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Versions: 00 01 02 03 04 05 06 07 RFC 2440

Network Working Group                                        Jon Callas
Category: INTERNET-DRAFT                            Pretty Good Privacy
draft-ietf-openpgp-formats-00.txt                      Lutz Donnerhacke
Expires May 1998                     IN-Root-CA Individual Network e.V.
November 1997                                                Hal Finney
                                                    Pretty Good Privacy
                                                          Rodney Thayer
                                                       Sable Technology

                  OP Formats - OpenPGP Message Format
                   draft-ietf-openpgp-formats-00.txt

Copyright 1997 by The Internet Society.  All Rights Reserved.

Status of this Memo

This document is an Internet-Draft.  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."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).

Abstract

This document is maintained in order to publish all necessary
information needed to develop interoperable applications based on the
OP 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 storing and implementation questions albeit it is
necessary to avoid security flaws.

OP (Open-PGP) software uses a combination of strong public-key and
conventional 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 OP.







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Table of Contents

1. Introduction
1.1 Terms
2. General functions
2.1 Confidentiality
2.2 Digital signature
2.3 Compression
2.4 Conversion to Radix-64
2.4.1 Forming ASCII Armor
2.4.2 Encoding Binary in Radix-64
2.4.3 Decoding Radix-64
2.4.4 Examples of Radix-64
2.5 Example of an ASCII Armored Message
2.6 Cleartext signature framework
3.0 Data Element Formats
3.1 Scalar numbers
3.2 Multi-Precision Integers
3.3 Counted Strings
3.4 Time fields
3.5 String-to-key (S2K) specifiers
3.5.1 String-to-key (S2k) specifier types
3.5.1.1 Simple S2K
3.5.1.2 Salted S2K
3.5.1.3 Iterated and Salted S2K
3.5.2 String-to-key usage
3.5.2.1 Secret key encryption
3.5.2.2 Conventional message encryption
3.5.3 String-to-key algorithms
3.5.3.1 Simple S2K algorithm
3.5.3.2 Salted S2K algorithm
3.5.3.3 Iterated-Salted S2K algorithm
4.0 Packet Syntax
4.1 Overview
4.2 Packet Headers
4.3 Packet Tags
5.0 Packet Types
5.1 Encrypted Session Key Packets (Tag 1)
5.2 Signature Packet (Tag 2)
5.2.1 Version 3 Signature Packet Format
5.2.2 Version 4 Signature Packet Format
5.2.2.1 Signature Subpacket Specification
5.2.2.2 Signature Subpacket Types
5.2.3 Signature Types
5.3 Conventional Encrypted Session-Key Packets (Tag 3)
5.4 One-Pass Signature Packets (Tag 4)
5.5 Key Material Packet
5.5.1 Key Packet Variants
5.5.1.1 Public Key Packet (Tag 6)
5.5.1.2 Public Subkey Packet (Tag 14)
5.5.1.3 Secret Key Packet (Tag 5)
5.5.1.4 Secret Subkey Packet (Tag 7)
5.5.2 Public Key Packet Formats


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5.5.3 Secret Key Packet Formats
5.6 Compressed Data Packet (Tag 8)
5.7 Symmetrically Encrypted Data Packet (Tag 9)
5.8 Marker Packet (Obsolete Literal Packet) (Tag 10)
5.9 Literal Data Packet (Tag 11)
5.10 Trust Packet (Tag 12)
5.11 User ID Packet (Tag 13)
5.12 Comment Packet (Tag 16)
6. Constants
6.1 Public Key Algorithms
6.2 Symmetric Key Algorithms
6.3 Compression Algorithms
6.4 Hash Algorithms
7. Packet Composition
7.1 Transferable Public Keys
7.2 OP Messages
8. Enhanced Key Formats
8.1 Key Structures
8.4  V4 Key IDs and Fingerprints
9. Security Considerations
10. Authors and Working Group Chair
11. References
12. Full Copyright Statement

1.  Introduction

This document provides information on the message-exchange packet
formats used by OP to provide encryption, decryption, signing, key
management and functions.  It builds on the foundation provided RFC
1991 "PGP Message Exchange Formats" [1].

1.1 Terms

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

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

PGP 2.6.x - This version of PGP has many variants, hence the term PGP
2.6.x.  It used only RSA and IDEA for its cryptography.

PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
community and also in the predecessor of this document, RFC1991.  It
has new formats and corrects a number of problems in the PGP 2.6.x.  It
is referred to here as PGP 5.x because that software was the first
release of the "PGP 3" code base.

"PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
Pretty Good Privacy, Inc.


2.  General functions


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

-digital signature -encryption -compression -radix-64 conversion

In addition, OP provides key management and certificate services.

2.1 Confidentiality via Encryption

OP offers two encryption options to provide confidentiality:
conventional (symmetric-key) encryption and public key encryption.
With public-key encryption, the message is actually encrypted using a
conventional encryption algorithm.  In this mode, each conventional key
is used only once.  That is, a new key is generated as a random number
for each message.  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 OP 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 OP 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 OP decrypts the session key using the recipient's
        private key.
  6. The receiving OP decrypts the message using the session key.
        If the message was compressed, it will be decompressed.

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 conventional session key.  Finally, the session key is
encrypted using public-key encryption and prepended to the encrypted
block.

2.2 Authentication via Digital signature

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

  1. The sender creates a message.
  2. The sending software generates a hash code of the message
  3. The sending software generates a signature from the hash code using
     the sender's private key.
  4. The binary signature is attached to the message.
  5. The receiving software keeps a copy of the message signature.
  6. The receiving software generates a new hash code for the received
     message and verifies it using the message's signature. If the


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     verification is successful, the message is accepted as authentic.

2.3 Compression

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

2.4 Conversion to Radix-64

OP'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 OP's native raw binary octets through email channels,
a printable encoding of these binary octets is needed.  OP 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.

