File: rfc2440.txt

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Network Working Group                                         J. Callas
Request for Comments: 2440                           Network Associates
Category: Standards Track                                L. Donnerhacke
                                     IN-Root-CA Individual Network e.V.
                                                              H. Finney
                                                     Network Associates
                                                              R. Thayer
                                                        EIS Corporation
                                                          November 1998


                         OpenPGP Message Format

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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.

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,



Callas, et. al.             Standards Track                     [Page 1]

RFC 2440                 OpenPGP Message Format            November 1998


   however, discuss implementation issues necessary to avoid security
   flaws.

   Open-PGP 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.

Table of Contents

            Status of this Memo                                       1
            IESG Note                                                 1
            Abstract                                                  1
            Table of Contents                                         2
   1.       Introduction                                              4
   1.1.     Terms                                                     5
   2.       General functions                                         5
   2.1.     Confidentiality via Encryption                            5
   2.2.     Authentication via Digital signature                      6
   2.3.     Compression                                               7
   2.4.     Conversion to Radix-64                                    7
   2.5.     Signature-Only Applications                               7
   3.       Data Element Formats                                      7
   3.1.     Scalar numbers                                            8
   3.2.     Multi-Precision Integers                                  8
   3.3.     Key IDs                                                   8
   3.4.     Text                                                      8
   3.5.     Time fields                                               9
   3.6.     String-to-key (S2K) specifiers                            9
   3.6.1.   String-to-key (S2k) specifier types                       9
   3.6.1.1. Simple S2K                                                9
   3.6.1.2. Salted S2K                                               10
   3.6.1.3. Iterated and Salted S2K                                  10
   3.6.2.   String-to-key usage                                      11
   3.6.2.1. Secret key encryption                                    11
   3.6.2.2. Symmetric-key message encryption                         11
   4.       Packet Syntax                                            12
   4.1.     Overview                                                 12
   4.2.     Packet Headers                                           12
   4.2.1.   Old-Format Packet Lengths                                13
   4.2.2.   New-Format Packet Lengths                                13
   4.2.2.1. One-Octet Lengths                                        14
   4.2.2.2. Two-Octet Lengths                                        14
   4.2.2.3. Five-Octet Lengths                                       14
   4.2.2.4. Partial Body Lengths                                     14
   4.2.3.   Packet Length Examples                                   14



Callas, et. al.             Standards Track                     [Page 2]

RFC 2440                 OpenPGP Message Format            November 1998


   4.3.     Packet Tags                                              15
   5.       Packet Types                                             16
   5.1.     Public-Key Encrypted Session Key Packets (Tag 1)         16
   5.2.     Signature Packet (Tag 2)                                 17
   5.2.1.   Signature Types                                          17
   5.2.2.   Version 3 Signature Packet Format                        19
   5.2.3.   Version 4 Signature Packet Format                        21
   5.2.3.1. Signature Subpacket Specification                        22
   5.2.3.2. Signature Subpacket Types                                24
   5.2.3.3. Signature creation time                                  25
   5.2.3.4. Issuer                                                   25
   5.2.3.5. Key expiration time                                      25
   5.2.3.6. Preferred symmetric algorithms                           25
   5.2.3.7. Preferred hash algorithms                                25
   5.2.3.8. Preferred compression algorithms                         26
   5.2.3.9. Signature expiration time                                26
   5.2.3.10.Exportable Certification                                 26
   5.2.3.11.Revocable                                                27
   5.2.3.12.Trust signature                                          27
   5.2.3.13.Regular expression                                       27
   5.2.3.14.Revocation key                                           27
   5.2.3.15.Notation Data                                            28
   5.2.3.16.Key server preferences                                   28
   5.2.3.17.Preferred key server                                     29
   5.2.3.18.Primary user id                                          29
   5.2.3.19.Policy URL                                               29
   5.2.3.20.Key Flags                                                29
   5.2.3.21.Signer's User ID                                         30
   5.2.3.22.Reason for Revocation                                    30
   5.2.4.   Computing Signatures                                     31
   5.2.4.1. Subpacket Hints                                          32
   5.3.     Symmetric-Key Encrypted Session-Key Packets (Tag 3)      32
   5.4.     One-Pass Signature Packets (Tag 4)                       33
   5.5.     Key Material Packet                                      34
   5.5.1.   Key Packet Variants                                      34
   5.5.1.1. Public Key Packet (Tag 6)                                34
   5.5.1.2. Public Subkey Packet (Tag 14)                            34
   5.5.1.3. Secret Key Packet (Tag 5)                                35
   5.5.1.4. Secret Subkey Packet (Tag 7)                             35
   5.5.2.   Public Key Packet Formats                                35
   5.5.3.   Secret Key Packet Formats                                37
   5.6.     Compressed Data Packet (Tag 8)                           38
   5.7.     Symmetrically Encrypted Data Packet (Tag 9)              39
   5.8.     Marker Packet (Obsolete Literal Packet) (Tag 10)         39
   5.9.     Literal Data Packet (Tag 11)                             40
   5.10.    Trust Packet (Tag 12)                                    40
   5.11.    User ID Packet (Tag 13)                                  41
   6.       Radix-64 Conversions                                     41



