Freaks Macintosh Archive/Freaks Macintosh Archive.bin/Freaks Macintosh Archives/Crypto⁄Encryption/PGP/MacPGP263i.sit/MacPGP263i/Documentation/pgformat.doc

Text File | 1996-01-12 | 37KB | 842 lines

File Formats Used by PGP 2.x
============================

***Note: Packets generated with PGP 2.6.3i normally contain a version
         byte of 3.  However, by using the +legal_kludge=off option, you
         can force PGP to use a version byte of 2 instead.  This will
         make all messages and keys generated with PGP 2.6.3i compatible
         with any PGP 2.x version.

This appendix describes the file formats used externally by Pretty
Good Privacy (PGP), the RSA public key cryptography application.  The
intended audience includes software engineers trying to port PGP to
other hardware environments or trying to implement other PGP-
compatible cryptography products, or anyone else who is curious.

[To be included: a description of ASCII armor.  An ASCII armored
file is just like a binary file described here, but with an extra
layer of encoding added, framing lines, and a 24-bit CRC at the end.]


Byte Order
----------

All integer data used by PGP is externally stored most significant byte
(MSB) first, regardless of the byte order used internally by the host
CPU architecture.  This is for portability of messages and keys between
hosts.  This covers multiprecision RSA integers, bit count prefix
fields, byte count prefix fields, checksums, key IDs, and timestamps.

The MSB-first byte order for external packet representation was
chosen only because many other crypto standards use it.


Multiprecision Integers
-----------------------

RSA arithmetic involves a lot of multiprecision integers, often
having hundreds of bits of precision.  PGP externally stores a
multiprecision integer (MPI) with a 16-bit prefix that gives the
number of significant bits in the integer that follows.  The integer
that follows this bitcount field is stored in the usual byte order,
with the MSB padded with zero bits if the bitcount is not a multiple
of 8.  The bitcount always specifies the exact number of significant
bits.  For example, the integer value 5 would be stored as these
three bytes:

    00 03 05

An MPI with a value of zero is simply stored with the 16-bit bitcount
prefix field containing a 0, with no value bytes following it.



Key ID
------

Some packets use a "key ID" field.  The key ID is the least
significant 64 bits of the RSA public modulus that was involved in
creating the packet.  For all practical purposes it unique to each
RSA public key.


User ID
-------

Some packets contain a "user ID", which is an ASCII string that
contains the user's name.  Unlike a C string, the user ID has a
length byte at the beginning that has a byte count of the rest of the
string.  This length byte does not include itself in the count.


Timestamp
---------

Some packets contain a timestamp, which is a 32-bit unsigned integer
of the number of seconds elapsed since 1970 Jan 1 00:00:00 GMT.  This
is the standard format used by Unix timestamps.  It spans 136 years.



Cipher Type Byte (CTB)
----------------------

Many of these data structures begin with a Cipher Type Byte (CTB),
which specifies the type of data structure that follows it.  The CTB
bit fields have the following meaning (bit 0 is the LSB, bit 7 is the
MSB):

Bit 7:     Always 1, which designates this as a CTB
Bit 6:     Reserved.
Bits 5-2:  CTB type field, specifies type of packet that follows
           0001 - public-key-encrypted packet
           0010 - secret-key-encrypted (signature) packet
           0101 - Secret key certificate
           0110 - Public key certificate
           1000 - Compressed data packet
           1001 - Conventional-Key-Encrypted data
           1011 - Raw literal plaintext data, with filename and mode
           1100 - Keyring trust packet
           1101 - User ID packet, associated with public or secret key
           1110 - Comment packet
           Other CTB packet types are unimplemented.
Bits 1-0:  Length-of-length field:
           00 - 1 byte packet length field follows CTB
           01 - 2 byte packet length field follows CTB
           10 - 4 byte packet length field follows CTB
           11 - no length field follows CTB, unknown packet length.
           The 8-, 16-, or 32-bit packet length field after the CTB
           gives the length in bytes of the rest of the packet, not
           counting the CTB and the packet length field.



