preferences                               61
   12.3.    Plaintext                                                62
   12.4.    RSA                                                      62
   12.5.    Elgamal                                                  62
   12.6.    DSA                                                      63
   12.7.    Reserved Algorithm Numbers                               63
   12.8.    OpenPGP CFB mode                                         63
   13.      Security Considerations                                  65
   14.      Implementation Nits                                      66
   15.      Authors and Working Group Chair                          67
   16.      References                                               68
   17.      Full Copyright Statement                                 70


























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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 RFC2440, "OpenPGP
   Message Format", which itself replaces RFC 1991, "PGP Message
   Exchange Formats."

1.1. Terms

     * OpenPGP - This is a definition 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, RFC1991, 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,
       RFC1991. 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.

     * GPG - GNU Privacy Guard, also called GNUpg. GPG is an OpenPGP
       implementation that avoids all encumbered algorithms.
       Consequently, early versions of GPG did not include RSA public
       keys. GPG 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
   Network Associates, Inc. and are used with permission.

   This document uses the terms "MUST", "SHOULD", and "MAY" as defined
   in RFC2119, along with the negated forms of those terms.

2. General functions

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

     - digital signatures

     - encryption

     - compression




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     - 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.

   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:

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   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.

   Note that while all past implementations of PGP properly handle
   messages that have not been compressed, they all have compressed
   messages by default. If an implementation does not implement
   compression, its authors should be aware that most PGP messages in
   the world are compressed. Thus, it may even be wise for a
   space-constrained implementation to implement decompression, but not
   compression.

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-comformant only in that they
   omit encryption.


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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]).

3.2. Multi-Precision Integers

   Multi-Precision 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].

   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.




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3.4. Text

   Unless otherwise specified, the character set for text is the UTF-8
   [RFC2279] encoding of Unicode [ISO10646].

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, as
   follows:

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
   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).


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   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

   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

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   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
   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.

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

       0:           secret data is unencrypted (no pass phrase)
       255:         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.



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   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.

   Note that the most significant bit is the left-most 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, either format may be
   used. Note that old format packets have four bits of content 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 -- content tag
         Bits 1-0 - length-type


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   New format packets contain:

         Bits 5-0 -- content 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.

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 from 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;




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4.2.2.2. Two-Octet Lengths

   A two-octet Body Length header encodes a length of from 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.

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 (of one of the three types
   -- one octet, two-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.

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 coded 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.


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   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. 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.

   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:

       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 Encrypted Session Key packets
   (either Public-Key or Symmetric-Key) may precede a Symmetrically

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   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.

   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
       implementation should accept, but not generate a version of 2,
       which is equivalent to V3 in all other respects.

     - An eight-octet number that gives the key ID of the public key
       that the session key is encrypted to. 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 [RFC2437] to form the "m"
   value used in the formulas above.

   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 new PKCS-1 encoding for each key.



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   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.

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 MUST accept V3 signatures. Implementations SHOULD
   generate V4 signatures.  Implementations MAY generate a V3 signature
   that can be verified by PGP 2.6.x.

   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
   specified in a signature type octet in any given signature. These
   meanings are:

   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> and
       trailing blanks removed.

   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.

   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.
       Note that all PGP "key signatures" are this type of

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       certification.

   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.

       Please note that the vagueness of these certification claims is
       not a flaw, but a feature of the system. Because PGP places
       final authority for validity upon the receiver of a
       certification, it may be that one authority's casual
       certification might be more rigorous than some other authority's
       positive certification. These classifications allow a
       certification authority to issue fine-grained claims.

   0x18: Subkey Binding Signature
       This signature is a statement by the top-level signing key
       indicates that it owns the subkey. This signature is calculated
       directly on the subkey itself, 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.

   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.



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   0x30: Certification revocation signature
       This signature revokes an earlier user ID certification
       signature (signature class 0x10 through 0x13). It should be
       issued by the same key that issued the revoked signature or an
       authorized revocation key The signature should have a later
       creation date than the signature it revokes.