Callas, et. al.             Standards Track                     [Page 3]

RFC 2440                 OpenPGP Message Format            November 1998


   6.1.     An Implementation of the CRC-24 in "C"                   42
   6.2.     Forming ASCII Armor                                      42
   6.3.     Encoding Binary in Radix-64                              44
   6.4.     Decoding Radix-64                                        46
   6.5.     Examples of Radix-64                                     46
   6.6.     Example of an ASCII Armored Message                      47
   7.       Cleartext signature framework                            47
   7.1.     Dash-Escaped Text                                        47
   8.       Regular Expressions                                      48
   9.       Constants                                                49
   9.1.     Public Key Algorithms                                    49
   9.2.     Symmetric Key Algorithms                                 49
   9.3.     Compression Algorithms                                   50
   9.4.     Hash Algorithms                                          50
   10.      Packet Composition                                       50
   10.1.    Transferable Public Keys                                 50
   10.2.    OpenPGP Messages                                         52
   10.3.    Detached Signatures                                      52
   11.      Enhanced Key Formats                                     52
   11.1.    Key Structures                                           52
   11.2.    Key IDs and Fingerprints                                 53
   12.      Notes on Algorithms                                      54
   12.1.    Symmetric Algorithm Preferences                          54
   12.2.    Other Algorithm Preferences                              55
   12.2.1.  Compression Preferences                                  56
   12.2.2.  Hash Algorithm Preferences                               56
   12.3.    Plaintext                                                56
   12.4.    RSA                                                      56
   12.5.    Elgamal                                                  57
   12.6.    DSA                                                      58
   12.7.    Reserved Algorithm Numbers                               58
   12.8.    OpenPGP CFB mode                                         58
   13.      Security Considerations                                  59
   14.      Implementation Nits                                      60
   15.      Authors and Working Group Chair                          62
   16.      References                                               63
   17.      Full Copyright Statement                                 65

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 builds on the foundation provided in
   RFC 1991 "PGP Message Exchange Formats."







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1.1. Terms

     * OpenPGP - This is a definition for security software that uses
       PGP 5.x as a basis.

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

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

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

   "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 RFC 2119, 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

     - 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 uses two encryption methods to provide confidentiality:
   symmetric-key encryption and public key encryption. With public-key
   encryption, the object is encrypted using a symmetric encryption
   algorithm.  Each symmetric key is used only once. A new "session key"
   is generated as a random number for each message. Since it is used



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   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:

   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.



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   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 MAY compress the message after applying the
   signature but before encryption.

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.

   Note that many applications, particularly messaging applications,
   will want more advanced features as described in the OpenPGP-MIME
   document, RFC 2015. An application that implements OpenPGP for
   messaging SHOULD implement OpenPGP-MIME.