RSA public-key-encrypted packet
-------------------------------

Offset  Length  Meaning
0       1       CTB for RSA public-key-encrypted packet
1       2       16-bit (or maybe 8-bit) length of packet
3    1    Version byte, may affect rest of fields that follow.
                (=2) for PGP versions <= 2.5
                (=3) for PGP versions >= 2.6
4       8       64-bit Key ID
12    1    Algorithm byte for RSA (=1 for RSA).
        --Algorithm byte affects field definitions that follow.
13      ?       RSA-encrypted integer, encrypted conventional key
                packet.  (MPI with bitcount prefix)

The conventionally-encrypted ciphertext packet begins right after the
RSA public-key-encrypted packet that contains the conventional key.



Signature packet
----------------

Offset  Length  Meaning
0       1       CTB for secret-key-encrypted (signed) packet
1       2       16-bit (or maybe 8-bit) length of packet
3    1    Version byte, may affect rest of fields that follow.
                (=2) for PGP versions <= 2.5
                (=3) for PGP versions >= 2.6
4    1    Length of following material that is implicitly included
        in MD calculation (=5).
5    1    Signature classification field (see below).
        Implicitly append this to message for MD calculation.
6    4    32-bit timestamp of when signature was made.
        Implicitly append this to message for MD calculation.
10      8       64-bit Key ID
18    1    Algorithm byte for public key scheme (RSA=0x01).
        --Algorithm byte affects field definitions that follow.
19    1    Algorithm byte for message digest (MD5=0x01).
20    2    First 2 bytes of the Message Digest inside the
        RSA-encrypted integer, to help us figure out if we
        used the right RSA key to check the signature.
22      ?       RSA-encrypted integer, encrypted message digest
                (MPI with bitcount prefix).

If the plaintext that was signed is included in the same file as the
signature packet, it begins right after the RSA secret-key-signed
packet that contains the message digest.  The plaintext has a
"literal" CTB prefix.

The original idea had a variable length field following the length
of following material byte, before the Key ID.  In particular, the
possibility of a 2-byte validity period was defined, although no
previous version of PGP ever generated those bytes.

Owing to the way the MD5 is computed for the signature, if that field
is variable length, it is possible to generate two different messages
with the same MD5 hash.  One would be a file of length N, with a 7-byte
following section consisting of a signature type byte, 4 bytes of
timestamp, and 2 of validity period, while the other would be a file of
length N+2, whose last two bytes would be the siganture type byte and
the first byte of timestamp, and the last three bytes of timestamp and
the validity period would instead be interpreted as a signature type
byte and a timestmap.

It should be emphasized that the messages are barely different and
special circumstances must arise for this to be possible, so it is
extremely unlilely that this would be exploitable, but it is a
potential weakness.  It has been plugged by allowing only the currently
implemented 5-byte option.  Validity periods will be added later with
a different format.

The signature classification field describes what kind of
signature certificate this is.  There are various hex values:
    00 -    Signature of a message or document, binary image.
    01 -    Signature of a message or document, canonical text.
    10 -    Key certification, generic.  Only version of key
        certification supported by PGP 2.5.
        Material signed is public key pkt and User ID pkt.
    11 -    Key certification, persona.  No attempt made at all
        to identify the user with a real name.
        Material signed is public key pkt and User ID pkt.
    12 -    Key certification, casual identification.  Some
        casual attempt made to identify user with his name.
        Material signed is public key pkt and User ID pkt.
    13 -    Key certification, positive ID.  Heavy-duty
        identification efforts, photo ID, direct contact
        with personal friend, etc.
        Material signed is public key pkt and User ID pkt.
    20 -     Key compromise.  User signs his own compromise
        certificate.  Independent of user ID associations.
        Material signed is public key pkt ONLY.
    30 -     Key/userid revocation.  User can sign his own
        revocation to dissolve an association between a key
        and a user ID, or certifier may revoke his previous
        certification of this key/userid pair.
        Material signed is public key pkt and User ID pkt.
    40 -    Timestamping a signature certificate made by someone
        else.  Can be used to apply trusted timestamp, and
        log it in notary's log.  Signature of a signature.
        (Planned, not implemented.)

When a signature is made to certify a key/UserID pair, it is computed
across two packets-- the public key packet, and the separate User ID
packet.  See below.

The packet headers (CTB and length fields) for the public key packet
and the user ID packet are both omitted from the signature
calculation for a key certification.

A key compromise certificate may be issued by someone to revoke his
own key when his secret key is known to be compromised.  If that
happens, a user would sign his own key compromise certificate with
the very key that is being revoked.  A key revoked by its own
signature means that this key should never be used or trusted again,
in any form, associated with any user ID.  A key compromise
certificate issued by the keyholder shall take precedence over any
other key certifications made by anyone else for that key.  A key
compromise signed by someone other than the key holder is invalid.