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

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.

     - One-octet public key algorithm.

     - One-octet hash algorithm.

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

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

   The data being signed is hashed, and then the signature type and
   creation time from the signature packet are hashed (5 additional
   octets).  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.



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

   Algorithm Specific Fields for ElGamal signatures:

     - MPI of ElGamal value a = g**k mod p.

     - MPI of ElGamal value b = (h-a*x)/k mod p-1.

   The hash h is PKCS-1 padded exactly the same way as for the above

   described RSA signatures.

   With RSA signatures, the hash value is encoded as described in
   PKCS-1 section 9.2.1 encoded using PKCS-1 encoding type
   EMSA-PKCS1-v1_5 [RFC2437].  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:

     - MD2:        0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02

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

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

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

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

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

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

   The ASN.1 OIDs are:

     - MD2:        1.2.840.113549.2.2

     - MD5:        1.2.840.113549.2.5

     - RIPEMD-160: 1.3.36.3.2.1

     - SHA-1:      1.3.14.3.2.26

     - SHA256:     2.16.840.1.101.3.4.2.1

     - SHA384:     2.16.840.1.101.3.4.2.2




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     - SHA512:     2.16.840.1.101.3.4.2.3

   The full hash prefixes for these are:

       MD2:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
                   0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02, 0x05, 0x00,
                   0x04, 0x10

       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

       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 with a size of 160 bits, to match q,
   the size of the group generated by the DSA key's generator value.
   The hash function result is treated as a 160 bit 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.


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     - Hashed subpacket data. (two or more subpackets)

     - Two-octet scalar octet count for 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. (zero or more subpackets)

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

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

   The data being signed is hashed, and then 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 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

   The subpacket fields consist of zero or more signature subpackets.
   Each set of subpackets is preceded by a two-octet scalar count of
   the length of the set of 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. Note that this is
   the same encoding used for signature subpackets

   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

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       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:

       2 = signature creation time
       3 = signature expiration time
       4 = exportable certification
       5 = trust signature
       6 = regular expression
       7 = revocable
       9 = key expiration time
       10 = placeholder for backward compatibility
       11 = preferred symmetric algorithms
       12 = revocation key
       16 = issuer key ID
       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 URL
       27 = key flags
       28 = signer's user id
       29 = reason for revocation
       30 = features
       31 = revocation identification

   100 to 110 = internal or user-defined

   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.




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   Implementations SHOULD implement "preferences" and the "reason for
   revocation" subpackets. 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.

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 user
   id 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 the
   signature refers to. 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 username, 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 usernames, Alice and Bob. Suppose that Alice prefers the
   symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. 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,
   then algorithm of the default user id of the key provides the
   default symmetric algorithm.

   Revoking a self-signature has a defined semantic meaning. 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 user themselves revokes their
   self-signature, it means the user no longer goes by that name, no

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   longer has that email address, etc. Revoking a binding signature
   effectively retires that subkey. Please see the "Reason for
   Revocation" subpacket below 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.

   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

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

5.2.3.8. Preferred hash algorithms

   (array of one-octet values)


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   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 6.
   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 6. 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.
   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.

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5.2.3.12. Revocable

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

   Signature's revocability status.  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.

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 package. A description of
   the syntax is found in a section below.

5.2.3.15. Revocation key

   (1 octet of class, 1 octet of algid, 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.




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   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.

   All undefined flags MUST be zero. Defined flags are:

       First octet: 0x80 = human-readable. This note value is text, a
                           note from one person to another, and need
                           not have meaning to software.
       Other octets: none.

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

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

   Names in the user name space 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 of bad form to create a new name in a DNS
   space that you don't own.

   Since the user name space 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.

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5.2.3.17. Key server preferences

   (N octets of flags)

   This is a list of 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.