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.

3. Data Element Formats

   This section describes the data elements used by OpenPGP.








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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].

3.3. Key IDs

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

3.4. Text

   The default character set for text is the UTF-8 [RFC2279] encoding of
   Unicode [ISO10646].




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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. 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.

3.6.1. String-to-key (S2k) specifier types

   There are three types of S2K specifiers currently supported, as
   follows:

3.6.1.1. 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 or equal to the
   session key size, the high-order (leftmost) octets of the hash are
   used as the key.

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

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





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3.6.1.2. 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.

3.6.1.3. 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
   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.



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   After the hashing is done the data is unloaded from the hash
   context(s) as with the other S2K algorithms.

3.6.2. String-to-key usage

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

3.6.2.1. 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 8-octet Initial Vector for the decryption of
   the secret values, if they are encrypted, and then the secret key
   values themselves.

3.6.2.2. 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.

   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.










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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;

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

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.




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   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.

   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




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       13       -- User ID Packet
       14       -- Public Subkey 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
   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.

     - 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.






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   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 padded as described in
   PKCS-1 block type 02 [RFC2313] 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 padding for each key.

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

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.
         Typically, this means the signer owns it, created it, or
         certifies that it has not been modified.




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   0x01: Signature of a canonical text document.
         Typically, 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
         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.






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   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.

   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.



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     - 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.

   Algorithm Specific Fields for DSA signatures:

     - MPI of DSA value r.

     - MPI of DSA value s.

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

   With RSA signatures, the hash value is encoded as described in PKCS-1
   section 10.1.2, "Data encoding", producing an ASN.1 value of type
   DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313].
   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








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   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

   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

   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. (zero 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.

   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:




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

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

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

   The value of the subpacket type octet may be:

       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
       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.






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

   Implementations SHOULD implement "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 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.

   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.









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5.2.3.3. Signature creation time

   (4 octet time field)

   The time the signature was made.

   MUST be present in the hashed area.

5.2.3.4. Issuer

   (8 octet key ID)

   The OpenPGP key ID of the key issuing the signature.

5.2.3.5. 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.

5.2.3.6. 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.

5.2.3.7. Preferred hash algorithms

   (array of one-octet values)

   Message digest algorithm numbers that indicate which algorithms the
   key holder prefers to receive. Like the preferred symmetric
   algorithms, the list is ordered. Algorithm numbers are in section 6.
   This is only found on a self-signature.









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5.2.3.8. 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.

5.2.3.9. 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.

5.2.3.10. 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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5.2.3.11. 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.

5.2.3.12. 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.

5.2.3.13. 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.

5.2.3.14. 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



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   is found on a self-signature.

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

5.2.3.15. 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 is text, a note
                           from one person to another, and has no
                           meaning to software.
       Other octets: none.

5.2.3.16. 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.





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5.2.3.17. 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.

5.2.3.18. 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.

5.2.3.19. Policy URL

   (String)

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

5.2.3.20. Key Flags

   (Octet string)

   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.




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       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.

5.2.3.21. Signer's User ID

   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.

5.2.3.22. 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:







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       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 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.

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. 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, without any
   header. A V4 certification hashes the constant 0xb4 (which is an
   old-style packet header with the length-of-length set to zero), a
   four-octet number giving the length of the username, and then the
   username 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.






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   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.

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



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   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
   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:





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

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

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

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

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

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

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

5.5. Key Material Packet

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

5.5.1. Key Packet Variants

5.5.1.1. Public Key Packet (Tag 6)

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

5.5.1.2. Public Subkey Packet (Tag 14)

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

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



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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:

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

         - an MPI of RSA public encryption exponent e.






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   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 where x is secret).

       Algorithm Specific Fields for Elgamal public keys:

         - MPI of Elgamal prime p;

         - MPI of Elgamal group generator g;



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         - MPI of Elgamal public key value y (= g**x 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
       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, a one-octet
       symmetric encryption algorithm.