Note that a key compromise certificate just includes the key packet
in its signature calculation, because it kills the whole key without
regard to any userid associations.  It isn't tied to any particular
userid association.  It should be inserted after the key packet,
before the first userid packet.

When a key compromise certificate is submitted to PGP, PGP will place
it on the public keyring.  A key compromise certificate is always
accompanied in its travels by the public key and userIDs it affects.
If the affected key is NOT already on the keyring, the compromise
certificate (and its key and user ID) is merely added to the keyring
anywhere.  If the affected key IS already on the keyring, the
compromise certificate is inserted after the affected key packet.
This assumes that the actual key packet is identical to the one
already on the key ring, so no duplicate key packet is needed.
If a key has been revoked, PGP will not allow its use to encipher any
messages, and if an incoming signature uses it, PGP will display a
stern warning that this key has been revoked.

NOTE:  Key/userid revocation certificates ARE NOT SUPPORTED in
this version of PGP.  But if we ever get around to supporting them,
here are some ideas on how they should work...

A key/userid revocation certificate may be issued by someone to
dissolve the association between his own key and a user ID.  He would
sign it with the very key that is being revoked.  A key/userid
revocation certificate issued by the keyholder shall take precedence
over any other key certifications made by anyone else for that
key/userid pair.  Also, a third party certifier may revoke his own
previous certification of this key/userid pair by issuing a
key/userid revocation certificate.  Such a revocation should not
affect the certifications by other third parties for this same
key/userid pair.

When a key/userid revocation certificate is submitted to PGP, PGP
will place it on the public keyring.  A key/userid revocation
certificate is always accompanied in its travels by the public key it
affects (the key packet and user ID packet precedes the revocation
certificate).  If the affected key is NOT already on the keyring, the
revocation certificate (and its key and user ID) is merely added to
the keyring anywhere.  If the affected key IS already on the keyring,
the revocation certificate is integrated in with the key's other
certificates as though it were just another key certification.  This
assumes that the actual key packet is identical to the one already on
the key ring, so no duplicate key packet is needed.



Message digest "packet"
-----------------------

The Message digest has no CTB packet framing.  It is stored
packetless and naked, with padding, encrypted inside the MPI in the
Signature packet.

PGP versions 2.3 and later use a new format for encoding the message
digest into the MPI in the signature packet, a format which is
compatible with RFC1425 (formerly RFC1115).  This format is accepted
but not written by version 2.2.  The older format used by versions 2.2
is acepted by versions up to 2.4, but the RSAREF code in 2.5 is
not capable of parsing it.

PGP versions 2.2 and earlier encode the MD into the MPI as follows:

        MSB             .   .   .                LSB

         0   1   MD(16 bytes)   0   FF(n bytes)   1

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

PGP versions 2.3 and later encode the MD into the MPI as follows:

        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:

        3020300c06082a864886f70d020505000410

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

All this mainly affects the rsa_private_encrypt() and rsa_public_decrypt()
functions in rsaglue.c.

There is no checksum included.  The padding serves to verify that the
correct RSA key was used.


Conventional Data Encryption Key (DEK) "packet"
-----------------------------------------------

The DEK has no CTB packet framing.  The DEK is stored packetless and
naked, with padding, encrypted inside the MPI in the RSA
public-key-encrypted packet.

PGP versions 2.3 and later use a new format for encoding the message
digest into the MPI in the signature packet.  (This format is not
presently based on any RFCs due to the use of the IDEA encryption
system.)  This format is accepted but not written by version 2.2.  The
older format used by versions 2.2 and earlier is also accepted by
versions up to 2.4, but the RSAREF code in 2.5 is unable to cope
with it.

PGP versions 2.2 and earlier encode the DEK into the MPI as follows:

        MSB                     .   .   .                          LSB

         0   1   DEK(16 bytes)   CSUM(2 bytes)   0   RND(n bytes)   2

CSUM refers to a 16-bit checksum appended to the high end of the DEK.
RND is a string of NONZERO pseudorandom bytes, enough to make the length
of this whole string equal to the number of bytes in the modulus.