5.2.3.18. Preferred key server

   (String)

   This is a URL 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 URL, 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 URL

   (String)

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



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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:

       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.

       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)



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   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.

   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:

       0x00 - No reason specified (key revocations or cert revocations)
       0x01 - Key is superceded (key revocations)
       0x02 - Key material has been compromised (key revocations)
       0x03 - Key is retired and no longer used (key revocations)
       0x20 - User id information is no longer valid (cert revocations)

   Following the revocation code is a string of octets which 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
   superceded 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 an 0x20 subpacket.

   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 no longer is a part of validity calculations.

5.2.3.24. Features

   (N octets of flags)

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   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.

   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:

       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.

5.2.3.25. Revocation Target

   (1 octet PK algorithm, 1 octet hash algorithm, N octets hash)

   This subpacket identifies a specific target signature that a
   revocation signature revokes. This provides explicit designation of
   a revocation. All arguments are an identifier of that 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.

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.

   The signature data is simple to compute for document signatures
   (types 0x00 and 0x01), for which the document itself is the data.
   For standalone signatures, this is a null string.

   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 signature (type 0x18)
   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 name packet,

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   without any header. A V4 certification hashes the constant 0xb4 for
   user ID certifications or the constant 0xd1 for User Attribute
   certifications (which are old-style packet headers with the
   length-of-length set to zero), 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.

   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
   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; 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, 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

   An implementation SHOULD put the two mandatory subpackets, creation
   time and issuer, as the first subpackets in the subpacket list,
   simply to make it easier for the implementer to find them.

   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 an 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.



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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 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.

   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 pass phrases. 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 insure 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

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   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.

   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.

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   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
   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 2 packets are identical in
   format to Version 3 packets, but are generated by PGP 2.5 or before.
   V2 packets are deprecated and they MUST NOT be generated.

   PGP 5.0 introduced version 4 packets, with new fields and semantics.
    PGP 2.6.x will not accept key-material packets with versions
   greater than 3.

   OpenPGP implementations SHOULD create keys with version 4 format. An
   implementation MAY generate a V3 key to ensure interoperability with
   old software; note, however, that V4 keys correct some security
   deficiencies in V3 keys. These deficiencies are described below. An
   implementation MUST NOT create a V3 key with a public key algorithm
   other than RSA.

   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 multi-precision integers comprising the key
       material:




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

         - an MPI of RSA public encryption exponent e.

   V3 keys SHOULD only be used for backward compatibility because of
   three weaknesses in them. 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, which increases the opportunity for fingerprint
   collisions. Third, there are minor weaknesses in the MD5 hash
   algorithm that make developers prefer other algorithms. See below
   for a fuller discussion of key IDs and fingerprints.

   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 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 multi-precision 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:




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         - MPI of Elgamal prime p;

         - MPI of Elgamal group generator g;

         - 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, in encrypted form.

   The packet contains:

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

     - One octet indicating string-to-key usage conventions.  0
       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 specifier.

     - [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, Initial Vector (IV) of
       the same length as the cipher's block size.

     - Encrypted multi-precision integers comprising the secret key
       data. These algorithm-specific fields are as described below.

     - If the string-to-key usage octet was 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.

       Algorithm Specific Fields for RSA secret keys:

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

       - MPI of RSA secret prime value p.

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




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       - 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 SHOULD use a string-to-key
   specifier; the simple hash is for backward compatibility. 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 16-bit 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 on the private
   key that can undetectably modify the secret key. Using a SHA-1 hash
   prevents this.

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 literal data
   packets.

   The body of this packet consists of:




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

     - The remainder of the packet is compressed data.

   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 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 RFC1950 ZLIB-style
   blocks.

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 literal data packets or compressed data
   packets, 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 PGP's variant of Cipher Feedback (CFB) mode.

   The symmetric cipher used may be specified in an 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.

   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-length 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.




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   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.

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 re-assigned 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.

   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.

