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

Network Working Group                                        Jon Callas
Internet-Draft                                          PGP Corporation
Intended status: Standards Track
Expires October 2007                                   Lutz Donnerhacke
Apr 2007

Obsoletes: 1991, 2440                                        Hal Finney
                                                         PGP Corporation

                                                              David Shaw

                                                           Rodney Thayer

                          OpenPGP Message Format
                    draft-ietf-openpgp-rfc2440bis-22


Status of this Memo

    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.

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

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

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

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

Copyright Notice

    Copyright (C) The IETF Trust (2007).

Abstract

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

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















































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Table of Contents

             Status of this Memo                                       1
             Copyright Notice                                          1
             Abstract                                                  1
             Table of Contents                                         3
    1.       Introduction                                              7
    1.1.     Terms                                                     7
    2.       General functions                                         7
    2.1.     Confidentiality via Encryption                            8
    2.2.     Authentication via Digital signature                      9
    2.3.     Compression                                               9
    2.4.     Conversion to Radix-64                                    9
    2.5.     Signature-Only Applications                              10
    3.       Data Element Formats                                     10
    3.1.     Scalar numbers                                           10
    3.2.     Multiprecision Integers                                  10
    3.3.     Key IDs                                                  11
    3.4.     Text                                                     11
    3.5.     Time fields                                              11
    3.6.     Keyrings                                                 11
    3.7.     String-to-key (S2K) specifiers                           11
    3.7.1.   String-to-key (S2K) specifier types                      11
    3.7.1.1. Simple S2K                                               12
    3.7.1.2. Salted S2K                                               12
    3.7.1.3. Iterated and Salted S2K                                  12
    3.7.2.   String-to-key usage                                      13
    3.7.2.1. Secret key encryption                                    13
    3.7.2.2. Symmetric-key message encryption                         14
    4.       Packet Syntax                                            14
    4.1.     Overview                                                 14
    4.2.     Packet Headers                                           14
    4.2.1.   Old-Format Packet Lengths                                15
    4.2.2.   New-Format Packet Lengths                                15
    4.2.2.1. One-Octet Lengths                                        16
    4.2.2.2. Two-Octet Lengths                                        16
    4.2.2.3. Five-Octet Lengths                                       16
    4.2.2.4. Partial Body Lengths                                     16
    4.2.3.   Packet Length Examples                                   17
    4.3.     Packet Tags                                              17
    5.       Packet Types                                             18
    5.1.     Public-Key Encrypted Session Key Packets (Tag 1)         18
    5.2.     Signature Packet (Tag 2)                                 19
    5.2.1.   Signature Types                                          20
    5.2.2.   Version 3 Signature Packet Format                        22
    5.2.3.   Version 4 Signature Packet Format                        24
    5.2.3.1. Signature Subpacket Specification                        25
    5.2.3.2. Signature Subpacket Types                                27
    5.2.3.3. Notes on Self-Signatures                                 27
    5.2.3.4. Signature creation time                                  28
    5.2.3.5. Issuer                                                   28
    5.2.3.6. Key expiration time                                      28

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    5.2.3.7. Preferred symmetric algorithms                           28
    5.2.3.8. Preferred hash algorithms                                29
    5.2.3.9. Preferred compression algorithms                         29
    5.2.3.10.Signature expiration time                                29
    5.2.3.11.Exportable Certification                                 29
    5.2.3.12.Revocable                                                30
    5.2.3.13.Trust signature                                          30
    5.2.3.14.Regular expression                                       30
    5.2.3.15.Revocation key                                           31
    5.2.3.16.Notation Data                                            31
    5.2.3.17.Key server preferences                                   32
    5.2.3.18.Preferred key server                                     32
    5.2.3.19.Primary User ID                                          32
    5.2.3.20.Policy URI                                               33
    5.2.3.21.Key Flags                                                33
    5.2.3.22.Signer's User ID                                         34
    5.2.3.23.Reason for Revocation                                    34
    5.2.3.24.Features                                                 35
    5.2.3.25.Signature Target                                         35
    5.2.3.26.Embedded Signature                                       36
    5.2.4.   Computing Signatures                                     36
    5.2.4.1. Subpacket Hints                                          37
    5.3.     Symmetric-Key Encrypted Session Key Packets (Tag 3)      37
    5.4.     One-Pass Signature Packets (Tag 4)                       38
    5.5.     Key Material Packet                                      39
    5.5.1.   Key Packet Variants                                      39
    5.5.1.1. Public Key Packet (Tag 6)                                39
    5.5.1.2. Public Subkey Packet (Tag 14)                            39
    5.5.1.3. Secret Key Packet (Tag 5)                                39
    5.5.1.4. Secret Subkey Packet (Tag 7)                             40
    5.5.2.   Public Key Packet Formats                                40
    5.5.3.   Secret Key Packet Formats                                41
    5.6.     Compressed Data Packet (Tag 8)                           43
    5.7.     Symmetrically Encrypted Data Packet (Tag 9)              44
    5.8.     Marker Packet (Obsolete Literal Packet) (Tag 10)         44
    5.9.     Literal Data Packet (Tag 11)                             45
    5.10.    Trust Packet (Tag 12)                                    46
    5.11.    User ID Packet (Tag 13)                                  46
    5.12.    User Attribute Packet (Tag 17)                           46
    5.12.1.  The Image Attribute Subpacket                            47
    5.13.    Sym. Encrypted Integrity Protected Data Packet (Tag 18)  47
    5.14.    Modification Detection Code Packet (Tag 19)              50
    6.       Radix-64 Conversions                                     51
    6.1.     An Implementation of the CRC-24 in "C"                   51
    6.2.     Forming ASCII Armor                                      52
    6.3.     Encoding Binary in Radix-64                              54
    6.4.     Decoding Radix-64                                        55
    6.5.     Examples of Radix-64                                     56
    6.6.     Example of an ASCII Armored Message                      56
    7.       Cleartext signature framework                            56
    7.1.     Dash-Escaped Text                                        57
    8.       Regular Expressions                                      58