PGP versions 2.3 and later encode the DEK into the MPI as follows:

        MSB                     .   .   .                   LSB

         0   2   RND(n bytes)   0   1   DEK(16 bytes)   CSUM(2 bytes)

CSUM refers to a 16-bit checksum appended to the high end of the DEK.
RND is a string of NONZERO pseudorandom bytes, enough to make the length
of this whole string equal to the number of bytes in the modulus.

For both versions, the 16-bit checksum is computed on the rest of the
bytes in the DEK key material, and does not include any other material
in the calculation.  In the above MSB-first representation, the
checksum is also stored MSB-first.  The checksum is there to help us
determine if we used the right RSA secret key for decryption.


All this mainly affects the rsa_public_encrypt() and rsa_private_decrypt()
functions in rsaglue.c.



Conventional Key Encrypted data packet
--------------------------------------

Offset  Length  Meaning
0       1       CTB for Conventional-Key-Encrypted data packet
1       4       32-bit (or maybe 16-bit) length of packet
5    ?    conventionally-encrypted data.
        plaintext has 64 bits of random data prepended,
        plus 16 bits prepended for "key check" purposes

The decrypted ciphertext may contain a compressed data packet or a
literal plaintext packet.

After decrypting the conventionally-encrypted data, a special 8-byte
random prefix and 2 "key check" bytes are revealed.  The random prefix
and key check prefix are inserted before encryption and discarded after
decryption.  This prefix group is visible after decrypting the
ciphertext in the packet.

The random prefix serves to start off the cipher feedback chaining
process with 64 bits of random material.  It may be discarded after
decryption.  The first 8 bytes is the random prefix material, followed
by the 2-byte "key-check" prefix.

The key-check prefix is composed of two identical copies of the last
2 random bytes in the random prefix, in the same order.  During
decryption, the 9th and 10th bytes of decrypted plaintext are checked
to see if they match the 7th and 8th bytes, respectively.  If these
key-check bytes meet this criterion, then the conventional key is
assumed to be correct.

One unusual point about the way encryption is done.  Using the IDEA
cipher in CFB mode, the first 10 bytes are decrypted normally,
but bytes 10 to 17, the first 8 bytes of the data proper, are
encrypted using bytes 2 to 9 (the last 8 bytes of the key check
prefix) as the IV.  This is essentially using CFB-16 for one
part of the encryption, while CFB-64 is used elsewhere.


Compressed data packet
----------------------

Offset  Length  Meaning
0       1       CTB for Compressed data packet
1    1    Compression algorithm selector byte (1=ZIP)
2    ?    compressed data

The compressed data begins right after the algorithm selector byte.
The compressed data may decompress into a raw literal plaintext data
packet with its own CTB.  Currently, compressed data packets
are always the last ones in their enclosing object, and the decompressor
knows when to stop, so the length field is omitted.  The low two bits
of the CTB are set to 11.  This is the only case in PGP where this
is currently done.


Literal data packet, with filename and mode
-------------------------------------------

Offset  Length  Meaning
0       1       CTB for raw literal data packet
1       4       32-bit (or maybe 16-bit) length of packet
5    1    mode byte, 'b'= binary or 't'= canonical text
6    ?    filename, with leading string length byte
?    4    Timestamp of last-modified date, or 0, or right now
?    ?    raw literal plaintext data

The timestamp may be have to be derived in a system dependent manner.
ANSI C functions should be used to get it if available, otherwise
store the current time in it.  Or maybe store 0 if it's somehow not
applicable.

Whne calculating a signature on a literal packet, the signature
calculation only includes the raw literal plaintext data that begins
AFTER the header fields in the literal packet-- after the CTB, the
length, the mode byte, the filename, and the timestamp.  The reason
for this is to guarantee that detached signatures are exactly the
same as attached signatures prefixed to the message.  Detached
signatures are calculated on a separate file that has no packet
encapsulation.



Comment packet
--------------

A comment packet is generally just skipped over by PGP, although it
may be displayed to the user when processed.  It can be put in a
keyring, or anywhere else.

Offset  Length  Meaning
0       1       CTB for Comment packet
1       1       8-bit length of packet
2       ?       ASCII comment, size is as in preceding length byte

Comment packets are currently not generated by PGP.



