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Network Working Group                                          D. Atkins
INTERNET_DRAFT                                                       MIT
Category: Informational                                     W. Stallings
                                                    Comp-Comm Consulting
                                                           P. Zimmermann
                                            Boulder Software Engineering
                                                               July 1995


                     PGP Message Exchange Formats

Status of this memo

   This document is  an  Internet-Draft.   Internet-Drafts  are  working
   documents  of  the Internet Engineering Task Force (IETF), its areas,
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   Distribution of this memo is unlimited.  Please send comments to  the
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Table of Contents

   1.    Introduction............................................2
   2.    PGP Services............................................3
   2.1   Digital signature.......................................3
   2.2   Confidentiality.........................................3
   2.3   Compression.............................................4
   2.4   Radix-64 conversion.....................................4
   2.4.1 ASCII Armor Formats.....................................5
   3.    Data Element Formats....................................6
   3.1   Byte strings............................................6
   3.2   Whole number fields.....................................7
   3.3   Multiprecision fields...................................7
   3.4   String fields...........................................8
   3.5   Time fields.............................................8
   4.    Common Fields...........................................8
   4.1   Packet structure fields.................................8
   4.2   Number ID fields.......................................10
   4.3   Version fields.........................................10
   5.    Packets................................................10
   5.1   Overview...............................................10
   5.2   General Packet Structure...............................11
   5.2.1 Message component......................................11
   5.2.2 Signature component....................................11
   5.2.3 Session key component..................................11
   6.    PGP Packet Types.......................................12
   6.1   Literal data packets...................................12
   6.2   Signature packets......................................13
   6.2.1 Message-digest-related fields..........................14
   6.2.2 Public-key-related fields..............................15
   6.2.3 RSA signatures.........................................16
   6.2.4 Miscellaneous fields...................................16
   6.3   Compressed data packets................................16
   6.4   Conventional-key-encrypted data packets................17
   6.4.1 Conventional-encryption type byte......................18
   6.5   Public-key-encrypted packets...........................18
   6.5.1 RSA-encrypted data encryption key (DEK)................19
   6.6   Public-key Packets.....................................19
   6.7   User ID packets........................................19
   7.    Transferable Public Keys...............................20
   8.    Acknowledgments........................................20
   9.    Security Considerations................................20
   10.   Authors' Addresses.....................................21

1. Introduction

   PGP (Pretty  Good   Privacy) uses a  combination    of public-key and
   conventional encryption to  provide security services for  electronic
   mail messages and data files.  These services include confidentiality
   and digital  signature.   PGP is widely  used  throughout  the global
   computer  community.    This document describes  the   format of "PGP
   files", i.e., messages  that have been  encrypted and/or signed  with
   PGP.

   PGP was created by  Philip Zimmermann and  first released, in Version
   1.0, in 1991. Subsequent versions have  been designed and implemented
   by an all-volunteer collaborative effort under the design guidance of
   Philip Zimmermann.  PGP  and  Pretty Good Privacy are   trademarks of
   Philip Zimmermann.

   This document describes  versions 2.x of PGP.  Specifically, versions
   2.6 and  2.7 conform to this specification.   Version 2.3 conforms to
   this specification with minor differences.

   A new  release of PGP, known as  PGP 3.0, is  anticipated in 1995. To
   the maximum extent possible, this version will be upwardly compatible
   with version 2.x. At a minimum, PGP 3.0 will be able to read messages
   and signatures produced by version 2.x.

2. PGP Services

   PGP provides four services related to the format of messages and data
   files: digital signature,  confidentiality, compression, and radix-64
   conversion.

2.1 Digital signature

   The digital signature  service involves the  use of  a hash code,  or
   message digest, algorithm, and a public-key encryption algorithm. The
   sequence is as follows:

     -the sender creates a message
     -the sending PGP generates a hash code of the message
     -the sending PGP encrypts the hash code using the sender's private
      key
     -the encrypted hash code is prepended to the message
     -the receiving PGP decrypts the hash code using the sender's public
      key
     -the receiving PGP generates a new hash code for the received
      message and compares it to the decrypted hash code. If the two
      match, the message is accepted as authentic

   Although signatures  normally are  found  attached to the  message or
   file that they sign, this is not always the case: detached signatures
   are supported. A  detached signature  may  be stored  and transmitted
   separately from the   message it signs.   This is  useful  in several
   contexts. A user may wish to maintain a separate signature log of all
   messages  sent or received.  A detached   signature of an  executable
   program  can  detect  subsequent  virus  infection. Finally, detached
   signatures can be used when more than one party must sign a document,
   such as a legal contract.  Each person's signature is independent and
   therefore is applied   only to  the document. Otherwise,   signatures
   would have  to  be nested, with   the second signer  signing both the
   document and the first signature, and so on.

