RFC1991 - PGP Message Exchange Formats

Network Working Group D. Atkins
Request for Comments: 1991 MIT
Category: Informational W. Stallings
Comp-Comm Consulting
P. Zimmermann
Boulder Software Engineering
August 1996
PGP Message Exchange Formats
Status of This Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Table of Contents
1. IntrodUCtion............................................2
2. PGP Services............................................2
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................................17
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........................................20
7. Transferable Public Keys...............................20
8. Acknowledgments........................................20
9. Security Considerations................................21
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