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    9.       Constants                                                58
    9.1.     Public Key Algorithms                                    59
    9.2.     Symmetric Key Algorithms                                 59
    9.3.     Compression Algorithms                                   60
    9.4.     Hash Algorithms                                          60
    10.      IANA Considerations                                      60
    10.1.    New String-to-Key specifier types                        60
    10.2.    New Packets                                              61
    10.2.1.  User Attribute Types                                     61
    10.2.1.1.Image Format Subpacket Types                             61
    10.2.2.  New Signature Subpackets                                 61
    10.2.2.1.Signature Notation Data Subpackets                       61
    10.2.2.2.Key Server Preference Extensions                         62
    10.2.2.3.Key Flags Extensions                                     62
    10.2.2.4.Reason For Revocation Extensions                         62
    10.2.2.5.Implementation Features                                  62
    10.2.3.  New Packet Versions                                      62
    10.3.    New Algorithms                                           63
    10.3.1.  Public Key Algorithms                                    63
    10.3.2.  Symmetric Key Algorithms                                 63
    10.3.3.  Hash Algorithms                                          63
    10.3.4.  Compression Algorithms                                   64
    11.      Packet Composition                                       64
    11.1.    Transferable Public Keys                                 64
    11.2.    Transferable Secret Keys                                 65
    11.3.    OpenPGP Messages                                         65
    11.4.    Detached Signatures                                      66
    12.      Enhanced Key Formats                                     66
    12.1.    Key Structures                                           66
    12.2.    Key IDs and Fingerprints                                 67
    13.      Notes on Algorithms                                      68
    13.1.    PKCS#1 Encoding In OpenPGP                               68
    13.1.1.  EME-PKCS1-v1_5-ENCODE                                    69
    13.1.2.  EME-PKCS1-v1_5-DECODE                                    69
    13.1.3.  EMSA-PKCS1-v1_5                                          70
    13.2.    Symmetric Algorithm Preferences                          71
    13.3.    Other Algorithm Preferences                              71
    13.3.1.  Compression Preferences                                  71
    13.3.2.  Hash Algorithm Preferences                               72
    13.4.    Plaintext                                                72
    13.5.    RSA                                                      72
    13.6.    DSA                                                      73
    13.7.    Elgamal                                                  73
    13.8.    Reserved Algorithm Numbers                               73
    13.9.    OpenPGP CFB mode                                         74
    13.10.   Private or Experimental Parameters                       75
    13.11.   Extension of the MDC System                              75
    13.12.   Meta-Considerations for Expansion                        76
    14.      Security Considerations                                  76
    15.      Implementation Nits                                      79
    16.      Authors' Addresses                                       80
    17.      References (Normative)                                   81

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    18.      References (Informative)                                 83
    19.      Full Copyright Statement                                 84
    20.      Intellectual Property                                    84


















































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1. Introduction

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

1.1. Terms

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

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

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

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

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

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

    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
    "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
    document are to be interpreted as described in RFC 2119.

    The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
    FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
    APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
    this document when used to describe namespace allocation are to be
    interpreted as described in RFC 2434.

2. General functions

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


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      - digital signatures

      - encryption

      - compression

      - radix-64 conversion

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

2.1. Confidentiality via Encryption

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

    1.  The sender creates a message.

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

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

    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

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    block.