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

OP's Radix-64 encoding is composed of two parts: a base64 encoding of
the binary data, and a checksum.  The base64 encoding is identical to
the MIME base64 content-transfer-encoding [RFC 2045, Section 6.8].  An
OP implementation MAY use ASCII Armor to protect the raw binary data.

The checksum is a 24-bit CRC converted to four characters of radix-64
encoding by the same MIME base64 transformation, preceded by an equals
sign (=).  The CRC is computed by using the generator 0x864CFB and an
initialization of 0xB704CE.  The accumulation is done on the data
before it is converted to radix-64, rather than on the converted data.
(For more information on CRC functions, see chapter 19 of [CAMPBELL].)

{{Editor's note:  This is old text, dating back to RFC 1991.  I have
never liked the glib way the CRC has been dismissed, but I also know
that this is no place to start a discussion of CRC theory.  Should we
construct a sample implementation in C and put it in an appendix? --
jdcc}}

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.

2.4.1 Forming ASCII Armor

When OP encodes data into ASCII Armor, it puts specific headers around
the data, so OP can reconstruct the data later.  OP informs the user
what kind of data is encoded in the ASCII armor through the use of the


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headers.

Concatenating the following data creates ASCII Armor:

- An Armor Header Line, appropriate for the type of data - Armor
Headers - A blank (zero-length, or containing only whitespace) line -
The ASCII-Armored data - An Armor Checksum - The Armor Tail, which
depends on the Armor Header Line.

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

BEGIN PGP MESSAGE used for signed, encrypted, or compressed files

BEGIN PGP PUBLIC KEY BLOCK used for armoring public keys

BEGIN PGP PRIVATE KEY BLOCK used for armoring private keys

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

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

BEGIN PGP SIGNATURE used for detached signatures, OP/MIME signatures,
and signatures following clearsigned messages

The Armor Headers are pairs of strings that can give the user or the
receiving OP message block 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.  OP
should consider improperly formatted Armor Headers to be corruption of
the ASCII Armor.  Unknown keys should be reported to the user, but OP
should continue to process the message.  Currently defined Armor Header
Keys include "Version" and "Comment", which define the OP Version used
to encode the message and a user-defined comment.

The "MessageID" Armor Header specifies 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 chosen with enough internal randomness that no two
messages would have the same MessageID string.

The MessageID should not appear unless it is in a multi-part message.
If it appears at all, it should be computed from the message in a


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deterministic fashion, rather than contain a purely random value.  This
is to allow anyone to determine that the MessageID cannot serve as a
covert means of leaking cryptographic key information.

{{Editor's note:  This needs to be cleaned up, with a table of the
defined headers.  Also, the MessageID description is too vague about
how random the id has to be.}}

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."

2.4.2 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 byte, and the eighth bit will be the low-order bit in the
first 8-bit byte, and so on.

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

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

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



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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:

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

- 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.

- 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.

2.4.3 Decoding Radix-64

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

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

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


2.4.4 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     63
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 =


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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      =      =


2.5 Example of an ASCII Armored Message

  -----BEGIN PGP MESSAGE-----
  Version: OP V0.0

  owFbx8DAYFTCWlySkpkHZDKEFCXmFedmFhdn5ucpZKdWFiv4hgaHKPj5hygUpSbn
  l6UWpabo8XIBAA==
  =3m1o
  -----END PGP MESSAGE-----

Note that this example is indented by two spaces.

2.6 Cleartext signature framework

Sometimes it is necessary to sign a textual octet stream without ASCII
armoring the stream itself, so the signed text is still readable
without special software.  In order to bind a signature to such a
cleartext, this framework is used. (Note that RFC 2015 defines another
way to clear sign messages for environments that support MIME.)

The cleartext signed message consists of:
  - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
    single line,
  - Zero or more "Hash" Armor Headers (3.1.2.4),
  - 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 Armor Header and Armor
    Tail Lines.

If the "Hash" armor header is given, the specified message digest
algorithm is used for the signature.  If this header is missing, SHA-1
is assumed.  If more than one message digest is used in the signature,
the "Hash" armor header contains a comma-delimited list of used message
digests.  As an abbreviation, the "Hash" armor header may be placed on
the cleartext header line, inserting a comma after the word 'MESSAGE',
as follows:

'-----BEGIN PGP SIGNED MESSAGE, Hash:  MD5, SHA1'.

{{Editor's note:  Should the above armor header line stay or go?
There's no reason that the "Hash:" armor header can't have multiple


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hashes in it.  I think anything that reduces parsing complexity is a
Good Thing. --jdcc}}

Current message digest names are:

        - "SHA1"
        - "MD5"
        - "RIPEMD160"

Dash escaped cleartext is the ordinary cleartext where every line
starting with a dash '-' (0x2D) is prepended by the sequence dash '-'
(0x2D) and space ' ' (0x20).  This prevents the parser from recognizing
armor headers of the cleartext itself.  The message digest is computed
using the cleartext itself, not the dash escaped form.

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

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

3.  Data Element Formats

This section describes the data elements used by OP.

3.1 Scalar numbers

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

3.2 Multi-Precision Integers

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

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

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

Examples:

(all numbers are in hexadecimal)



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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.

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


3.3 Counted Strings

A counted string consists of a length and then N octets of string data.
Its default character set is UTF-8 [RFC2044] encoding of Unicode
[ISO10646].

3.4 Time fields

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

3.5 String-to-key (S2K) specifiers

String-to-key (S2K) specifiers are used to convert passphrase strings
into conventional 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
conventionally encrypted messages.

3.5.1 String-to-key (S2k) specifier types

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

3.5.1.1 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

3.5.1.2 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


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3.5.1.3 Iterated and Salted S2K

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

        Octet 0:                0x03
        Octet 1:                hash algorithm
        Octets 2-9:             8-octet salt value
        Octet 10:               count, in special format (described below)

3.5.2 String-to-key usage

Implementations MUST implement simple S2K and salted S2K specifiers.
Implementations MAY implement iterated and salted S2K specifiers.
Implementations SHOULD use salted S2K specifiers, as simple S2K
specifiers are more vulnerable to dictionary attacks.