2.2 Confidentiality

   PGP provides confidentiality by encrypting messages to be transmitted
   or data files to be stored  locally using conventional encryption. In
   PGP, each conventional key  is used only once. That  is, a new key is
   generated as a random 128-bit number for each message. Since it is to
   be 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:

     -the sender creates a message
     -the sending PGP generates a random number to be used as a session
      key for this message only
     -the sending PGP encrypts the message using the session key
     -the session key is encrypted using the recipient's public key and
      prepended to the encrypted message
     -the receiving PGP decrypts the session key using the recipient's
      private key
     -the receiving PGP decrypts the message using the session key

   Both digital signature and confidentiality services may be applied to
   the same message. First, a signature is generated for the message and
   prepended 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.

2.3 Compression

   As a default, PGP compresses the message after applying the signature
   but before encryption.

2.4 Radix-64 conversion

   When PGP is used, usually   part of the   block to be transmitted  is
   encrypted. If  only the signature  service is used,  then the message
   digest  is  encrypted (with  the   sender's   private key). If    the
   confidentiality service   is  used, the   message plus  signature (if
   present) are encrypted (with a one-time conventional key). Thus, part
   or all of the resulting block consists of a stream of arbitrary 8-bit
   bytes.  However, many electronic mail  systems only permit the use of
   blocks consisting of ASCII text. To accommodate this restriction, PGP
   provides the  service of converting the  raw 8-bit binary stream to a
   stream of printable ASCII characters, called ASCII Armor.

   The scheme  used for this purpose  is radix-64 conversion. Each group
   of three bytes of binary data is mapped into 4 ASCII characters. This
   format   also  appends a  CRC  to detect  transmission  errors.  This
   radix-64 conversion, also called Ascii Armor, is a wrapper around the
   binary PGP  messages,  and is  used to  protect  the  binary messages
   during transmission over non-binary channels, such as Internet Email.

   The following table defines the mapping.  The characters used are the
   upper-  and lower-case  letters,  the  digits  0 through  9,  and the
   characters + and  /.   The carriage-return and  linefeed   characters
   aren't used in the conversion, nor is  the tab or any other character
   that might be altered by the  mail system. The  result is a text file
   that is "immune" to the modifications inflicted by mail systems.

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

   It is possible   to use PGP to  convert  any arbitrary file to  ASCII
   Armor.  When this  is done, PGP tries to  compress the data before it
   is converted to Radix-64.

2.4.1 ASCII Armor Formats

   When  PGP  encodes data into  ASCII Armor,  it  puts specific headers
   around the data, so PGP  can reconstruct the data  at a future  time.
   PGP tries  to inform the user  what kind  of  data is  encoded in the
   ASCII armor through the use of the headers.

   ASCII Armor is created by concatenating the following data:

        - An Armor Headerline, appropriate for the type of data
        - Armor Headers
        - A blank line
        - The ASCII-Armored data
        - An Armor Checksum
        - The Armor Tail (which depends on the Armor Headerline).

   An Armor Headerline is composed  by taking the appropriate headerline
   text  surrounded by  five  (5)  dashes  (-)  on  either side of   the
   headerline text.  The headerline text  is chosen based upon the  type
   of data that is being encoded in Armor, and how  it is being encoded.
   Headerline texts include the following strings:

    BEGIN PGP MESSAGE -- used for signed, encrypted, or compressed files
    BEGIN PGP PUBLIC KEY BLOCK -- used for transferring public keys
    BEGIN PGP MESSAGE, PART X/Y -- used for multi-part messages, where
                                    the armor is split amongst Y files,
                                    and this is the Xth file out of Y.

   The Armor Headers are pairs of strings that  can give the user or the
   receiving PGP program 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  should not be  used to  convey any important
   information, since they can be changed in transport.