2.2. Authentication via Digital signature

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

    1.  The sender creates a message.

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

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

    4.  The binary signature is attached to the message.

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

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

2.3. Compression

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

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

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

2.4. Conversion to Radix-64

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


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    Implementations SHOULD provide Radix-64 conversions.

2.5. Signature-Only Applications

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

3. Data Element Formats

    This section describes the data elements used by OpenPGP.

3.1. Scalar numbers

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

3.2. Multiprecision Integers

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

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

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

    Examples:

    (all numbers are in hexadecimal)

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

    Additional rules:

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

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



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    Unused bits of an MPI MUST be zero.

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

3.3. Key IDs

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

3.4. Text

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

3.5. Time fields

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

3.6. Keyrings

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

3.7. String-to-key (S2K) specifiers

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

3.7.1. String-to-key (S2K) specifier types

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

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




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    These are described as follows:

3.7.1.1. Simple S2K

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

        Octet 0:        0x00
        Octet 1:        hash algorithm

    Simple S2K hashes the passphrase to produce the session key. The
    manner in which this is done depends on the size of the session key
    (which will depend on the cipher used) and the size of the hash
    algorithm's output. If the hash size is greater than the session key
    size, the high-order (leftmost) octets of the hash are used as the
    key.

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

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

3.7.1.2. Salted S2K

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

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

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

3.7.1.3. Iterated and Salted S2K

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


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

3.7.2. String-to-key usage

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

3.7.2.1. Secret key encryption

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

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



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

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

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

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

3.7.2.2. Symmetric-key message encryption

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

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

4. Packet Syntax

    This section describes the packets used by OpenPGP.

4.1. Overview

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

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

4.2. Packet Headers

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



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    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, the new packet format
    is RECOMMENDED. Note that old format packets have four bits of
    packet tags, and new format packets have six; some features cannot
    be used and still be backward-compatible.

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

    Old format packets contain:

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

    New format packets contain:

          Bits 5-0 -- packet tag

4.2.1. Old-Format Packet Lengths

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

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

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

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

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

4.2.2. New-Format Packet Lengths

    New format packets have four possible ways of encoding length:

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

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

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

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

4.2.2.1. One-Octet Lengths

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

        bodyLen = 1st_octet;

4.2.2.2. Two-Octet Lengths

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

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

4.2.2.3. Five-Octet Lengths

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

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

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

4.2.2.4. Partial Body Lengths

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




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        partialBodyLen = 1 << (1st_octet & 0x1f);

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

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

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

4.2.3. Packet Length Examples

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

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

    A packet with length 1723 may have its length 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.

    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:



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

5. Packet Types

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

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

    The body of this packet consists of:

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

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

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

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

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    Algorithm Specific Fields for RSA encryption

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

    Algorithm Specific Fields for Elgamal encryption:

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

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

    The value "m" in the above formulas is derived from the session key
    as follows. First the session key is prefixed with a one-octet
    algorithm identifier that specifies the symmetric encryption
    algorithm used to encrypt the following Symmetrically Encrypted Data
    Packet. Then a two-octet checksum is appended which is equal to the
    sum of the preceding session key octets, not including the algorithm
    identifier, modulo 65536. This value is then encoded as described in
    PKCS#1 block encoding EME-PKCS1-v1_5 in Section 12.1 of RFC 3447 to
    form the "m" value used in the formulas above. See Section 13.1 of
    this document for notes on OpenPGP's use of PKCS#1.

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

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

5.2. Signature Packet (Tag 2)

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

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

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

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




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5.2.1. Signature Types

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

    These meanings are:

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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




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    0x50: Third-Party Confirmation signature.
        This signature is a signature over some other OpenPGP signature
        packet(s). It is analogous to a notary seal on the signed data.
        A third-party signature SHOULD include Signature Target
        subpacket(s) to give easy identification. Note that we really do
        mean SHOULD. There are plausible uses for this (such as a blind
        party that only sees the signature, not the key nor source
        document) that cannot include a target subpacket.

5.2.2. Version 3 Signature Packet Format

    The body of a version 3 Signature Packet contains:

      - One-octet version number (3).

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

          - One-octet signature type.

          - Four-octet creation time.

      - Eight-octet key ID of signer.

      - One-octet public key algorithm.

      - One-octet hash algorithm.

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

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

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

    Algorithm Specific Fields for RSA signatures:

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

    Algorithm Specific Fields for DSA signatures:

      - MPI of DSA value r.

      - MPI of DSA value s.