3.5.2.1 Secret key encryption

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

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

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

        0               secret data is unencrypted (no pass phrase)
        255             followed by algorithm octet and S2K specifier
        Cipher alg      use Simple S2K algorithm using MD5 hash

This last possibility, the cipher algorithm number with an implicit use
of MD5 is provided for backward compatibility; it should be understood,
but not generated.

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

3.5.2.2 Conventional message encryption

PGP 2.X always used IDEA with Simple string-to-key conversion when


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conventionally encrypting a message.  PGP 5 can create a Conventional
Encrypted Session Key packet at the front of a message.  This can be
used to allow S2K specifiers to be used for the passphrase conversion,
to allow other ciphers than IDEA to be used, or to create messages with
a mix of conventional ESKs and public key ESKs.  This allows a message
to be decrypted either with a passphrase or a public key.

3.5.3 String-to-key algorithms

3.5.3.1 Simple S2K algorithm

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

3.5.3.2 Salted S2K algorithm

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.

3.5.3.3 Iterated-Salted S2K algorithm

{{Editor's note:  This is very complex, with bizarre things like an
8-bit floating point format.  Should we just drop it? --jdcc}}

Iterated-Salted S2K hashes the passphrase and salt data multiple times.
The total number of octets to be hashed is encoded in the count octet
that follows the salt in the S2K specifier.  The count value is stored
as a normalized floating-point value with 4 bits of exponent and 4 bits
of mantissa.  The formula to convert from the count octet to a count of
the number of octets to be hashed is as follows, letting the high 4
bits of the count octet be CEXP and the low four bits be CMANT:

        count of octets to be hashed = (16 + CMANT) << (CEXP + 6)

This allows encoding hash counts as low as 16 << 6 or 1024 (using an


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octet value of 0), and as high as 31 << 21 or 65011712 (using an octet
value of 0xff).  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.

4.  Packet Syntax

This section describes the packets used by OP.

4.1 Overview

An OP 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 OP message, keyring, certificate, and
so forth consists of a number of packets.  Some of those packets may
contain other OP packets (for example, a compressed data packet, when
uncompressed, contains OP packets).

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

4.2 Packet Headers

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

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

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


PGP 2.6.X only uses old format packets.  Thus, software that
interoperates with those versions of PGP must only use old format
packets.  If interoperability is not an issue, either format may be
used.

Old format packets contain:
  Bits 5-2 -- content tag


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  Bits 1-0 - length-type

New format packets contain:
  Bits 5-0 -- content tag

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 byte long,
and the application 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 application should not use indeterminate
length packets except where the end of the data will be clear from the
context.

New format packets have three possible ways of encoding length.  A
one-octet Body Length header encodes packet lengths of up to 191
octets, and a two-octet Body Length header encodes packet lengths of
192 to 8383 octets.  For cases where longer packet body lengths are
needed, or where the length of the packet body is not known in advance
by the issuer, Partial Body Length headers can be used.  These are
one-octet length headers that encode the length of only part of the
data packet.

Each Partial Body Length header is followed by a portion of the packet
body data.  The Partial Body Length header specifies this portion's
length.  Another length header (of one of the three types) follows that
portion.  The last length header in the packet must always be a regular
Body Length header.  Partial Body Length headers may only be used for
the non-final parts of the packet.

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 = length_octet;

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) * 256 + (2nd_octet) + 192

A Partial Body Length header is one octet long and encodes a length
which is a power of 2, from 1 to 2147483648 (2 to the 31st power).  It
is recognized because its one octet value is greater than or equal to
224.  The partial body length is equal to:



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

Examples:

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


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 packets can only have tags less than 16, whereas new format
packets can have tags as great as 63.  The defined tags (in decimal)
are:

0        -- Reserved. A packet must not have a tag with this value.
1        -- Encrypted Session Key Packet
2        -- Signature Packet
3        -- Conventionally 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       -- Name Packet
14       -- Subkey Packet
15       -- Reserved
16       -- Comment Packet
60 to 63 -- Private or Experimental Values

5.  Packet Types

5.1 Encrypted Session Key Packets (Tag 1)



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An Encrypted Session Key packet holds the key used to encrypt a message
that is itself encrypted with a public key.  Zero or more Encrypted
Session Key packets and/or Conventional Encrypted Session Key packets
may precede a Symmetrically Encrypted Data Packet, which 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 Conventional Encrypted Session Key packet.  The
Symmetrically Encrypted Data Packet is preceded by one Encrypted
Session Key packet for each OP key to which the message is encrypted.
The recipient of the message finds a session key that is encrypted to
their public key, decrypts the session key, and then uses the session
key to decrypt the message.

The body of this packet consists of:

        - A one-octet number giving the version number of the packet type.
          The currently defined value for packet version is 3. An
        implementation should accept, but not generate a version of 2,
        which is equivalent to V3 in all other respects.
        - An eight-octet number that gives the key ID of the public key that
          the session key is encrypted to.
        - A one-octet number giving the public key algorithm used.
        - A string of octets that is the encrypted session key. This
          string takes up the remainder of the packet, and its contents are
          dependent on the public key algorithm used.

    Algorithm Specific Fields for RSA encryption
        - multiprecision integer (MPI) of RSA encrypted value m**e.

    Algorithm Specific Fields for Elgamal encryption:
        - MPI of DSA value g**k.
        - MPI of DSA value m * y**k.

The encrypted value "m" in the above formulas is derived from the
session key as follows.  First the session key is prepended with a
one-octet algorithm identifier that specifies the conventional
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 octets, including the algorithm
identifier and session key, modulo 65536.  This value is then padded as
described in PKCS-1 block type 02 [PKCS1] to form the "m" value used in
the formulas above.