   Early versions of PGP also defined a value of 'l' as a 'local' mode
   for machine-local conversions. RFC 1991 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 file name),
       if the encrypted data should be saved as a file.

   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.



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     - A four-octet number that indicates the modification date of the
       file, or the creation time of the packet, or a zero that
       indicates the present 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.

   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.

5.11. User ID Packet (Tag 13)

   A User ID packet consists of data that is intended to represent the
   name and email address of the key holder.  By convention, it
   includes an RFC822 mail name, but there are no restrictions on its
   content.  The packet length in the header specifies the length of
   the user id. If it is text, it is encoded in UTF-8.

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 (by convention) 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:



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     - 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-byte numerical values in this
   document, due to an historical accident this value is encoded as a
   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 and 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.



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   There is a corresponding feature in the features signature subpacket
   that denotes that an implementation can properly use this packet
   type. An implementation SHOULD NOT use this packet when encrypting
   to a recipient that does not state it can use this packet, and
   SHOULD prefer this to older Symmetrically Encrypted Data Packet when
   possible.

   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 literal data packets or compressed data packets).  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.

   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.  Specifically,
   the input to the hash function does not include the prefix data
   described above; it includes all of the plaintext, and then also

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   includes two octets of values 0xD0, 0x14.  These represent the
   encoding of a Modification Detection Code packet tag and length
   field of 20 octets.

   The resulting hash value is stored in a Modification Detection Code
   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,
   not including the prefix data but including 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 difference in hash values is an indication that the
   message has been modified and SHOULD be reported to the user.
   Likewise, the absence of an MDC packet, or an MDC packet in any
   position other than the end of the plaintext, also represent message
   modifications and SHOULD also be reported.

   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.

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 which 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.

   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, not
       including prefix data but including the tag and length byte 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.

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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 [RFC 2045].
   An OpenPGP implementation MAY use ASCII Armor to protect the raw
   binary data.

   The checksum is a 24-bit CRC converted to four characters of
   radix-64 encoding by the same MIME base64 transformation, preceded
   by an equals 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.

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;
               }

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           }
           return crc & 0xffffffL;
       }

6.2. Forming ASCII Armor

   When OpenPGP encodes data into ASCII Armor, it puts specific headers
   around the data, so OpenPGP can reconstruct the data later. 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:

   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
       signatures following clearsigned messages. Note that PGP 2.x

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       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 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.

   Currently defined Armor Header Keys are:

     - "Version", that states the OpenPGP 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.
       Consquently, if a comment has character 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.




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

   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

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         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.

   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

   Any characters outside of the base64 alphabet are ignored in
   Radix-64 data. Decoding software must ignore all line breaks or
   other characters not found in the table above.

   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.

   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.

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

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       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 is indented by two spaces.

7. Cleartext signature framework

   It is desirable 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 RFC 3156 defines
   another way to clear sign messages for environments that support
   MIME.)

   The cleartext signed message consists of:

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




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     - 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 is used for the signature. If there are no such headers,
   MD5 is used, an implementation MAY omit them for 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.

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. 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.

   Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of
   any line is ignored when the cleartext signature is calculated.

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

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   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.

   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)
       2          - RSA Encrypt-Only
       3          - RSA Sign-Only
       16         - Elgamal (Encrypt-Only), see [ELGAMAL]
       17         - DSA (Digital Signature Standard) [SCHNEIER]
       18         - Reserved for Elliptic Curve
       19         - Reserved for ECDSA
       20         - 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.
   Implementations MAY implement any other algorithm.




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9.2. Symmetric Key Algorithms

       ID           Algorithm
       --           ---------
       0          - Plaintext or unencrypted data
       1          - IDEA [IDEA]
       2          - Triple-DES (DES-EDE, [SCHNEIER] -
                    168 bit key derived from 192)
       3          - CAST5 (128 bit key, as per RFC2144)
       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
       5          - SAFER-SK128 (13 rounds) [SAFER]
       6          - Reserved for DES/SK
       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 Triple-DES. Implementations SHOULD
   implement AES-128 and CAST5. Implementations that interoperate with
   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)
       100 to 110 - Private/Experimental algorithm.