   The format  of  an Armor  Header is that   of a key-value  pair,  the
   encoding  of  RFC-822  headers.     PGP should  consider   improperly
   formatted Armor Headers to be corruption of the ASCII Armor.  Unknown
   Keys should be   reported to the  user,  but so  long as the  RFC-822
   formatting is correct, PGP   should continue to process the  message.
   Currently defined Armor  Header Keys include "Version" and "Comment",
   which  define   the PGP  Version used  to  encode the   message and a
   user-defined comment.

   The Armor Checksum   is a  24-bit CRC   converted  to four  bytes  of
   radix-64 encoding, prepending an   equal-sign  (=) to the   four-byte
   code.  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,  the reader is asked to
   look  at  chapter 19  of  the book  "C  Programmer's  Guide to Serial
   Communications," by Joe Campbell.

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

3. Data Element Formats

3.1 Byte strings

   The objects considered  in this document are  all "byte strings."   A
   byte string is a finite sequence of bytes.  The concatenation of byte
   string X of length M with byte string Y of  length N is a byte string
   Z of length M + N; the first  M bytes of Z are  the bytes of X in the
   same order, and the remaining N bytes of Z are the bytes  of Y in the
   same order.

   Literal byte strings  are written from left  to right,  with pairs of
   hex nibbles  separated by spaces,  enclosed  by angle  brackets:  for
   instance, <05  ff 07> is a byte  string of length  3 whose bytes have
   numeric  values  5, 255,  and  7 in that  order.  All numbers in this
   document outside angle brackets are written in decimal.

   The byte string of length 0 is called "empty" and written <>.

3.2 Whole number fields

   Purpose.  A whole number field can represent any nonnegative integer,
   in a format where the field length is known in advance.

   Definition.  A whole  number field is  any byte string.  It is stored
   in radix-256 MSB-first format.  This means  that a whole number field
   of length N with bytes b_0 b_1 ...  b_{N-2} b_{N-1} in that order has
   value

      b_0 * 256^{N-1} + b_1 * 256^{N-2} + ... + b_{N-2} * 256 + b_{N-1}.

   Examples.   The byte  string <00  0D  64 11 00 00>   is a valid whole
   number field with value 57513410560.  The byte string <FF> is a valid
   whole number field with value   255.  The byte  string  <00 00> is  a
   valid whole number field with value 0.  The empty byte string <> is a
   valid whole number field with value 0.

3.3 Multiprecision fields

   Purpose.   A  multiprecision   field can  represent  any  nonnegative
   integer which is not too large.   The field length  need not be known
   in advance.  Multiprecision fields  are designed to waste very little
   space: a small integer uses a short field.

   Definition.  A  multiprecision  field  is  the  concatenation of  two
   fields:

      (a) a whole number field of length 2, with value B;
      (b) a whole number field, with value V.

   Field (b) is of length [(B+7)/8], i.e., the greatest integer which is
   no larger than   (B+7)/8.  The value  of the  multiprecision field is
   defined to be V.  V  must be between 2^{B-1} and  2^B - 1  inclusive.
   In other words B must be exactly the number of significant bits in V.

   Some   implementations   may limit the  possible    range of  B.  The
   implementor  must  document  which values  of B   are   allowed by an
   implementation.

   Examples.  The byte string <00  00> is a valid multiprecision integer
   with value 0.  The  byte string <00  03 05> is a valid multiprecision
   field with value 5.  The byte  strings <00 03 85>  and <00 00 00> are
   not valid multiprecision fields.  The former is invalild because <85>
   has  8 significant bits,  not  3; the latter   is invalid because the
   second field has too many bytes of data given  the value of the first
   field.  The byte string <00 09 01 ff> is a valid multiprecision field
   with value 511.  The byte string <01 00 80 00 00 00 00 00 00 00 00 00
   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 07> is
   a valid multiprecision field with value 2^255 + 7.

3.4  String fields

   Purpose.  A  string field represents  any sequence of bytes of length
   between 0 and 255   inclusive.   The length  need   not be  known  in
   advance.  By convention,  the content of a  string field  is normally
   interpreted as ASCII codes when it is displayed.

   Definition.  A string field is the concatenation of the following:

     (a) a whole number field of length 1, with value L;
     (b) a byte string of length L.

   The content of the string field is defined to be field (b).

   Examples: <05  48 45 4c 4c 4f>  is a valid  string field  which would
   normally be  displayed as the string  HELLO.  <00> is a  valid string
   field which would normally be displayed as the empty string.  <01 00>
   is a valid string field.