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.


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Implementations MUST accept V3 signatures.  Implementations SHOULD
generate V4 signatures, unless there is a need to generate a signature
that can be verified by PGP 2.6.x.

5.2.1 Version 3 Signature Packet Format

A version 3 Signature packet contains:
        - One-octet version number (3).
        - One-octet length of following hashed material.  MUST be 5.
        - One-octet signature type.
        - Four-octet creation time.
        - Eight-octet key ID of signer.
        - One-octet public key algorithm.
        - One-octet hash algorithm.
        - Two-octet field holding left 16 bits of signed hash value.
        - One or more multi-precision integers comprising the signature.
          This portion is algorithm specific, as described below.

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

    Algorithm Specific Fields for RSA signatures:
        - multiprecision integer (MPI) of RSA signature value m**d.

    Algorithm Specific Fields for DSA signatures:
        - MPI of DSA value r.
        - MPI of DSA value s.

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

With RSA signatures, the hash value is encoded as described in PKCS-1
section 10.1.2, "Data encoding", producing an ASN.1 value of type
DigestInfo, and then padded using PKCS-1 block type 01 [PKCS1].  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:

        - SHA-1:            0x2b, 0x0e, 0x03, 0x02, 0x1a
        - MD5:              0x2a, 0x86, 0x48, 0x86, 0xf7, 0x0d, 0x02, 0x05
        - RIPEMD-160:   0x2b, 0x24, 0x03, 0x02, 0x01

The ASN.1 OIDs are:
        - MD5:       1.2.840.113549.2.5
        - SHA-1:     1.3.14.3.2.26
        - RIPEMD160: 1.3.36.3.2.1


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DSA signatures SHOULD use hashes with a size of 160 bits, to match q,
the size of the group generated by the DSA key's generator value.  The
hash function result is treated as a 160 bit number and used directly
in the DSA signature algorithm.

5.2.2 Version 4 Signature Packet Format

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 octet count for following hashed subpacket data.
        - Hashed subpacket data.
        - Two-octet octet count for following unhashed subpacket data.
        - Unhashed subpacket data.
        - Two-octet field holding left 16 bits of signed hash value.
        - One or more multi-precision integers comprising the signature.
          This portion is algorithm specific, as described above.

The data being signed is hashed, and then the signature data from the
version number through the hashed subpacket data 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 above.

5.2.2.1 Signature Subpacket Specification

The subpacket fields consist of zero or more signature subpackets.
Each set of subpackets is preceded by a two-octet count of the length
of the set of subpackets.

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

        - subpacket length (1 or 2 octets):
          Length includes the type octet but not this length,
          1st octet <  192, then length is octet value
          1st octet >= 192, then length is 2 octets and equal to
            (1st octet - 192) * 256 + (2nd octet) + 192
        - subpacket type (1 octet):
          If bit 7 is set, subpacket understanding is critical,


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           2 = signature creation time,
           3 = signature expiration time,
           4 = exportable,
           5 = trust signature,
           6 = regular expression,
           7 = revocable,
           9 = key expiration time,
          10 = additional recipient request,
          11 = preferred symmetric algorithms,
          12 = revocation key,
          16 = issuer key ID,
      20 = notation data,
      21 = preferred hash algorithms,
      22 = preferred compression algorithms,
      23 = key server preferences,
      24 = preferred key server

        - subpacket specific data:

Bit 7 of the subpacket type is the "critical" bit.  If set, it implies
that it is critical that the subpacket be one which is understood by
the software.  If a subpacket is encountered which is marked critical
but the software does not understand, the handling depends on the
relationship between the issuing key and the key that is signed.  If
the signature is a valid self-signature (for which the issuer is the
key that is being signed, either directly or via a username binding),
then the key should not be used.  In other cases, the signature
containing the critical subpacket should be ignored.

5.2.2.2 Signature Subpacket Types

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

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

The descriptions below describe whether a subpacket is typically found
in the hashed or unhashed subpacket sections.  If a subpacket is not
hashed, then it cannot be trusted.

    Signature creation time (4 octet time field) (Hashed)


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The time the signature was made.  Always included with new signatures.

    Issuer (8 octet key ID) (Non-hashed)

The OP key ID of the key issuing the signature.

    Key expiration time (4 octet time field) (Hashed)

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.

    Preferred symmetric algorithms (array of one-octet values) (Hashed)

Symmetric algorithm numbers that indicate which algorithms the key
holder prefers to use.  This is an ordered list of octets with the most
preferred listed first.  It should be assumed that only algorithms
listed are supported by the recipient's software.  Algorithm numbers in
section 6. This is only found on a self-signature.

    Preferred hash algorithms (array of one-octet values) (Hashed)

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

{{Editor's note:  The above preference (hash algs) is controversial.  I
included it in for symmetry, because if someone wants to build a
minimal OP implementation, there needs to be a way to tell someone that
you won't be able to verify a signature unless it's made with some set
of algorithms.  It also permits to prefer DSA with RIPEMD-160, for
example. If you have an opinion, please state it.}}

    Preferred compression algorithms (array of one-octet values)
        (Hashed)

Compression algorithm numbers that indicate which algorithms the key
holder prefers to use.  Like the preferred symmetric algorithms, the
list is ordered.  Algorithm numbers are in section 6.  If this
subpacket is not included, ZIP is preferred. A zero denotes that no
compression is preferred; the key holder's software may not have
compression software. This is only found on a self-signature.

    Signature expiration time (4 octet time field) (Hashed)

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.

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


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               (Hashed)

Signature's exportability status.  Packet body contains a boolean flag
indicating whether the signature is exportable. Signatures which are
not exportable are ignored during export and import operations.  If
this packet is not present the signature is assumed to be exportable.

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

Signature's revocability status.  Packet body contains a boolean flag
indicating whether the signature is revocable.  Signatures which are
not revocable get 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 assumed to be revocable.