Secret key certificate
----------------------

Offset  Length  Meaning
0       1       CTB for secret key certificate
1       2       16-bit (or maybe 8-bit) length of packet
3    1    Version byte, may affect rest of fields that follow.
                (=2) for PGP versions <= 2.5
                (=3) for PGP versions >= 2.6
4       4       Timestamp
8       2       Validity period, in number of DAYS (0 means forever)
10    1    Algorithm byte for RSA (=1 for RSA).
        --Algorithm byte affects field definitions that follow.
?       ?       MPI of RSA public modulus n
?       ?       MPI of RSA public encryption exponent e

?    1    Algorithm byte for cipher that protects following
        secret components (0=unencrypted, 1=IDEA cipher)
?    8    Cipher Feedback IV for cipher that protects secret
        components (not present if unencrypted)
?       ?       MPI of RSA secret decryption exponent d
?       ?       MPI of RSA secret factor p
?       ?       MPI of RSA secret factor q
?       ?       MPI of RSA secret multiplicative inverse u
                (All MPI's have bitcount prefixes)
?    2    16-bit checksum of all preceding secret component bytes

All secret fields in the secret key certificate may be password-
encrypted, including the checksum.  The checksum is calculated from
all of the bytes of the unenciphered secret components.  The public
fields are not encrypted.  The encrypted fields are done in CFB mode,
and the checksum is used to tell if the password was good.  The CFB
IV field is just encrypted random data, assuming the "true" IV was
zero.

NOTE:  The secret key packet does not contain a User ID field.  The
User ID is enclosed in a separate packet that always follows the secret
key packet on a keyring or in any other context.


Public key certificate
----------------------

Offset  Length  Meaning
0       1       CTB for public key certificate
1       2       16-bit (or maybe 8-bit) length of packet
3    1    Version byte, may affect rest of fields that follow.
                (=2) for PGP versions <= 2.5
                (=3) for PGP versions >= 2.6
4       4       Timestamp of key creation
8       2       Validity period, in number of DAYS (0 means forever)
10    1    Algorithm byte for RSA (=1 for RSA).
        --Algorithm byte affects field definitions that follow.
?       ?       MPI of RSA public modulus n
?       ?       MPI of RSA public encryption exponent e
                (All MPI's have bitcount prefixes)

NOTE:  The public key packet does not contain a User ID field.  The
User ID is enclosed in a separate packet that always follows
somewhere after the public key packet on a keyring or in any other
context.

The validity period is currently always set to 0.



User ID packet
--------------

Offset  Length  Meaning
0       1       CTB for User ID packet
1       1       8-bit length of packet
2       ?       User ID string, size is as in preceding length byte

The User ID packet follows a public key on a public key ring.  It
also follows a secret key on a secret key ring.

When a key is certified by a signature, the signature covers both the
public key packet and the User ID packet.  The signature certificate
thereby logically "binds" together the user ID with the key.  The
user ID packet is always associated with the most recently occurring
public key on the key ring, regardless of whether there are other
packet types appearing between the public key packet and the
associated user ID packet.

There may be more than one User ID packet after a public key packet.
They all would be associated with the preceding public key packet.


Keyring trust packet
--------------------

The three different forms of this packet each come after: a public key
packet, a user ID packet, or a signature packet on the public key
ring.  They exist only on a public key ring, and are never extracted
with a key.  Don't copy this separate trust byte packet from keyring,
and do add it in back in when adding to keyring.

The meaning of the keyring trust packet is context sensitive.  The
trust byte has three different definitions depending on whether it
follows a key packet on the ring, or follows a user ID packet on the
ring, or follows a signature on the ring.

Offset  Length  Meaning
0       1       CTB for Keyring trust packet
1       1       8-bit length of packet (always 1 for now)
2       1       Trust flag byte, with context-sensitive bit
                definitions given below.


For trust bytes that apply to the preceding key packet, the following
bit definitions apply:

  Bits 0-2 - OWNERTRUST bits - Trust bits for this key owner.  Values are:
       000 - undefined, or uninitialized trust.
       001 - unknown, we don't know the owner of this key.
       010 - We usually do not trust this key owner to sign other keys.
       011 - reserved
       100 - reserved
       101 - We usually do trust this key owner to sign other keys.
       110 - We always trust this key owner to sign other keys.
       111 - This key is also present in the secret keyring.
  Bits 3-4 - Reserved.
  Bit 5 - DISABLED bit - Means that this key is disabled, and
          should not be used.
  Bit 6 - Reserved
  Bit 7 - BUCKSTOP bit - Means this key also appears in secret key ring.
          Signifies the ultimately-trusted "keyring owner".
          "The buck stops here".  This bit computed from looking
          at secret key ring.  If this bit is set, then all the
          KEYLEGIT fields are set to maximum for all the user IDs for
          this key, and OWNERTRUST is also set to ultimate trust.