3.5  Time fields

   Purpose.  A time field represents the number of seconds elapsed since
   1970   Jan   1 00:00:00   GMT.  It  is   compatible  with   the usual
   representation of times under UNIX.

   Definition.  A time field  is a whole  number field of length 4, with
   value  V.  The time represented  by the time  field is the one-second
   interval beginning V seconds after 1970 Jan 1 00:00:00 GMT.

4. Common Fields

   This section defines fields found in more than one packet format.

4.1  Packet structure fields

   Purpose.  The packet structure  field distinguishes between different
   types of packets, and indicates the length of packets.

   Definition.  A packet structure  field is a  byte string of length 1,
   2, 3, or 5.  Its first byte is the cipher type  byte (CTB), with bits
   labeled  76543210,  7 the most    significant  bit and  0  the  least
   significant bit.   As   indicated  below the  length   of  the packet
   structure field is determined by the CTB.

   CTB bits 76 have values listed in the following table:

      10 - normal CTB
      11 - reserved for future experimental work
      all others - reserved

   CTB  bits 5432, the "packet  type  bits", have  values  listed in the
   following table:

      0001 - public-key-encrypted packet
      0010 - signature packet
      0101 - secret-key certificate packet
      0110 - public-key certificate packet
      1000 - compressed data packet
      1001 - conventional-key-encrypted packet
      1011 - literal data packet
      1100 - keyring trust packet
      1101 - user id packet
      1110 - comment packet     (*)
      all others - reserved

   CTB bits  10, the "packet-length length  bits", have values listed in
   the following table:

      00 - 1-byte packet-length field
      01 - 2-byte packet-length field
      10 - 4-byte packet-length field
      11 - no packet length supplied, unknown packet length

   As indicated  in this table,  depending  on the packet-length  length
   bits, the remaining 1, 2, 4, or 0 bytes of the packet structure field
   are a "packet-length   field".  The packet-length   field is a  whole
   number field.  The value of the packet-length field  is defined to be
   the value of the whole number field.

   A   value of 11   is  currently  used   in  one place: on  compressed
   data. That is,  a compressed data block currently  looks like <A3  01
   . .  .>, where   <A3>, binary  10 1000   11, is an  indefinite-length
   packet. The proper interpretation is "until the  end of the enclosing
   structure", although it  should  never  appear outermost (where   the
   enclosing structure is a file).

   Options marked with an   asterisk (*) are  not implemented   yet; PGP
   2.6.2 will never output this packet type.

4.2  Number ID fields

   Purpose.  The ID of a whole number is  its 64 least significant bits.
   The ID is a convenient way  to distinguish between large numbers such
   as keys, without having to transmit the number itself. Thus, a number
   that may be hundreds or thousands of decimal  digits in length can be
   identified with a 64-bit identifier. Two keys may have the same ID by
   chance or by  malice; although  the  probability that two large  keys
   chosen at random would have the same ID is extremely small.

   Definition.  A number ID  field is a whole  number field of length 8.
   The value of  the number ID field  is defined to be  the value of the
   whole number field.

4.3  Version fields

   Many packet  types include a version number  as the first byte of the
   body.  The format   and meaning of   the body depend on  the  version
   number.   More versions of packets,  with new version numbers, may be
   defined in the  future.   An implementation  need  not  support every
   version of each packet type.  However,  the implementor must document
   which versions   of   each  packet  type   are supported     by   the
   implementation.

   A version  number of  2 or 3   is currently allowed  for each  packet
   format.  New versions will probably be  numbered sequentially up from
   3.  For   backwards compatibility,  implementations will   usually be
   expected to  support  version  N of  a  packet  whenever they support
   version N+1.  Version 255 may be used for experimental purposes.

5. Packets

5.1 Overview

   A packet  is a  digital envelope with  data inside.   A PGP file,  by
   definition, is the concatenation of one or more packets. In addition,
   one  or    more of the  packets   in  a file   may  be   subject to a
   transformation using encryption, compression, or radix-64 conversion.

   A packet is the concatenation of the following:

      (a) a packet structure field;
      (b) a byte string of some length N.

   Byte string (b) is called the "body" of the packet.  The value of the
   packet-length field inside the  packet structure field (a) must equal
   N, the length of the body.

   Other characteristics of the packet are determined by the type of the
   packet.  See the  definitions of particular  packet types for further
   details.  The CTB packet-type bits inside the packet structure always
   indicate the packet type.