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

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    With RSA signatures, the hash value is encoded as described in
    PKCS#1 section 9.2.1 of RFC 3447 encoded using PKCS#1 encoding type
    EMSA-PKCS1-v1_5 as described in section 12.1 of RFC 3447. This
    requires inserting the hash value as an octet string into an ASN.1
    structure. The object identifier for the type of hash being used is
    included in the structure. The hexadecimal representations for the
    currently defined hash algorithms are:

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

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

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

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

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

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

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

    The ASN.1 OIDs are:

      - MD5:        1.2.840.113549.2.5

      - RIPEMD-160: 1.3.36.3.2.1

      - SHA-1:      1.3.14.3.2.26

      - SHA224:     2.16.840.1.101.3.4.2.4

      - SHA256:     2.16.840.1.101.3.4.2.1

      - SHA384:     2.16.840.1.101.3.4.2.2

      - SHA512:     2.16.840.1.101.3.4.2.3

    The full hash prefixes for these are:

        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




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

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

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

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

    DSA signatures MUST use hashes that are equal in size to the number
    of bits of q, the group generated by the DSA key's generator value.
    If the output size of the chosen hash is larger than the number of
    bits of q, the hash result is truncated to fit by taking the number
    of leftmost bits equal to the number of bits of q. This (possibly
    truncated) hash function result is treated as a number and used
    directly in the DSA signature algorithm.

5.2.3. Version 4 Signature Packet Format

    The body of a version 4 Signature Packet contains:

      - One-octet version number (4).

      - One-octet signature type.

      - One-octet public key algorithm.

      - One-octet hash algorithm.

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

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

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

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




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

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

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

    There are two fields consisting of signature subpackets. The first
    field is hashed with the rest of the signature data, while the
    second is unhashed. The second set of subpackets is not
    cryptographically protected by the signature and should include only
    advisory information.

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

5.2.3.1. Signature Subpacket Specification

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

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

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

      - the subpacket type (1 octet)

    and is followed by the subpacket specific data.

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

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

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




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

    The value of the subpacket type octet may be:

        0 = reserved
        1 = reserved
        2 = signature creation time
        3 = signature expiration time
        4 = exportable certification
        5 = trust signature
        6 = regular expression
        7 = revocable
        8 = reserved
        9 = key expiration time
        10 = placeholder for backward compatibility
        11 = preferred symmetric algorithms
        12 = revocation key
        13 = reserved
        14 = reserved
        15 = reserved
        16 = issuer key ID
        17 = reserved
        18 = reserved
        19 = reserved
        20 = notation data
        21 = preferred hash algorithms
        22 = preferred compression algorithms
        23 = key server preferences
        24 = preferred key server
        25 = primary User ID
        26 = policy URI
        27 = key flags
        28 = signer's User ID
        29 = reason for revocation
        30 = features
        31 = signature target
        32 = embedded signature

    100 to 110 = private or experimental

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

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



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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" and the "reason for
    revocation" subpackets. Note, however, that if an implementation
    chooses not to implement some of the preferences, it is required to
    behave in a polite manner to respect the wishes of those users who
    do implement these preferences.

5.2.3.2. Signature Subpacket Types

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

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

5.2.3.3. Notes on Self-Signatures

    A self-signature is a binding signature made by the key the
    signature refers to. There are three types of self-signatures, the
    certification signatures (types 0x10-0x13), the direct-key signature
    (type 0x1f), and the subkey binding signature (type 0x18). For
    certification self-signatures, each User ID may have a
    self-signature, and thus different subpackets in those
    self-signatures. For subkey binding signatures, each subkey in fact
    has a self-signature. Subpackets that appear in a certification
    self-signature apply to the username, and subpackets that appear in
    the subkey self-signature apply to the subkey. Lastly, subpackets on
    the direct-key signature apply to the entire key.

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

    Revoking a self-signature or allowing it to expire has a semantic
    meaning that varies with the signature type. Revoking the
    self-signature on a User ID effectively retires that user name. The

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    self-signature is a statement, "My name X is tied to my signing key
    K" and is corroborated by other users' certifications. If another
    user revokes their certification, they are effectively saying that
    they no longer believe that name and that key are tied together.
    Similarly, if the user themselves revokes their self-signature, it
    means the user no longer goes by that name, no longer has that email
    address, etc. Revoking a binding signature effectively retires that
    subkey. Revoking a direct-key signature cancels that signature.
    Please see the "Reason for Revocation" subpacket below for more
    relevant detail.

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

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

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

5.2.3.4. Signature creation time

    (4 octet time field)

    The time the signature was made.

    MUST be present in the hashed area.

5.2.3.5. Issuer

    (8 octet key ID)

    The OpenPGP key ID of the key issuing the signature.

5.2.3.6. Key expiration time

    (4 octet time field)

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

5.2.3.7. Preferred symmetric algorithms

    (array of one-octet values)


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    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 are in section 9. This is only found on a
    self-signature.