For trust bytes that apply to the preceding user ID packet, the
following bit definitions apply:

  Bit 0-1 - KEYLEGIT bits - Validity bits for this key.
          Set if we believe the preceding key is legitimately owned by
          who it appears to belong to, specified by the preceding user
          ID.  Computed from various signature trust packets that
          follow.  Also, always fully set if BUCKSTOP is set.
          To define the KEYLEGIT byte does not require that
          OWNERTRUST be nonzero, but OWNERTRUST nonzero does require
          that KEYLEGIT be fully set to maximum trust.
       00 - unknown, undefined, or uninitialized trust.
       01 - We do not trust this key's ownership.
       10 - We have marginal confidence of this key's ownership.
            Totally useless for certifying other keys, but may be useful
            for checking message signatures with an advisory warning
            to the user.
       11 - We completely trust this key's ownership.
        This requires either:
        - 1 ultimately trusted signature (a signature from
          yourself, SIGTRUST=111)
        - COMPLETES_NEEDED completely trusted signatures
          (SIGTRUST=110)
        - MARGINALS_NEEDED marginally trusted signatures
          (SIGTRUST=101)
        COMPLETES_NEEDED and MARGINALS_NEEDED are configurable
        constants.
  Bit 7 - WARNONLY bit - If the user wants to use a not fully validated
      key for encryption, he is asked if he really wants to use this
      key.  If the user answers 'yes', the WARNONLY bit gets set,
      and the next time he uses this key, only a warning will be
      printed. This bit gets cleared during the maintenance pass.

For a trust byte that applies to the preceding signature, the
following bit definitions apply:

  Bits 0-2 - SIGTRUST bits - Trust bits for this signature.  Value is
             copied directly from OWNERTRUST bits of signer:
       000 - undefined, or uninitialized trust.
       001 - unknown
       010 - We do not trust this signature.
       011 - reserved
       100 - reserved
       101 - We reasonably trust this signature.
       110 - We completely trust this signature.
       111 - ultimately trusted signature (from the owner of the ring)
  Bits 3-6 - Reserved.
  Bit 6 - CHECKED bit - This means that the key checking pass (pgp -kc,
          also invoked automatically whenever keys are added to the
          keyring) has tested this signature and found it good.  If
          this bit is not set, the maintenance pass considers this
          signature untrustworthy.
  Bit 7 - CONTIG bit - Means this signature leads up a contiguous trusted
          certification path all the way back to the ultimately-
          trusted keyring owner, where the buck stops.  This bit is derived
          from other trust packets.  It is currently not used for anything
          in PGP.

The OWNERTRUST bits are set by the user.  PGP does not modify them.
PGP computes the BUCKSTOP bit by checking to see if the key is on the
secret key ring.  If it is, it was created by this user, and thus
controlled by him.

All other trust is derived from the BUCKSTOP keys in a special
maintenance pass over the keyring.  Any good signature made by a given
key has its SIGTRUST equal to the key's OWNERTRUST.  Based on
COMPLETES_NEEDED and MARGINALS_NEEDED, if enough trusted signatures are
on a key/userID pair, the key/userid association is considered
legitimate.

To be precise, an ultimately trusted key has weight 1, a completely
trusted key has weight 1/COMPLETES_NEEDED (or 0 if COMPLETES_NEEDED
is 0), and a marginally trsuted key has weight 1/MARGINALS_NEEDED.
Other trust values have weight 0.  If the total weight of the signatures
on a key/userid pair is 1 or more, the userid is considered legitimate.

When a key has a legitimate userid, the user is asked to set the
OWNERTRUST for the corresponding key.  Ths idea is that the userid
identifies someone the user knows, at least by reputation, so once it
has been established who holds the secret key, that person's
trustworthiness as an introducer can be established and assigned to the
key.

Once that is done, the key's signatures then have weight establishing
other key/userid associations.

There is a limit to the depth to which this can go.  Keys on the secret
keyring are at depth 0.  Keys signed by those keys are at depth 1.
Keys which are fully certified using only signatures from keys at depth
1 or less are at depth 2.  Keys which are fully certified using only
signatures from keys at depth 2 or less are at depth 3, and so on.