   Note that packets may  be nested: one digital  envelope may be placed
   inside another.   For  example,  a conventional-key-encrypted  packet
   contains a disguised packet, which in turn might be a compressed data
   packet.

5.2  General packet structure

   A pgp file  consists  of  three components:  a  message  component, a
   signature (optional), and a session key component (optional).

5.2.1 Message component

   The  message   component includes  the  actual  data  to be stored or
   transmitted  as well as a   header that includes control  information
   generated by PGP. The message component consists  of a single literal
   data packet.

5.2.2 Signature component

   The  signature component is the  signature of  the message component,
   formed using a hash code of the message component  and the public key
   of the sending  PGP entity.  The  signature component consists  of  a
   single signature packet.

   If the   default option  of compression  is   chosen, then the  block
   consisting of the  literal data  packet  and the signature  packet is
   compressed to form a compressed data packet.

5.2.3 Session key component

   The session key component includes the encrypted  session key and the
   identifier of the recipients public key used by the sender to encrypt
   the  session key.  The  session key  component  consists  of a single
   public-key-encrypted packet for each recipient of the message.

   If compression has been used, then conventional encryption is applied
   to the  compressed data  packet  formed from  the compression  of the
   signature packet and the literal data packet. Otherwise, conventional
   encryption is applied to the block consisting of the signature packet
   and the  literal  data packet.  In   either case,  the  cyphertext is
   referred to as a conventional-key-encrypted data packet.

6.  PGP Packet Types

   PGP includes the following types of packets:

       -literal data packet
       -signature packet
       -compressed data packet
       -conventional-key-encrypted data packet
       -public-key-encrypted packet
       -public-key packet
       -User ID packet

6.1 Literal data packets

   Purpose.  A literal data packet is the lowest  level of contents of a
   digital envelope.   The  data  inside a  literal  data packet  is not
   subject to any further interpretation by PGP.

   Definition.  A    literal data packet  is   the concatenation  of the
   following fields:

      (a) a packet structure field;
      (b) a byte, giving a mode;
      (c) a string field, giving a filename;
      (d) a time field;
      (e) a byte string of literal data.

   Fields (b), (c), and (d) suggest how the data should  be written to a
   file. Byte (b) is either  ASCII b <62>, for binary,  or ASCII t <74>,
   for text. Byte (b) may also  take on the value  ASCII 1, indicating a
   machine-local conversion. It is not defined how PGP will convert this
   across platforms.

   Field (c) suggests a filename. Field (d) should be  the time at which
   the file was last modified, or the time at  which the data packet was
   created, or 0.

   Note that  only  field  (e) of a  literal  data  packet is fed   to a
   message-digest  function   for  the formation  of    a signature. The
   exclusion of the  other fields ensures  that detached signatures  are
   exactly the same   as attached signatures  prefixed to   the message.
   Detached signatures  are calculated on  a separate file that has none
   of the literal data packet header fields.

6.2 Signature packet

   Purpose.  Signatures  are attached to data, in  such a way  that only
   one  entity,  called the "writer,"   can  create the signature.   The
   writer  must first create a  "public key"  K  and distribute it.  The
   writer keeps  certain  private  data related   to  K.  Only   someone
   cooperating  with  the writer can sign  data  using K, enveloping the
   data  in  a signature  packet  (also known as a private-key-encrypted
   packet).  Anyone   can look through the   glass  in the  envelope and
   verify that the signature was  attached to the data  using K.  If the
   data is altered in any way then the verification will fail.

   Signatures have different meanings.   For example, a signature  might
   mean "I wrote  this  document," or  "I received   this document."   A
   signature packet    includes a "classification"  which  expresses its
   meaning.

   Definition.  A signature packet, version 2 or 3, is the concatenation
   of the following fields:

      (a) packet structure field (2, 3, or 5 bytes);
      (b) version number = 2 or 3 (1 byte);
      (c) length of following material included in MD calculation
          (1 byte, always the value 5);
      (d1) signature classification (1 byte);
      (d2) signature time stamp (4 bytes);
      (e) key ID for key used for singing (8 bytes);
      (f) public-key-cryptosystem (PKC) type (1 byte);
      (g) message digest algorithm type (1 byte);
      (h) first two bytes of the MD output, used as a checksum
          (2 bytes);
      (i) a byte string of encrypted data holding the RSA-signed digest.