     - [Optional] If string-to-key usage octet was 255, 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, eight-octet Initial
       Vector (IV).

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

     - Two-octet checksum of the plaintext of the algorithm-specific
       portion (sum of all octets, mod 65536).

       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).

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

       Algorithm Specific Fields for DSA secret keys:

       - MPI of DSA secret exponent x.




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       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.

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:

     - 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.





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

   ZLIB-compressed packets are compressed with RFC 1950 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 will
   typically contain other packets (often literal data packets or
   compressed data packets).

   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 10-octet string to
   the data before it is encrypted.  The first eight octets are random,
   and the 9th and 10th octets are copies of the 7th and 8th octets,
   respectively. After encrypting the first 10 octets, the CFB state is
   resynchronized if the cipher block size is 8 octets or less.  The
   last 8 octets of ciphertext are passed through the cipher and the
   block boundary is reset.

   The repetition of 16 bits in the 80 bits of 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.



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   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.  RFC
   1991 also defined a value of 'l' as a 'local' mode for machine-local
   conversions.  This use is now 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.

     - 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,



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   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 RFC 822 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.

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 [RFC2231,
   Section 6.8]. 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.



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

       #define CRC24_INIT 0xb704ceL
       #define CRC24_POLY 0x1864cfbL

       typedef long crc24;
       crc24 crc_octets(unsigned char *octets, size_t len)
       {
           crc24 crc = CRC24_INIT;
           int i;

           while (len--) {
               crc ^= (*octets++) << 16;
               for (i = 0; i < 8; i++) {
                   crc <<= 1;
                   if (crc & 0x1000000)
                       crc ^= CRC24_POLY;
               }
           }
           return crc & 0xffffffL;
       }

6.2. Forming ASCII Armor

   When OpenPGP encodes data into ASCII Armor, it puts specific headers
   around the 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:



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   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
       natures following clearsigned messages. Note that PGP 2.x s BEGIN
       PGP MESSAGE for detached signatures.

   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.

     - "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






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       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.

     - "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 by
       default. 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 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.

   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.








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

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

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

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

   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.




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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
       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      =      =





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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 2015 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,

     - 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.



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   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 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.






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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)
       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.

9.2. Symmetric Key Algorithms

       ID           Algorithm
       --           ---------
       0          - Plaintext or unencrypted data
       1          - IDEA [IDEA]
       2          - Triple-DES (DES-EDE, as per spec -
                    168 bit key derived from 192)
       3          - CAST5 (128 bit key, as per RFC 2144)
       4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
       5          - SAFER-SK128 (13 rounds) [SAFER]
       6          - Reserved for DES/SK
       7          - Reserved for AES with 128-bit key



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       8          - Reserved for AES with 192-bit key
       9          - Reserved for AES with 256-bit key
       100 to 110 - Private/Experimental algorithm.

   Implementations MUST implement Triple-DES. Implementations SHOULD
   implement IDEA and CAST5.Implementations MAY implement any other
   algorithm.

9.3. Compression Algorithms

       ID           Algorithm
       --           ---------
       0          - Uncompressed
       1          - ZIP (RFC 1951)
       2          - ZLIB (RFC 1950)
       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)
       5          - MD2                                    "MD2"
       6          - Reserved for TIGER/192                 "TIGER192"
       7          - Reserved for HAVAL (5 pass, 160-bit)
       "HAVAL-5-160"
       100 to 110 - Private/Experimental algorithm.

   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 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:



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     - 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 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.

   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.

   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.





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

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

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

   In addition, decrypting a Symmetrically Encrypted Data packet and

   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.





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           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 ...] ...]
              [[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.

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

   In a key that has a main key and subkeys, the primary key MUST be a
   key capable of signing. 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.



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   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);

  e.3) MPI of DSA group generator g;

  e.4) MPI of DSA public key value y (= g**x 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.

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".