   Implementations MUST implement uncompressed data. Implementations
   SHOULD implement ZIP. Implementations MAY implement ZLIB.

9.4. Hash Algorithms

       ID           Algorithm                             Text Name
       --           ---------                             ---- ----
       1          - MD5                                   "MD5"
       2          - SHA-1                                 "SHA1"
       3          - RIPE-MD/160                           "RIPEMD160"
       4          - Reserved for double-width SHA (experimental,
       obviated)
       5          - MD2                                   "MD2"
       6          - Reserved for TIGER/192                "TIGER192"
       7          - Reserved for HAVAL (5 pass, 160-bit)  "HAVAL-5-160"
       8          - SHA256                                "SHA256"
       9          - SHA384                                "SHA384"
       10         - SHA512                                "SHA512"
       100 to 110 - Private/Experimental algorithm.

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   Implementations MUST implement SHA-1. Implementations SHOULD
   implement MD5.

10. 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.

10.1. Transferable Public Keys

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

     - One Public Key packet

     - Zero or more revocation signatures

     - 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, 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

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   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 packets there may be one 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.

   Each Subkey packet must be followed by one Signature packet, which
   should be a subkey binding signature issued by 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.

10.2. 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

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

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



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   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.

10.3. 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 that they are a signature of.

11. Enhanced Key Formats

11.1. 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.

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

           Primary-Key
              [Revocation Self Signature]
              [Direct Key Self 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 V4 is preferred, of
   course.



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   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 key that has a main key and subkeys, 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 used for certifying subkeys that are used for encryption
   and signatures.

11.2. 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.

   A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet
   Tag, 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)-(f) (1 octet)

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

    b) version number = 4 (1 octet);

    c) time stamp 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);




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  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
   byte (even though this is not a valid packet ID for a public
   subkey).

12. Notes on Algorithms

12.1. 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 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

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   would ideally decrypt it anyway.)

   An implementation that is striving for backward compatibility MAY
   consider a V3 key with a V3 self-signature to be an implicit
   preference for IDEA, and no ability to do TripleDES. This is
   technically non-compliant, but an implementation MAY violate the
   above rule in this case only and use IDEA to encrypt the message,
   provided that the message creator is warned. Ideally, though, the
   implementation would follow the rule by actually generating two
   messages, because it is possible that the OpenPGP user's
   implementation does not have IDEA, and thus could not read the
   message. Consequently, an implementation MAY, but SHOULD NOT use
   IDEA in an algorithm conflict with a V3 key.

12.2. 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.

12.2.1. 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.

12.2.2. 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

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   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.

12.3. 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.

12.4. RSA

   There are algorithm types for RSA-signature-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
   768 bits.

   It is permissible for an implementation to support RSA merely for
   backward compatibility; for example, such an implementation would
   support V3 keys with IDEA symmetric cryptography. Note that this is
   an exception to the other MUST-implement rules. An implementation
   that supports RSA in V4 keys MUST implement the MUST-implement
   features.

12.5. Elgamal

   If an Elgamal key [ELGAMAL] is to be used for both signing and
   encryption, extra care must be taken in creating the key.

   An Elgamal key consists of a generator g, a prime modulus p, a
   secret exponent x, and a public value y = g^x mod p.

   The generator and prime must be chosen so that solving the discrete
   log problem is intractable.  The group g should generate the
   multiplicative group mod p-1 or a large subgroup of it, and the
   order of g should have at least one large prime factor.  A good
   choice is to use a "strong" Sophie-Germain prime in choosing p, so
   that both p and (p-1)/2 are primes. In fact, this choice is so good
   that implementers SHOULD do it, as it avoids a small subgroup
   attack.