   The message digest is taken of the bytes of the file, followed by the
   bytes of field  (d). It was originally intended  that  the length (c)
   could vary, but now  it seems that it will  alwaye remain  a constant
   value of 5, and that is the only value that will  be accepted.  Thus,
   only the fields (d1) and (d2) will be hashed into the signature along
   with the main message.

6.2.1 Message-digest-related fields

   The message digest algorithm is specified  by the message digest (MD)
   number of field (g). The following MD numbers are currently defined:

      1 - MD5 (output length 16)
      255 - experimental

   More MD numbers may be defined in the future.  An implementation need
   not  support every MD number.  The   implementor must document the MD
   numbers understood by an implementation.

   A  message digest algorithm reads a   byte string of  any length, and
   writes a byte string of some fixed  length, as indicated in the table
   above.

   The input to  the message digest algorithm   is the concatenation  of
   some "primary input" and some "appended input."

   The appended input is specified by field (c), which gives a number of
   bytes to  be taken from the following  fields: (d1), (d2), and so on.
   The current   implementation uses the value 5   for  this number, for
   fields (d1)  and (d2).  Any field not  included in the appended input
   is not "signed" by field (i).

   The primary input is determined by  the signature classification byte
   (d1).   Byte  (d1) is  one of the  following hex  numbers, with these
   meanings:

      <00> - document signature, binary image ("I wrote this document")
      <01> - document signature, canonical text ("I wrote this document")
      <10> - public key packet and user ID packet, generic certification
           ("I think this key was created by this user, but I won't say
           how sure I am")
      <11> - public key packet and user ID packet, persona certification
           ("This key was created by someone who has told me that he is
           this user") (#)
      <12> - public key packet and user ID packet, casual certification
           ("This key was created by someone who I believe, after casual
           verification, to be this user")  (#)
      <13> - public key packet and user ID packet, positive certification
           ("This key was created by someone who I believe, after
           heavy-duty identification such as picture ID, to be this
           user")  (#)
      <20> - public key packet, key compromise ("This is my key, and I
           have revoked it")
      <30> - public key packet and user ID packet, revocation ("I retract
           all my previous statements that this key is related to this
           user")  (*)
      <40> - time stamping ("I saw this document") (*)

   More classification numbers  may be defined in  the future to  handle
   other meanings of signatures, but only the  above numbers may be used
   with version  2 or version  3 of  a signature  packet.   It should be
   noted that PGP 2.6.2  has not implemented the  packets marked with an
   asterisk (*), and the packets marked with a  hash  (#) are not output
   by PGP 2.6.2.

   Signature packets are used in two  different contexts. One (signature
   type <00>   or <01>) is  of text  (either  the contents of  a literal
   packet or a separate file), while types <10> through <1F> appear only
   in key files, after the keys and user  IDs that they sign.  Type <20>
   appears in  key files, after  the keys that it   signs, and type <30>
   also appears after a key/userid combination. Type <40> is intended to
   be a signature of a signature, as a notary seal on a signed document.

   The output of  the message digest algorithm is  a message digest,  or
   hash code. Field i contains the cyphertext produced by encrypting the
   message digest with  the signer's private key.  Field h contains  the
   first two bytes  of the unencrypted  message digest. This enables the
   recipient to determine if the correct public key  was used to decrypt
   the message  digest  for authentication, by comparing  this plaintext
   copy of the first two byes with the first  two bytes of the decrypted
   digest. These two bytes  also serve as a  16-bit frame check sequence
   for the message.

6.2.2 Public-key-related fields

   The  message  digest is signed by  encrypting  it using  the writer's
   private key. Field (e) is the ID of the corresponding public key.

   The  public-key-encryption algorithm  is specified by  the public-key
   cryptosystem (PKC) number of field (f). The following PKC numbers are
   currently defined:

      1 - RSA
      255 - experimental

   More PKC numbers  may be defined   in the future.   An implementation
   need not support every PKC number.  The implementor must document the
   PKC numbers understood by an implementation.

   A PKC number identifies both   a public-key encryption method and   a
   signature method.  Both of these methods are fully defined as part of
   the definition of the PKC number.  Some cryptosystems are usable only
   for encryption, or only  for signatures; if  any such PKC numbers are
   defined in the future, they will be marked appropriately.

6.2.3 RSA signatures

   An RSA-signed byte string is a multiprecision field that is formed by
   taking the message  digest and filling in an  ASN structure, and then
   encrypting the whole byte string in the RSA key of the signer.