5.2.3.8. Preferred hash algorithms

    (array of one-octet values)

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

5.2.3.9. Preferred compression algorithms

    (array of one-octet values)

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

5.2.3.10. Signature expiration time

    (4 octet time field)

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

5.2.3.11. Exportable Certification

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

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

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

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

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

5.2.3.12. Revocable

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

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

5.2.3.13. Trust signature

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

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

5.2.3.14. Regular expression

    (null-terminated regular expression)

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



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5.2.3.15. Revocation key

    (1 octet of class, 1 octet of PK algorithm ID, 20 octets of
    fingerprint)

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

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

5.2.3.16. Notation Data

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

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

    All undefined flags MUST be zero. Defined flags are:

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

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

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

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


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

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

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

5.2.3.17. Key server preferences

    (N octets of flags)

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

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

    This is found only on a self-signature.

5.2.3.18. Preferred key server

    (String)

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

5.2.3.19. Primary User ID

    (1 octet, Boolean)

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




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

5.2.3.20. Policy URI

    (String)

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

5.2.3.21. Key Flags

    (N octets of flags)

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

        First octet:

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

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

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

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

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

        0x20 - This key may be used for authentication.

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

    Usage notes:

    The flags in this packet may appear in self-signatures or in
    certification signatures. They mean different things depending on
    who is making the statement -- for example, a certification
    signature that has the "sign data" flag is stating that the
    certification is for that use. On the other hand, the
    "communications encryption" flag in a self-signature is stating a
    preference that a given key be used for communications. Note
    however, that it is a thorny issue to determine what is

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    "communications" and what is "storage." This decision is left wholly
    up to the implementation; the authors of this document do not claim
    any special wisdom on the issue, and realize that accepted opinion
    may change.

    The "split key" (0x10) and "group key" (0x80) flags are placed on a
    self-signature only; they are meaningless on a certification
    signature. They SHOULD be placed only on a direct-key signature
    (type 0x1f) or a subkey signature (type 0x18), one that refers to
    the key the flag applies to.

5.2.3.22. Signer's User ID

    (String)

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

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

5.2.3.23. Reason for Revocation

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

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

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

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

    Following the revocation code is a string of octets which gives
    information about the reason for revocation in human-readable form
    (UTF-8). The string may be null, that is, of zero length. The length
    of the subpacket is the length of the reason string plus one.

    An implementation SHOULD implement this subpacket, include it in all
    revocation signatures, and interpret revocations appropriately.
    There are important semantic differences between the reasons, and
    there are thus important reasons for revoking signatures.



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

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

5.2.3.24. Features

    (N octets of flags)

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

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

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

    Defined features are:

        First octet:

        0x01 - Modification Detection (packets 18 and 19)

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

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

5.2.3.25. Signature Target

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

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


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

5.2.3.26. Embedded Signature

    (1 signature packet body)

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

5.2.4. Computing Signatures

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

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

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

    A certification signature (type 0x10 through 0x13) hashes the User
    ID being bound to the key into the hash context after the above
    data. A V3 certification hashes the contents of the User ID or
    attribute packet packet, without any header. A V4 certification
    hashes the constant 0xb4 for User ID certifications or the constant
    0xd1 for User Attribute certifications, followed by a four-octet
    number giving the length of the User ID or User Attribute data, and
    then the User ID or User Attribute data.

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

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

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

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

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

5.2.4.1. Subpacket Hints

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

    Some apparent conflicts may actually make sense -- for example,
    suppose a keyholder has 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 Public-Key Encrypted Session Key packets and/or
    Symmetric-Key Encrypted Session Key packets may precede a
    Symmetrically Encrypted Data Packet that holds an encrypted message.
    The message is encrypted with a session key, and the session key is
    itself encrypted and stored in the Encrypted Session Key packet or
    the Symmetric-Key Encrypted Session Key packet.

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

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    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:

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

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



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      - 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.

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.



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5.5.1.4. Secret Subkey Packet (Tag 7)

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

5.5.2. Public Key Packet Formats

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

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

    A version 3 public key or public subkey packet contains:

      - A one-octet version number (3).

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

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

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

      - A series of multiprecision integers comprising the key material:

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

          - an MPI of RSA public encryption exponent e.

    V3 keys are deprecated. They contain three weaknesses 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, there is an increased
    opportunity for fingerprint collisions. Third, there are weaknesses
    in the MD5 hash algorithm that make developers prefer other
    algorithms. See below for a fuller discussion of key IDs and
    fingerprints.

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

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


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    A version 4 packet contains:

      - A one-octet version number (4).

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

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

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

        Algorithm Specific Fields for RSA public keys:

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

          - MPI of RSA public encryption exponent e.