    Trust signature (1 octet of "level" (depth), 1 octet of trust amount)
     (Hashed)
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.

    Regular expression (null-terminated regular expression) (Hashed)

Used in conjunction with trust signature packets (of level > 0) to
limit the scope of trust which is extended.  Only signatures by the
target key on user IDs which match the regular expression in the body
of this packet have trust extended by the trust packet.

    Additional recipient request (1 octet of class, 1 octet of algid,
                                  20 octets of fingerprint) (Hashed)

Key holder requests encryption to additional recipient when data is
encrypted to this username.  If the class octet contains 0x80, then the
key holder strongly requests that the additional recipient be added to
an encryption.  Implementing software may treat this subpacket in any
way it sees fit. This is found only on a self-signature.

    Revocation key (1 octet of class, 1 octet of algid, 20 octets of
                      fingerprint) (Hashed)

Authorizes the specified key to issue revocation self-signatures on
this key.  Class octet must have bit 0x80 set, other bits are for


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future expansion to other kinds of signature authorizations. This is
found on a self-signature.

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

This subpacket describes a "notation" on the signature that the issuer
wishes to make.  The notation has a name and a value, each of which are
strings of octets.  There may be more than one notation in a signature.
Notations can be used for any extension the issuer of the signature
cares to make.  The "flags" field holds four octets of flags. All
undefined flags MUST be zero.  Defined flags are:
        First octet: 0x80 = human-readable. This note is text, a note
                            from one person to another, and has no
                            meaning to software.
        Other octets: none.

    Key server preferences (N octets of flags) (Hashed)

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

       First octet: 0x80 = No-modify -- the key holder requests that
                           this key only be modified or updated by the
                           key holder or an authorized administrator of
                           the key server.
This is found only on a self-signature.

    Preferred key server (String) (Hashed)

This is a URL of a key server that the key holder prefers be used for
updates.  Note that keys with multiple user names can have a preferred
key server for each user name. This is found only on a self-signature.

Implementations SHOULD implement a "preference" and MAY implement a
"request."

{{Editor's note:  None of the preferences have a way to specify a
negative preference (for example, I like Triple-DES, don't use
algorithm X).  Tacitly, the absence of an algorithm from a set is a
negative preference, but should there be an explicit way to give a
negative preference? -jdcc}}

{{Editor's note:  A missing feature is to invalidate (or revoke) a user
id, rather than the entire key.  Lots of people want this, and many
people have keys cluttered with old work email addresses.  There is
another related issue, that that is with key rollover -- suppose I'm
retiring an old key, but I don't want to have to lose all my
certification signatures.  It would be nice if there were a way for a
key to transfer itself to a new one.  Lastly, if either (or both) of
these is desirable, do we handle them with a new signature type, or


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with notations, which are an extension mechanism.  I think that it
makes sense to make a revocation type (because it's analogous to the
other forms of revocation), but rollover might be best implemented as
an extension. --jdcc}}

{{Editor's note:  PGP 3 designed, but never implemented a number of
other subpacket types.  They were:  A signature version number; A set
of key usage flags (signing key, encryption key for communication, and
encryption key for storage); User ID of the signer; Policy URL; net
location of the key.

Some of these options are things the WG has talked about as being a
Good Thing -- like flags denoting if a key is a comm key or a storage
key.  My design of such a feature would be different than the other
one, though. I think it would be a great idea to have a URL that's a
location to find the key, so people who prefer to have a web, ftp, or
finger location can use those.  However, some of them (like a URL) are
also perfect for designing in with extensions.  After all, we only have
128 subpacket constants.

--jdcc}}

5.2.3 Signature Types

There are a number of possible meanings for a signature, which are
specified in a signature type octet in any given signature.  These
meanings are:

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

        - 0x01: Signature of a canonical text document.
Typically, this means the signer owns it, created it, or certifies that
it has not been modified.  The signature will be calculated over the
textual 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.

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

        - 0x11: This is a persona certification of a User ID and
          Public Key packet.
It means that the issuer of this certification has not done any
verification of the claim that the owner of this key is the user ID


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specified.  Note that no released version of PGP has generated this
type of certification.

        - 0x12: This is the casual certification of a User ID and
          Public Key packet.
It means that the issuer of this certification has done some casual
verification of the claim of identity.  Note that no version of PGP has
generated this type of certification, nor is there any definition of
what constitutes a casual certification.

        - 0x13: This is the positive certification of a User ID and
          Public Key packet.
It means that the issuer of this certification has done substantial
verification of the claim of identity.  Note that no version of PGP has
generated this type of certification, nor is there any definition of
what constitutes a positive certification.  Please also note that the
vagueness of these certification systems is not a flaw, but a feature
of the system.  Because PGP places final authority for validity upon
the receiver of a certification, it may be that one authority's casual
certification might be more rigorous than some other authority's
positive certification.

{{Editor's note:  While there is a scale of identification signatures
in the range 0x10 to 0x13, most of them have never been implemented or
used.  Current implementations only use 0x10, the "generic
certification." Should the others be removed?  RFC 1991 went to some
trouble to explain which ones were defined but not implemented, or read
but not generated.  I think we should not do that.  If we define them,
they should be MAY features at the very least.  If we're not going to
use them, they shouldn't be in the spec. --jdcc}}

        - 0x18: This is used for a signature by a signature key to bind a
          subkey which will be used for encryption.
The signature is calculated directly on the subkey itself, not on any
User ID or other packets.

        - 0x20: This signature is used to revoke a key.
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.

        - 0x28: This is used to revoke a subkey.
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 which is bound to this subkey, or by an
authorized revocation key, should be considered.

        - 0x30: This signature revokes an earlier user ID certification
          signature (signature class 0x10 - 0x13).
It should be issued by the same key which issued the revoked signature,
and should have a later creation date.



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    - 0x40: Timestamp signature.