   PGP versions 2.3 and later encode the MD into a PKCS-format signature
   string, which has the following format:

          MSB               .   .   .                    LSB
          0   1   <FF>(n bytes)   0   ASN(18 bytes)   MD(16 bytes)

   See RFC1423 for an explanation of the meaning  of the ASN string.  It
   is the following 18 byte long hex value:

          <30 20 30 0C 06 08 2A 86 48 86 F7 0D 02 05 05 00 04 10>

   Enough bytes of  <FF> padding are added  to  make the length of  this
   whole string equal to the number of bytes in the modulus.

6.2.4 Miscellaneous fields

   The timestamp   field (d2) is  analogous to  the  date box next  to a
   signature  box on a  form.  It  represents a time  which is typically
   close to the moment that the signature packet  was created.  However,
   this is not a requirement.  Users may choose to date their signatures
   as they wish, just as they do now in handwritten signatures.

   If an application  requires the  creation  of trusted  timestamps  on
   signatures, a  detached  signature   certificate with  an   untrusted
   timestamp may  be submitted to a  trusted timestamp notary service to
   sign the  signature packet with  another signature packet, creating a
   signature of a signature.   The notary's signature's  timestamp could
   be used as the trusted "legal" time of the original signature.

6.3 Compressed data packets

   Purpose.  A   compressed  data packet  is an  envelope   which safely
   squeezes its contents into a small space.

   Definition.  A compressed  data  packet is  the concatenation of  the
   following fields:

      (a) a packet structure field;
      (b) a byte, giving a compression type;
      (c) a byte string of compressed data.

   Byte  string  (c) is a  packet  which  may be decompressed  using the
   algorithm  identified in  byte (b).   Typically,  the  data  that are
   compressed consist of  a literal  data  packet or  a signature packet
   concatenated to a literal data packet.

   A  compression type  selects  a  compression  algorithm  for use   in
   compressed  data  packets.   The   following compression  numbers are
   currently defined.

      1 - ZIP
      255 - experimental

   More  compression    numbers may   be defined  in    the  future.  An
   implementation need not support every   MD number.  The   implementor
   must  document   the     compression   numbers  understood   by    an
   implementation.

6.4 Conventional-key-encrypted data packets

   Purpose.  A    conventional-key-encrypted data packet   is  formed by
   encrypting a block  of data with  a conventional encryption algorithm
   using a one-time session key. Typically, the block to be encrypted is
   a compressed data packet.

   Definition.    A   conventional-key-encrypted data  packet    is  the
   concatenation of the following fields:

      (a) a packet structure field;
      (b) a byte string of encrypted data.

   The plaintext or compressed plaintext that is encrypted to form field
   (b) is   first prepended with  64 bits  of random  data  plus 16 "key
   check"  bits.  The  random prefix  serves  to  start off  the  cipher
   feedback  chaining process   with  64 bits of   random material; this
   serves    the same function as an    initialization vector (IV) for a
   cipher-block-chaining encryption   scheme.  The key  check  prefix is
   equal to the last 16 bits of the random  prefix. During decryption, a
   comparison is made to see  if the 7th  and 8th byte of the  decrypted
   material match the 9th  and 10th bytes.  If so, then the conventional
   session key used for decryption is assumed to be correct.

6.4.1 Conventional-encryption type byte

   Purpose.  The conventional-encryption type byte  is used to determine
   what conventional encryption algorithm is in use.  The algorithm type
   byte will also  define how long the  conventional encryption key  is,
   based upon the algorithm in use.

   Definition.  A conventional-encryption   type byte is a   single byte
   which  defines   the algorithm  in  use.   It   is possible that  the
   algorithm in use may  require further definition, such as key-length.
   It  is up to the implementor  to document the supported key-length in
   such a situation.

      1 - IDEA (16-byte key)
      255 - experimental

6.5 Public-key-encrypted packets

   Purpose.  The public-key-encrypted  packet   is the format   for  the
   session key component of a message. The  purpose of this packet is to
   convey the one-time  session key used to  encrypt the message  to the
   recipient in a secure manner. This  is done by encrypting the session
   key with the  recipient's public key, so that  only the recipient can
   recover the session key.