        Algorithm Specific Fields for DSA public keys:

          - MPI of DSA prime p;

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

          - MPI of DSA group generator g;

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

        Algorithm Specific Fields for Elgamal public keys:

          - MPI of Elgamal prime p;

          - MPI of Elgamal group generator g;

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

5.5.3. Secret Key Packet Formats

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

    The packet contains:

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

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

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

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

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

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

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

        Algorithm Specific Fields for RSA secret keys:

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

        - MPI of RSA secret prime value p.

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

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

        Algorithm Specific Fields for DSA secret keys:

        - MPI of DSA secret exponent x.

        Algorithm Specific Fields for Elgamal secret keys:

        - MPI of Elgamal secret exponent x.

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



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

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

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

5.6. Compressed Data Packet (Tag 8)

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

    The body of this packet consists of:

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

      - The remainder of the packet is compressed data.

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

    ZIP-compressed packets are compressed with raw 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.

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



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5.7. Symmetrically Encrypted Data Packet (Tag 9)

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

    The body of this packet consists of:

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

    The symmetric cipher used may be specified in 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, though this use is deprecated.

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

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

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

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

    An experimental version of PGP used this packet as the Literal
    packet, but no released version of PGP generated Literal packets
    with this tag. With PGP 5.x, this packet has been 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. The
    tag 'u' (0x75) means the same as 't', but also indicates that
    implementation believes that the literal data contains UTF-8 text.

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

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

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

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

      - The remainder of the packet is literal data.

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

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5.10. Trust Packet (Tag 12)

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

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

5.11. User ID Packet (Tag 13)

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

5.12. User Attribute Packet (Tag 17)

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

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

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

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

      - the subpacket type (1 octet)

    and is followed by the subpacket specific data.

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

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5.12.1. The Image Attribute Subpacket

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

    The image attribute subpacket begins with an image header. The first
    two octets of the image header contain the length of the image
    header. Note that unlike other multi-octet numerical values in this
    document, due to an historical accident this value is encoded as a
    little-endian number. The image header length is followed by a
    single octet for the image header version. The only currently
    defined version of the image header is 1, which is a 16 octet image
    header. The first three octets of a version 1 image header are thus
    0x10 0x00 0x01.

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

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

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

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

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

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



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

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

    The body of this packet consists of:

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

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

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

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

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

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

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    Code packet tag and length field of 20 octets.

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

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

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

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

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

        NON-NORMATIVE EXPLANATION

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

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

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

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



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

        Consequently, because it is a modest security system, it has
        modest requirements on the hash function(s) it employs. It does
        not rely on a hash function being collision-free, it relies on a
        hash function being one-way. If a forger, Frank, wishes to send
        Alice a (digitally) unsigned message that says, "I've always
        secretly loved you, signed Bob" it is far easier for him to
        construct a new message than it is to modify anything
        intercepted from Bob. (Note also that if Bob wishes to
        communicate secretly with Alice, but without authentication nor
        identification and with a threat model that includes forgers, he
        has a problem that transcends mere cryptography.)

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

        However, given the present state of hash function cryptanalysis
        and cryptography, it may be desirable to upgrade the MDC system
        to a new hash function. See section 10.5 in the IANA
        considerations for guidance.

5.14. Modification Detection Code Packet (Tag 19)

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

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

    The body of this packet consists of:

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



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

6. Radix-64 Conversions

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

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

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

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

    The checksum with its leading equal sign MAY appear on the first
    line after the Base64 encoded data.

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

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

        #define CRC24_INIT 0xb704ceL
        #define CRC24_POLY 0x1864cfbL

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




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

6.2. Forming ASCII Armor

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

    Concatenating the following data creates ASCII Armor:

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

      - Armor Headers

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

      - The ASCII-Armored data

      - An Armor Checksum

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

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

    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.


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

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

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

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

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

    Note that some transport methods are sensitive to line length. While
    there is a limit of 76 characters for the Radix-64 data (section
    6.3), there is no limit to the length of Armor Headers. Care should
    be taken that the Armor Headers are short enough to survive
    transport. One way to do this is to repeat an Armor Header key
    multiple times with different values for each so that no one line is
    overly long.

    Currently defined Armor Header Keys are:

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

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



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

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

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

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

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

6.3. Encoding Binary in Radix-64

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

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

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    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.

6.4. Decoding Radix-64

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




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    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      =      =

6.6. Example of an ASCII Armored Message

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

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

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

7. Cleartext signature framework

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

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

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

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

7.1. Dash-Escaped Text

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

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

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



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

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

8. Regular Expressions

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

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

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

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

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

9. Constants

    This section describes the constants used in OpenPGP.