{{Editor's note:  The timestamp signature is left over from RFC 1991,
and has never been fully designed nor implemented.  Is this the sort of
thing best handled by notations? --jdcc}}

{{Editor's note:  It would be nice to have a signature that applied to
the key alone, rather than a key plus a user name.  Perhaps this is
best done with a notation. --jdcc}}

{{Editor's note:  There is presently no way for a key-signer (a.k.a.
certifier) to sign a main key along with a subkey.  There are a number
of useful situations for a set of keys (main plus subkeys) to all be
signed together.  How do we solve this? --jdcc}}

5.3 Conventional Encrypted Session-Key Packets (Tag 3)

The Conventional Encrypted Session Key packet holds the
conventional-cipher encryption of a session key used to encrypt a
message.  Zero or more Encrypted Session Key packets and/or
Conventional 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 Conventional
Encrypted Session Key packet.

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

The body of this packet consists of:
        - A one-octet version number. The only currently defined version is
          4.
        - A one-octet number describing the symmetric algorithm used.
        - A string-to-key (S2K) specifier, length as defined above.
        - Optionally, the encrypted session key itself, which is decrypted
          with the string-to-key object.

If the encrypted session key is not present (which can be detected on
the basis of packet length and S2K specifier size), then the S2K
algorithm applied to the passphrase produces the session key for
decrypting the file, using the symmetric cipher algorithm from the
Conventional 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 conventional encryption algorithm used to encrypt the
following Symmetrically Encrypted Data Packet, followed by the session


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key octets themselves.

Note: because an all-zero IV is used for this decryption, the S2K
specifier MUST use a salt value, either a 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 5.2.3.
        - 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 which describes another
signature to be applied to the same message data.


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 OP key
(sometimes called an OP 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.


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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 backwards compatibility.

5.5.1.3 Secret Key Packet (Tag 5)

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

5.5.1.4 Secret Subkey Packet (Tag 7)

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

5.5.2 Public Key Packet Formats

There are two versions of key-material packets.  Version 3 packets were
first generated PGP 2.6.  Version 2 packets are identical in format to
Version 3 packets, but are generated by PGP 2.5 or before.  PGP 5.0
introduces version 4 packets, with new fields and semantics.  PGP 2.6.X
will not accept key-material packets with versions greater than 3.

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

A version 3 public key or public subkey packet contains:
    - A one-octet version number (3).
    - A four-octet number denoting the time that the key was created.
    - A two-octet number denoting the time in days that this key is
      valid. If this number is zero, then it does not expire.
    - A one-octet number denoting the public key algorithm of this key
    - A series of multi-precision integers comprising the key
      material:
    - a multiprecision integer (MPI) of RSA public modulus n;
    - an MPI of RSA public encryption exponent e.

The fingerprint of the 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.

The eight-octet key ID of the key consists of the low 64 bits of the
public modulus of an RSA key.

Since the release of V3 keys, there have been a number of improvements
desired in the key format.  For example, if the key ID is a function of
the public modulus, it is easy for a person to create a key that has
the same key ID as some existing key.  Similarly, MD5 is no longer the


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preferred hash algorithm, and not hashing the length of an MPI with its
body increases the chances of a fingerprint collision.

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 elsewhere.

A version 4 packet contains:
    - A one-octet version number (4).
    - A four-octet number denoting the time that the key was created.
    - A one-octet number denoting the public key algorithm of this key
    - A series of multi-precision integers comprising the key
      material.  This algorithm-specific portion is:

    Algorithm Specific Fields for RSA public keys:
    - multiprecision integer (MPI) of RSA public modulus n;
    - MPI of RSA public encryption exponent e.

    Algorithm Specific Fields for DSA public keys:
    - MPI of DSA prime p;
    - MPI of DSA group order q (q is a prime divisor of p-1);
    - MPI of DSA group generator g;
    - MPI of DSA public key value y (= g**x where x is secret).

    Algorithm Specific Fields for Elgamal public keys:
    - MPI of Elgamal prime p;
    - MPI of Elgamal group generator g;
    - MPI of Elgamal public key value y (= g**x where x
      is secret).


5.5.3 Secret Key Packet Formats

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

The packet contains:
    - A Public Key or Public Subkey packet, as described above
    - One octet indicating string-to-key usage conventions.  0 indicates
          that the secret key data is not encrypted.  255 indicates that a
          string-to-key specifier is being given.  Any other value
          is a conventional encryption algorithm specifier.
        - [Optional] If string-to-key usage octet was 255, a one-octet
          conventional encryption algorithm.
        - [Optional] If string-to-key usage octet was 255, a string-to-key
          specifier.  The length of the string-to-key specifier is implied
          by its type, as described above.
        - [Optional] If secret data is encrypted, eight-octet Initial Vector
      (IV).
        - Encrypted multi-precision integers comprising the secret key data.
          These algorithm-specific fields are as described below.


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

    Algorithm Specific Fields for RSA secret keys:
        - multiprecision integer (MPI) of RSA secret exponent d.
        - MPI of RSA secret prime value p.
        - MPI of RSA secret prime value q (p < q).
        - MPI of u, the multiplicative inverse of p, mod q.

    Algorithm Specific Fields for DSA secret keys:
        - MPI of DSA secret exponent x.

    Algorithm Specific Fields for Elgamal secret keys:
        - MPI of Elgamal secret exponent x.

Secret MPI values can be encrypted using a passphrase.  If a
string-to-key specifier is given, that describes the algorithm for
converting the passphrase to a key, else a simple MD5 hash of the
passphrase is used.  Implementations SHOULD use a string-to-key
specifier; the simple hash is for backwards compatibility.  The cipher
for encrypting the MPIs is specified in the secret key packet.

Encryption/decryption of the secret data is done in CFB mode using the
key created from the passphrase and the Initial Vector from the packet.
A different mode is used with RSA keys than with other key formats.
With RSA 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 non-RSA keys, a simpler method is used.  All secret MPI values are
encrypted in CFB mode, including the MPI bitcount prefix.