   Definition.   A public-key-encrypted packet, version   2 or 3, is the
   concatenation of the following fields:

      (a) a packet structure field;
      (b) a byte, giving the version number, 2 or 3;
      (c) a number ID field, giving the ID of a key;
      (d) a byte, giving a PKC number;
      (e) a byte string of encrypted data (DEK).

   Byte  string (e) represents  the value of  the session key, encrypted
   using the reader's public key K, under the cryptosystem identified in
   byte (d).

   The value of field (c) is the ID of K.

   Note that the packet does not actually  identify K: two keys may have
   the  same ID, by chance or  by malice.  Normally   it will be obvious
   from the context which   key K was used  to  create the packet.   But
   sometimes it is not obvious.  In this case field  (c) is useful.  If,
   for example,  a  reader has   created  several keys,  and receives  a
   message, then he should attempt to  decrypt the message only with the
   key whose ID matches the value of field  (c).  If he has accidentally
   generated two keys with the same ID,  then he must attempt to decrypt
   the message with both keys, but this case is highly unlikely to occur
   by chance.

6.5.1 RSA-encrypted data encryption key (DEK)

   The Data Encryption Key (DEK) is  a multiprecision field which stores
   an RSA encrypted byte string.  The byte string  is a PKCS encoding of
   the secret key used the encrypt the message,  with random padding for
   each Public-Key encrypted packet.

   PGP version  2.3 and later   encode the DEK   into an  MPI using  the
   following format:

     MSB                       .   .   .                       LSB
      0   2   RND(n bytes)   0  ALG(1 byte)  DEK(k bytes)  CSUM(2 bytes)

   ALG refers to the algorithm byte for the secret key algorithm used to
   encrypt the data packet.  The DEK is  the actual Data Encryption Key,
   and  its size is dependent upon  the encryption  algorithm defined by
   ALG.  For the IDEA encryption  algorithm, type byte 1,  the DEK is 16
   bytes long.  CSUM is a 16-bit checksum of  the DEK, used to determine
   that  the correct Private  key was used  to decrypt this packet.  The
   checksum is computed by the 16-bit sum of the  bytes in the DEK.  RND
   is random padding to  expand  the byte to  fill  the size of  the RSA
   Public Key that is used to encrypt the whole byte.

6.6 Public Key Packet

   Purpose.  A public key packet defines an RSA public key.

   Definition.     A public  key  packet  is   the concatenation of  the
   following fields:

      (a) packet structure field (2 or 3 bytes);
      (b) version number = 2 or 3 (1 byte);;
      (c) time stamp of key creation (4 bytes);
      (d) validity period in days (0 means forever) (2 bytes);
      (e) public-key-cryptosystem (PKC) type (1 byte);
      (f) MPI of RSA public modulus n;
      (g) MPI of RSA public encryption exponent e.

    The validity period is always set to 0.

6.7 User ID Packet

   Purpose.  A user ID packet identifies a user and is associated with a
   public or private key.

   Definition.  A  user ID packet is the  concatenation of the following
   fields:

      (a) packet structure field (2 bytes);
      (b) User ID string.

   The  User ID string may be  any string of printable ASCII characters.
   However, since the purpose of this packet is  to uniquely identify an
   individual, the  usual practice is for  the User ID string to consist
   of the user's name followed by  an e-mail address  for that user, the
   latter enclosed in angle brackets.

7. Transferable Public Keys

   Public keys may transferred between PGP users. The essential elements
   of a transferable public key are

      (a) One public key packet;
      (b) One or more user ID packets;
      (c) Zero 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.

8. Acknowledgments

   Philip Zimmermann is  the creator of  PGP 1.0, which is the precursor
   of PGP   2.x.   Major parts  of  later   versions of PGP    have been
   implemented  by  an  international collaborative effort   involving a
   large  number  of contributors, under the   design guidance of Philip
   Zimmermann.

9. Security Considerations

    Security issues are discussed throughout this memo.

10. Authors' Addresses

   Derek Atkins
   12 Rindge Ave. #1R
   Cambridge, MA
   Phone: +1 617 868-4469
   EMail: warlord@MIT.EDU

   William Stallings
   Comp-Comm Consulting
   P. O. Box 2405
   Brewster, MA 02631
   EMail: stallings@ACM.org

   Philip Zimmermann
   Boulder Software Engineering
   3021 Eleventh Street
   Boulder, Colorado 80304  USA
   Phone: +1-303-541-0140
   EMail: prz@acm.org


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