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




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

9.1. Public Key Algorithms

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

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

9.2. Symmetric Key Algorithms

        ID           Algorithm
        --           ---------
        0          - Plaintext or unencrypted data
        1          - IDEA [IDEA]
        2          - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
                     168 bit key derived from 192)
        3          - CAST5 (128 bit key, as per RFC 2144)
        4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
        5          - Reserved
        6          - Reserved
        7          - AES with 128-bit key [AES]
        8          - AES with 192-bit key
        9          - AES with 256-bit key
        10         - Twofish with 256-bit key [TWOFISH]
        100 to 110 - Private/Experimental algorithm.

    Implementations MUST implement TripleDES. Implementations SHOULD
    implement AES-128 and CAST5. Implementations that interoperate with
    PGP 2.6 or earlier need to support IDEA, as that is the only
    symmetric cipher those versions use. Implementations MAY implement
    any other algorithm.



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9.3. Compression Algorithms

        ID           Algorithm
        --           ---------
        0          - Uncompressed
        1          - ZIP [RFC 1951]
        2          - ZLIB [RFC 1950]
        3          - BZip2 [BZ2]
        100 to 110 - Private/Experimental algorithm.

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

9.4. Hash Algorithms

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

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

10. IANA Considerations

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

10.1. New String-to-Key specifier types

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



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10.2. New Packets

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

10.2.1. User Attribute Types

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

10.2.1.1. Image Format Subpacket Types

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

10.2.2. New Signature Subpackets

    OpenPGP signatures contain a mechanism for signed (or unsigned) data
    to be added to them for a variety of purposes in the signature
    subpackets as discussed in section 5.2.3.1. This specification
    creates a registry of signature subpacket types. The registry
    includes the signature subpacket type, the name of the subpacket and
    a reference to the defining specification. The initial values for
    this registry can be found in 5.2.3.1. Adding a new signature
    subpacket MUST be done through the IETF CONSENSUS method, as
    described in [RFC2434].

10.2.2.1. Signature Notation Data Subpackets

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

    This specification creates a registry of Signature Notation Data
    types. The registry includes the Signature Notation Data type, the
    name of the Signature Notation Data, its allowed values, and a

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    reference to the defining specification. The initial values for this
    registry can be found in 5.2.3.16. Adding a new Signature Notation
    Data subpacket MUST be done through the EXPERT REVIEW method, as
    described in [RFC2434].

10.2.2.2. Key Server Preference Extensions

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

10.2.2.3. Key Flags Extensions

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

10.2.2.4. Reason For Revocation Extensions

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

10.2.2.5. Implementation Features

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

    Also see section 10.6 for more information about when feature flags
    are needed.

10.2.3. New Packet Versions

    The core OpenPGP packets all have version numbers, and can be

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

10.3. New Algorithms

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

10.3.1. Public Key Algorithms

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

10.3.2. Symmetric Key Algorithms

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

10.3.3. Hash Algorithms

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


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10.3.4. Compression Algorithms

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

11. Packet Composition

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

11.1. Transferable Public Keys

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

      - One Public Key packet

      - Zero or more revocation signatures

      - One or more User ID packets

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

      - Zero or more User Attribute packets

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

      - Zero or more Subkey packets

      - After each Subkey packet, one signature packet, plus optionally
        a revocation.

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

    Immediately following each User ID packet, there are zero or more
    signature packets. Each signature packet is calculated on the
    immediately preceding User ID packet and the initial Public Key
    packet. The signature serves to certify the corresponding public key

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    and User ID. In effect, the signer is testifying to his or her
    belief that this public key belongs to the user identified by this
    User ID.

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

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

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

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

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

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

11.2. Transferable Secret Keys

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

11.3. OpenPGP Messages

    An OpenPGP message is a packet or sequence of packets that
    corresponds to the following grammatical rules (comma represents

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    sequential composition, and vertical bar separates alternatives):

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

    Compressed Message :- Compressed Data Packet.

    Literal Message :- Literal Data Packet.

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

    ESK Sequence :- ESK | ESK Sequence, ESK.

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

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

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

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

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

11.4. Detached Signatures

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

12. Enhanced Key Formats

12.1. Key Structures

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

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




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

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

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

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

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

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

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

12.2. Key IDs and Fingerprints

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

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

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

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    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 mod p where x is secret).

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

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

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

13. Notes on Algorithms

13.1. PKCS#1 Encoding In OpenPGP

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

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    needed changes in the conventions.