The 16-bit checksum that follows the algorithm-specific portion is the
algebraic sum, mod 65536, of the plaintext of all the
algorithm-specific octets (including MPI prefix and data).  With RSA
keys, the checksum is stored in the clear.  With non-RSA keys, the
checksum is encrypted like the algorithm-specific data.  This value is
used to check that the passphrase was correct.

5.6 Compressed Data Packet (Tag 8)

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

The body of this packet consists of:
        - One octet that gives the algorithm used to compress the packet.
        - The remainder of the packet is compressed data.

A Compressed Data Packet's body contains an RFC1951 DEFLATE block that


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compresses some set of packets.  See section 7 for details on how
messages are formed.

5.7 Symmetrically Encrypted Data Packet (Tag 9)

The Symmetrically Encrypted Data packet contains data encrypted with a
conventional (symmetric-key) algorithm.  When it has been decrypted, it
will typically contain other packets (often literal data packets or
compressed data packets).

The body of this packet consists of:

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

The conventional cipher used may be specified in an Encrypted Session
Key or Conventional Encrypted Session Key packet which precedes the
Symmetrically Encrypted Data Packet.  In that case, the cipher
algorithm octet is prepended to the session key before it is encrypted.
If no packets of these types precede the encrypted data, the IDEA
algorithm is used with the session key calculated as the MD5 hash of
the passphrase.

The data is encrypted in CFB mode, with a CFB shift size equal to the
cipher's block size [Ref].  The Initial Vector (IV) is specified as all
zeros.  Instead of using an IV, OP prepends a 10 octet string to the
data before it is encrypted.  The first eight octets are random, and
the 9th and 10th octets are copies of the 7th and 8th octets,
respectivelly. After encrypting the first 10 octets, the CFB state is
resynchronized if the cipher block size is 8 octets or less.  The last
8 octets of ciphertext are passed through the cipher and the block
boundary is reset.

The repetition of 16 bits in the 80 bits of random data prepended to
the message allows the receiver to immediately check whether the
session key is correct.

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

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

The body of this packet consists of:
        - The three octets 0x60, 0x47, 0x60 (which spell "PGP" in UTF-8).

Such a packet should be ignored on input.  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 necessary to
process the message.

5.9 Literal Data Packet (Tag 11)


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

The body of this packet consists of:
        - A one-octet field that describes how the data is formatted.
If it is a 'b' (0x62), then the literal packet contains binary data. If
it is a 't' (0x74), then it contains text data, and thus may need line
ends converted to local form, or other text-mode changes.  RFC 1991
also defined a value of 'l' as a 'local' mode for machine-local
conversions.  This use is now deprecated.

        - File name as a string (one-octet length, followed by file name),
          if the encrypted data should be saved as a file.
If the special name "_CONSOLE" is used, the message is considered to be
"for your eyes only".  This advises that the message data is unusually
sensitive, and the receiving program should process it more carefully,
perhaps avoiding storing the received data to disk, for example.

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

    - The remainder of the packet is literal data.

Text data is stored with <CR><LF> text endings.  This should be
converted to native line endings by the receiving software.

5.10 Trust Packet (Tag 12)

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

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

{{Editor's note:  I have brushed aside the description of the old PGP
trust packets for a number of reasons.  They are context dependent;
their meaning depends on the packet preceding them in a keyring.

There is also a security problem with trust packets.  For example,
malicious software can write a new public key into a user's key ring
with trust packets that make it trusted.

A number of us have discussed this problem, and think that trust
information should always be self-signed to act as an integrity check,
but other people may have other solutions.

My solution is to make trust packets implementation dependent.  They


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are not emitted on export and ignored on import.  Because of this, they
are arguably out of scope of this document anyway.  Given that the PGP
implementation of trust packets has security flaws, this seems to be
the best way to deal with them.

--jdcc}}

5.11 User ID Packet (Tag 13)

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

{{Editor's note:  PRZ thinks there should be more types of "user ids"
other than the traditional name, such as photos, and so on.  The above
definition, which assiduously avoids saying that the content of the
packet is a counted string, is one potential way to handle it.  Another
would be to explicitly state that this packet is a string, and
introduce a free-form user identification packet.

A related issue with this document is that sometimes it says "user id"
and sometimes "user name." We need some work here.  Present plan is to
use "User ID" everywhere. --jdcc}}

{{Editor's note:  Carl Ellison pointed out to me that if we have
non-exportable (local to one's own keyring) usernames that I can assign
to keys I use, then essentially we have SDSI naming in PGP.  This is a
Good Thing, in my opinion, but we have to have a way to define it.
--jdcc}}

5.12 Comment Packet (Tag 16)

A Comment packet is used for holding data that is not relevant to
software.  Comment packets should be ignored.

{{Editor's note: should?  Must?  What does it mean to ignore them?  For
example, if it's desirable to show a comment to a user, then how does
that interact with should/must and a suitable definition of "ignore." I
believe that they MUST be ignored, but displaying them to a user is
ignoring them.  Looking inside them for cryptographic content (like OP
packets) is *not* ignoring them.}}

{{Editor's note: should we put in an X.509 encapsulation packet type?}}

6.  Constants

This section describes the constants used in OP.

Note that these tables are not exhaustive lists; an implementation MAY
implement an algorithm not on these lists.



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6.1 Public Key Algorithms

1              - RSA (Encrypt or Sign)
2          - RSA Encrypt-Only
3          - RSA Sign-Only
16             - Elgamal
17             - DSA (Digital Signature Standard)
100 to 110 - Private/Experimental algorithm.

Implementations MUST implement DSA for signatures, and Elgamal for
encryption.  Implementations SHOULD implement RSA encryption.
Implementations MAY implement any other algorithm.