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   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 than protocol has been
   violated. (For example, suppose that Alice, above, has software that
   implements all algorithms in this specification. Nonetheless, she
   prefers subsets for work or home. If she is sent a message encrypted
   with IDEA, which is not in her preferences, the software warns her
   that someone sent her an IDEA-encrypted message, but it would ideally
   decrypt it anyway.)

   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.





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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.
   And 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)].

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 does not typically
   know who is going to be verifying the signature. This preference,
   though, allows a protocol based upon digital signatures ease in
   negotiation.

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

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.




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   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 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
   implementors 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.  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.

   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.






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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.

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 implementor 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.

   We have reserver three algorithm IDs for the US NIST's Advanced
   Encryption Standard. This algorithm will work with (at least) 128,
   192, and 256-bit keys. We expect that this algorithm will be selected
   from the candidate algorithms in the year 2000.

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
   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.

   OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
   prefixes the plaintext with ten octets of random data, such that
   octets 9 and 10 match octets 7 and 8.  It does a CFB "resync" after
   encrypting those ten octets.

   Note that for an algorithm that has a larger block size than 64 bits,
   the equivalent function will be done with that entire block.  For
   example, a 16-octet block algorithm would operate on 16 octets, and
   then produce two octets of check, and then work on 16-octet blocks.




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   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 8 octets of random data prefixed to
       the plaintext to produce C1-C8, the first 8 octets of ciphertext.

   4.  FR is loaded with C1-C8.

   5.  FR is encrypted to produce FRE, the encryption of the first 8
       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 C9-C10,
       the next two octets of ciphertext.

   7.  (The resync step) FR is loaded with C3-C10.

   8.  FR is encrypted to produce FRE.

   9.  FRE is xored with the first 8 octets of the given plaintext, now
       that we have finished encrypting the 10 octets of prefixed data.
       This produces C11-C18, the next 8 octets of ciphertext.

   10.  FR is loaded with C11-C18

   11.  FR is encrypted to produce FRE.

   12.  FRE is xored with the next 8 octets of plaintext, to produce the
       next 8 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.



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   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
   implementor 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.

   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 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.



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     * 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.

     * 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 keyid, 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.












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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

   Phone: +1 619-658-3510
   EMail: jwn2@qualcomm.com


   The principal authors of this memo are:

   Jon Callas
   Network Associates, Inc.
   3965 Freedom Circle
   Santa Clara, CA 95054, USA

   Phone: +1 408-346-5860
   EMail: jon@pgp.com, jcallas@nai.com


   Lutz Donnerhacke
   IKS GmbH
   Wildenbruchstr. 15
   07745 Jena, Germany

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


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

   EMail: hal@pgp.com


   Rodney Thayer
   EIS Corporation
   Clearwater, FL 33767, USA

   EMail: rodney@unitran.com





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RFC 2440                 OpenPGP Message Format            November 1998


   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, William Stallings, Mark Weaver, and
   Philip R. Zimmermann.

16. References

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

   [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>

   [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

   [ISO-10646]      ISO/IEC 10646-1:1993. International Standard --
                    Information technology -- Universal Multiple-Octet
                    Coded Character Set (UCS) -- Part 1: Architecture
                    and Basic Multilingual Plane.  UTF-8 is described in
                    Annex R, adopted but not yet published.  UTF-16 is
                    described in Annex Q, adopted but not yet published.

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




Callas, et. al.             Standards Track                    [Page 63]

RFC 2440                 OpenPGP Message Format            November 1998


   [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.

   [RFC2015]        Elkins, M., "MIME Security with Pretty Good Privacy
                    (PGP)", RFC 2015, October 1996.

   [RFC2231]        Borenstein, N. and N. Freed, "Multipurpose Internet
                    Mail Extensions (MIME) Part One: Format of Internet
                    Message Bodies.", RFC 2231, 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.

   [RFC2313]        Kaliski, B., "PKCS #1: RSA Encryption Standard
                    version 1.5", RFC 2313, March 1998.

   [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




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RFC 2440                 OpenPGP Message Format            November 1998


17.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  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.

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

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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