13.1.1. EME-PKCS1-v1_5-ENCODE

    Input:

    k = the length in octets of the key modulus

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

    Output:

   EM = encoded message, an octet string of length k

    Error:   "message too long"

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

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

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

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

    4.  Output EM.

13.1.2. EME-PKCS1-v1_5-DECODE

    Input:

   EM = encoded message, an octet string

    Output:

    M = message, an octet string

    Error:   "decryption error"

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

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

    If the first octet of EM does not have hexadecimal value 0x00, if
    the second octet of EM does not have hexadecimal value 0x02, if
    there is no octet with hexadecimal value 0x00 to separate PS from M,

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    or if the length of PS is less than 8 octets, output "decryption
    error" and stop. See also the security note in section 13 regarding
    differences in reporting between a decryption error and a padding
    error.

13.1.3. EMSA-PKCS1-v1_5

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

    Option:

   Hash hash function (hLen denotes the length in octets of the hash
        function output)

    Input:

    M = message to be encoded

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

    Output:

   EM = encoded message, an octet string of length emLen

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

    Steps:

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

        H = Hash(M).

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

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

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

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



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

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

     6. Output EM.

13.2. Symmetric Algorithm Preferences

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

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

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

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

13.3. Other Algorithm Preferences

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

13.3.1. Compression Preferences

    Compression has been an integral part of PGP since its first days.

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

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

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

13.3.2. Hash Algorithm Preferences

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

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

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

13.4. Plaintext

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

13.5. RSA

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

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

13.6. DSA

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

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

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

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

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

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

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

13.7. Elgamal

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

13.8. Reserved Algorithm Numbers

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



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

13.9. OpenPGP CFB mode

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

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

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

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

    Step by step, here is the procedure:

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

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

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

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

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

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


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

    8.  FR is encrypted to produce FRE.

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

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

   11.  FR is encrypted to produce FRE.

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

13.10. Private or Experimental Parameters

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

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

13.11. Extension of the MDC System

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

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

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

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

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

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

13.12. Meta-Considerations for Expansion

    If OpenPGP is extended in a way that is not backwards-compatible,
    meaning that old implementations will not gracefully handle their
    absence of a new feature, the extension proposal can be declared in
    the key holder's self-signature as part of the Features signature
    subpacket.

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

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

14. Security Considerations

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

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

      * Certain operations in this specification involve the use of
        random numbers. An appropriate entropy source should be used to
        generate these numbers. See RFC 4086.




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

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

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

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

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

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


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

        For example, suppose Alice DSA signs message M using hash
        algorithm H. Suppose that Mallet finds a message M' that has the
        same hash value as M with H'. Mallet can then construct a

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        signature block that verifies as Alice's signature of M' with
        H'. However, this would also constitute a weakness in either H
        or H' or both. Should this ever occur, a revision will have to
        be made to this document to revise the allowed hash algorithms.

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

15. Implementation Nits

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

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

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

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

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

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

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

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

16. Authors' Addresses

    The working group can be contacted via the current chair:

        Derek Atkins
        IHTFP Consulting, Inc.
        6 Farragut Ave
        Somerville, MA  02144  USA
        Email: derek@ihtfp.com
        Tel: +1 617 623 3745

    The principal authors of this draft are:




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        Jon Callas
        Email: jon@callas.org

        Lutz Donnerhacke
        IKS GmbH
        Wildenbruchstr. 15
        07745 Jena, Germany

        EMail: lutz@iks-jena.de

        Hal Finney
        Email: hal@finney.org

        David Shaw
        Email: dshaw@jabberwocky.com

        Rodney Thayer
        Email: rodney@canola-jones.com

    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, Ben
    Laurie, Raph Levien, Colin Plumb, Will Price, David Shaw, William
    Stallings, Mark Weaver, and Philip R. Zimmermann.

17. References (Normative)


    [AES]            NIST, FIPS PUB 197, "Advanced Encryption Standard
                             (AES)," November 2001.

http://csrc.nist.gov/publications/fips/fips197/
                             fips-197.{ps,pdf}

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

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

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

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

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

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

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

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

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

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

    [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Level", BCP 14, RFC 2119, March 1997.
    [RFC2045]        Borenstein, N. and N. Freed, "Multipurpose Internet
                     Mail Extensions (MIME) Part One: Format of Internet
                     Message Bodies.", RFC 2045, November 1996.

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

    [RFC2434]        Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 2434, October 1998.
    [RFC2822]        Resnick, P., "Internet Message Format", RFC 2822.

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

    [RFC3447]        B. Kaliski and J. Staddon, "PKCS #1: RSA
                     Cryptography Specifications Version 2.1",
                     RFC 3447, February 2003.


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

    [RFC4086]        Eastlake, D., Crocker, S. and J. Schiller,
                     "Randomness Recommendations for Security", RFC
                     4086, June 2005.

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

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


18. References (Informative)


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

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

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

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

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

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

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

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


19. Full Copyright Statement

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