   In addition, a result of Bleichenbacher [BLEICHENBACHER] shows that
   if the generator g has only small prime factors, and if g divides
   the order of the group it generates, then signatures can be forged.

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   In particular, choosing g=2 is a bad choice if the group order may
   be even. On the other hand, a generator of 2 is a fine choice for an
   encryption-only key, as this will make the encryption faster.

   While verifying Elgamal signatures, note that it is important to
   test that r and s are less than p.  If this test is not done then
   signatures can be trivially forged by using large r values of
   approximately twice the length of p.  This attack is also discussed
   in the Bleichenbacher paper.

   Details on safe use of Elgamal signatures may be found in [MENEZES],
   which discusses all the weaknesses described above. Please note that
   Elgamal signatures are controversial; because of the care that must
   be taken with Elgamal keys, many implementations forego them.

   If an implementation allows Elgamal signatures, then it MUST use the
   algorithm identifier 20 for an Elgamal public key that can sign.

   An implementation SHOULD NOT implement Elgamal keys of size less
   than 768 bits. For long-term security, Elgamal keys should be 1024
   bits or longer.

12.6. DSA

   An implementation SHOULD NOT implement DSA keys of size less than
   768 bits. Note that present DSA is limited to a maximum of 1024 bit
   keys, which are recommended for long-term use. Also, DSA keys MUST
   be an even multiple of 64 bits long.

12.7. 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 the Public Algorithms section 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.

   The reserved symmetric key algorithm, DES/SK (6), does not have
   semantics defined.

   The reserved hash algorithms, TIGER192 (6), and HAVAL-5-160 (7), do
   not have OIDs. The reserved algorithm number 4, reserved for a
   double-width variant of SHA1, is not presently defined.

12.8. OpenPGP CFB mode

   OpenPGP does symmetric encryption using a variant of Cipher Feedback
   Mode (CFB mode). This section describes the procedure it uses in

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   detail. This mode is what is used for Symmetrically Encrypted Data
   Packets; the mechanism used for encrypting secret key material is
   similar, but described in those 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 blocksize 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
   "resync" 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 blocksize 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.

   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 resync 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.


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  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. 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.
       Possession of the private key portion of a public-private key
       pair is assumed to be controlled 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 RFC 1750.

     * The MD5 hash algorithm has been found to have weaknesses
       (pseudo-collisions in the compress function) that make some
       people deprecate its use.  They consider the SHA-1 algorithm
       better.

     * 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.

     * The DSA algorithm will work with any 160-bit hash, but it is
       sensitive to the quality of the hash algorithm, if the hash
       algorithm is broken, it can leak the secret key. The Digital
       Signature Standard (DSS) specifies that DSA be used with SHA-1.
       RIPEMD-160 is considered by many cryptographers to be as strong.
       An implementation should take care which hash algorithms are
       used with DSA, as a weak hash can not only allow a signature to
       be forged, but could leak the secret key. These same
       considerations about the quality of the hash algorithm apply to
       Elgamal signatures.

     * 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.



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       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 Triple-DES. Other algorithms may have other controversies
       surrounding them.

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

14. 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.

     * PGP 5.x does not accept V4 signatures for anything other than
       key material.

     * PGP 5.x does not recognize the "five-octet" lengths in
       new-format headers or in signature subpacket lengths.

     * PGP 5.0 rejects an encrypted session key if the keylength
       differs from the S2K symmetric algorithm. This is a bug in its
       validation function.

     * PGP 5.0 does not handle multiple one-pass signature headers and
       trailers. Signing one will compress the one-pass signed literal
       and prefix a V3 signature instead of doing a nested one-pass
       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.


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     * In a clear-signed signature, PGP 5.0 will figure out the correct
       hash algorithm if there is no "Hash:" header, but it will reject
       a mismatch between the header and the actual algorithm used. The
       "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
       rejects the "Hash:" header and assumes MD5. There are a number
       of enhanced variants of PGP 2.6.x that have been modified for
       SHA-1 signatures.