{{Editor's note: reserve an algorithm for elliptic curve?  Note that
I've left Elgamal signatures completely unmentioned.  I think this is
good. --jdcc}}

6.2 Symmetric Key Algorithms

0              - Plaintext
1              - IDEA
2              - Triple-DES (DES-EDE, as per spec -
             168 bit key derived from 192)
3              - CAST5 (128 bit key)
4          - Blowfish (128 bit key)
5          - ROT-N (128 bit N)
6          - SAFER-SK128
7          - DES/SK
100 to 110 - Private/Experimental algorithm.

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

6.3 Compression Algorithms

0          - Uncompressed
1              - ZIP
100 to 110 - Private/Experimental algorithm.

Implementations MUST implement uncompressed data. Implementations
SHOULD implement ZIP.

6.4 Hash Algorithms

1              - MD5
2              - SHA-1
3              - RIPE-MD/160
4          - HAVAL
100 to 110 - Private/Experimental algorithm.

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


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7.  Packet Composition

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

7.1 Transferable Public Keys

OP 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
        - Zero or more Subkey packets
        - After each Subkey packet, one or more Signature packets

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
enjoy the use of more than one e-mail address, and construct a User ID
packet for each one.

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

After the User ID packets there may be one or more Subkey packets.
Subkeys are used in cases where the top-level public key is a
signature-only key.  The subkeys are then encryption-only keys that are
bound to the signature key.  Each Subkey packet must be followed by at
least one Signature packet, which should be of the subkey binding
signature type, and issued by the top level key.

{{Editor's note:  I think it is a good idea to have signature-only
subkeys, too (or even encrypt-and-sign subkeys), but no implementation
does this.  Should we generalize here? --jdcc}}

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 which
is authorized to issue revocations via a revocation key subpacket in a
self-signature by the top level key.

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


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7.2 OP Messages

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

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

   Compressed Message :- Compressed Data Packet.

   Literal Message :- Literal Data Packet.

   ESK :- Encrypted Session Key Packet |
          Conventionally Encrypted Session Key Packet.

   ESK Sequence :- ESK | ESK Sequence, ESK.

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

   One-Pass Signed Message :- One-Pass Signature Packet, OP Message,
                                   Signature Packet.

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

In addition, the decrypting a Symmetrically Encrypted Data packet and
decompressing a Compressed Data packet must yield a valid OP Message.


8.  Enhanced Key Formats

8.1 Key Structures

The format of V3 OP key using RSA is as follows.  Entries in square
brackets are optional and ellipses indicate repetition.

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

Each signature certifies the RSA public key and the preceding user ID.
The RSA public key can have many user IDs and each user ID can have
many signatures.

The format of an OP V4 key that uses two public keys is very similar
except that the second key is added to the end as a 'subkey' of the
primary key.

    Primary-Key


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

The subkey always has a single signature after it that is issued using
the primary key to tie the two keys together.  The new format can use
either the new signature packets or the old signature packets.

In an Elgamal/DSA key, the DSA public key is the primary key, the
Elgamal public key is the subkey, and either version 3 or 4 of the
signature packet can be used.  There may be other types 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, etc, or RSA primary key with an
Elgamal subkey.

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.


8.2 V4 Key IDs and Fingerprints

A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet Tag,
followed by the two-octet packet length, followed by the entire Public
Key packet starting with the version field.  The key ID is either the
low order 32 bits or 64 bits of the fingerprint.  Here are the fields
of the hash material, with the example of a DSA key:

    a.1) 0x99 (1 byte)
    a.2) high order length byte of (b)-(f) (1 byte)
    a.3) low order length byte of (b)-(f) (1 byte)
    b) version number = 4 (1 byte);
    c) time stamp of key creation (4 bytes);
    e) algorithm (1 byte):
         17 = DSA;
    f) Algorithm specific fields.

    Algorithm Specific Fields for DSA keys (example):
    f.1) MPI of DSA prime p;
    f.2) MPI of DSA group order q (q is a prime divisor of p-1);
    f.3) MPI of DSA group generator g;
    f.4) MPI of DSA public key value y (= g**x where x is secret).


9.  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.


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Possession of the private key portion of a public-private key pair is
assumed to be controlled by the proper party or parties.

Certain operations in this specification involve the use of random
numbers.  An appropriate entropy source should be used to generate
these numbers.  See RFC 1750.

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

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

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

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

10.  Authors and Working Group Chair

The working group can be contacted via the current chair:

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

The principal authors of this draft are (in alphabetical order):

Jon Callas Pretty Good Privacy, Inc. 555 Twin Dolphin Drive, #570
Redwood Shores, CA 94065, USA Email: jon@pgp.com Tel: +1-650-596-1960

Lutz Donnerhacke IKS GmbH Wildenbruchstr. 15 07745 Jena, Germany EMail:
lutz@iks-jena.de Tel: +49-3641-675642

Hal Finney Pretty Good Privacy, Inc. 555 Twin Dolphin Drive, #570
Redwood Shores, CA 94065, USA Email: hal@pgp.com Tel: +1-650-572-0430

Rodney Thayer Sable Technology Corporation 246 Walnut Street Newton, MA
02160 USA Email: rodney@sabletech.com Tel: +1-617-332-7292



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

11.  References


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[CAMPBELL} Campbell, Joe, "C Programmer's Guide to Serial
Communications"

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

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

[PKCS1] RSA Laboratories, "PKCS #1:  RSA Encryption Standard," version
1.5, November 1993

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

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

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

[RFC1750] Eastlake, Crocker, & Schiller., Randomness Recommendations
for Security.  December 1994.

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

[RFC1983] G.  Malkin., Internet Users' Glossary.  August 1996.

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

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

[RFC2044] F.  Yergeau., UTF-8, a transformation format of Unicode and
ISO 10646.  October 1996.

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

[RFC2119] Bradner, S., Key words for use in RFCs to Indicate
Requirement Level.  March 1997.

12.  Full Copyright Statement



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Copyright 1997 by The Internet Society.  All Rights Reserved.

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

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






































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