     * PGP 5.0 can read an RSA key in V4 format, but can only recognize
       it with a V3 key id, and can properly use only a V3 format RSA
       key.

     * Neither PGP 5.x nor PGP 6.0 recognize Elgamal Encrypt and Sign
       keys. They only handle Elgamal Encrypt-only keys.

     * 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.

     * PGP 2.6.X and 5.0 do not trim trailing whitespace from a
       "canonical text" signature. They only remove it from cleartext
       signatures. These signatures are not OpenPGP compliant --
       OpenPGP requires trimming the whitespace. If you wish to
       interoperate with PGP 2.6.X or PGP 5, you may wish to accept
       these non-compliant signatures.

15. Authors and Working Group Chair

   The working group can be contacted via the current chair:

       John W. Noerenberg, II
       Qualcomm, Inc
       6455 Lusk Blvd
       San Diego, CA 92131 USA
       Email: jwn2@qualcomm.com
       Tel: +1 (619) 658-3510

   The principal authors of this draft are:

       Jon Callas
       Wave Systems Corp.
       1601 S. DeAnza Blvd, Suite 200
       Cupertino, CA 95014, USA



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       Email: jon@callas.org, jcallas@wavesys.com
       Tel: +1 (408) 448-6801

       Lutz Donnerhacke
       IKS GmbH
       Wildenbruchstr. 15
       07745 Jena, Germany

       EMail: lutz@iks-jena.de
       Tel: +49-3641-675642

       Hal Finney
       Network Associates, Inc.
       3965 Freedom Circle
       Santa Clara, CA 95054, USA

       Email: hal@pgp.com

       Rodney Thayer

       Email: rodney@tillerman.to

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

16. References


   [AES]            Advanced Encryption Standards Questions and Answers
                    <http://csrc.nist.gov/encryption/aes/round2/
                    aesfact.html>

                    <http://csrc.nist.gov/encryption/aes/round2/
                    r2algs.html#Rijndael>

   [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>

   [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>


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   [DONNERHACKE]    Donnerhacke, L., et. al, "PGP263in - an improved
                    international version of PGP", ftp://ftp.iks-
                    jena.de/mitarb/lutz/crypt/software/pgp/

   [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.

   [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

   [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.

   [MENEZES]        Alfred Menezes, Paul van Oorschot, and Scott
                    Vanstone, "Handbook of Applied Cryptography," CRC
                    Press, 1996.

   [RFC822]         Crocker, D., "Standard for the format of ARPA
                    Internet text messages", STD 11, RFC 822, August
                    1982.

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

   [RFC1641]        Goldsmith, D. and M. Davis, "Using Unicode with
                    MIME", RFC 1641, July 1994.

   [RFC1750]        Eastlake, D., Crocker, S. and J. Schiller,
                    "Randomness Recommendations for Security", RFC
                    1750, December 1994.

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

   [RFC1983]        Malkin, G., "Internet Users' Glossary", FYI 18, RFC
                    1983, August 1996.

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

   [RFC2045]        Borenstein, N. and N. Freed, "Multipurpose Internet

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                    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 Level", BCP 14, RFC 2119, March 1997.

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

   [RFC2279]        Yergeau., F., "UTF-8, a transformation format of
                    Unicode and ISO 10646", RFC 2279, January 1998.

   [RFC2437]        B. Kaliski and J. Staddon, " PKCS #1: RSA
                    Cryptography Specifications Version 2.0",
                    RFC 2437, October 1998.

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

   [SAFER]          Massey, J.L. "SAFER K-64: One Year Later", B.
                    Preneel, editor, Fast Software Encryption, Second
                    International Workshop (LNCS 1008) pp212-241,
                    Springer-Verlag 1995

   [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.


17. Full Copyright Statement

   Copyright 2002 by The Internet Society. All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.



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   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.



















































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