If you know all of your trusted introducers personally, and have signed
their keys, then you will never have a key at a depth of greater than 2.
The maximum depth is limited my MAX_CERT_DPETH.  It never gets very large
in a well-connected "web of trust".

This redundant and decentralized method of determining public key
legitimacy is one of the principal strengths of PGP's key management
architecture, as compared with PEM, when used in social structures
that are not miltiary-style rigid hierarchies.

The trust of a key owner (OWNERTRUST) does not just reflect our
estimation of their personal integrity, it also reflects how competent
we think they are at understanding key management and using good
judgement in signing keys.  The OWNERTRUST bits are not computed from
anything -- it requires asking the user for his opinion.

To define the OWNERTRUST bits for a key owner, ask:
    Would you always trust "Oliver North"
    to certify other public keys?
    (1=Yes, 2=No, 3=Usually, 4=I don't know) ? _

When a key is added to the key ring the trust bytes are initialized
to zero (undefined).


[--manual setting of SIGTRUST/OWNERTRUST not implemented]
Normally, we derive the value of the SIGTRUST field by copying it
directly from the signer key's OWNERTRUST field.  Under special
circumstances, if the user explicitly requests it with a special PGP
command, we may let the user override the copied value for SIGTRUST
by displaying an advisory to him and asking him for ratification,
like so:
    This key is signed by "Oliver North",
    whom you usually trust to sign keys.
    Do you trust "Oliver North"
    to certify the key for "Daniel Ellsberg"?
    (1=Yes, 2=No, 3=Somewhat, 4=I don't know) ? _      <default is yes>

Or:
    This key is signed by "Oliver North",
    whom you usually do not trust to sign keys.
    Do you trust "Oliver North"
    to certify the key for "Daniel Ellsberg"?
    (1=Yes, 2=No, 3=Somewhat, 4=I don't know) ? _      <default is no>

An "I don't know" response to this question would have the same
effect as a response of "no".

If we had no information about the trustworthiness of the signer (the
OWNERTRUST field was uninitialized), we would leave the advisory note
off.


Certifying a public key is a serious matter, essentially promising to
the world that you vouch for this key's ownership.  But sometimes I
just want to make a "working assumption" of trust for someone's
public key, for my own purposes on my own keyring, without taking the
serious step of actually certifying it for the rest of the world.  In
that case, we can use a special PGP keyring management command to
manually set the KEYLEGIT field, without relying on it being computed
during a maintenance pass.  Later, if a maintenance pass discovers a
KEYLEGIT bit set that would not have been otherwise computed as set
by the maintenance pass logic, it alerts me and asks me to confirm
that I really want it set.

[--end of not implemented section]


During routine use of the public keyring, we don't actually check the
associated signatures certifying a public key.  Rather, we always
rely on trust bytes to tell us whether to trust the key in question.
We depend on a separate checking pass (pgp -kc) to actually check the key
signature certificates against the associated keys, and to set the
trust bytes accordingly.  This pass checks signatures, and if a
signature fails to verify, obnoxiously alerts the user and drops it from
the key ring.  Then it tuns a maintenance pass to calculate the
ring-wide effects of this.

A failed signature should be exceedingly rare, and it may not even
result in a KEYLEGIT field being downgraded.  Having several signatures
certifying each key should prevent damage from spreading too far from a
failed certificate.  But if dominoes do keep falling from this, it may
indicate the discovery of an important elaborate attack.

The maintenance pass is run every time the keyring changes, and
operates in a top-of-pyramid-down manner as follows.

If at any time during any of these steps the KEYLEGIT field goes from
not fully set to fully set, and the OWNERTRUST bits are still undefined,
the user is asked a question to define the OWNERTRUST bits.  First, for
all keys with BUCKSTOP set, check if they are really present in the
secret keyring, if not, the BUCKSTOP bit is cleared.  SIGTRUST and
KEYLEGIT is initialized to zero for non-buckstop keys.

The real maintenance pass is done in a recursive scan:  Start with
BUCKSTOP keys, find all userid/key pairs signed by a key and update
the trust value of these signatures by copying the OWNERTRUST of the
signer to the SIGTRUST of the signature.  If this makes a key fully
validated, start looking for signatures made by this key, and update
the trust value for them.  Repeat until everything has settled down.




Public Key Ring Overall Structure
=================================

A public key ring is comprised of a series of public key packets,
keyring trust packets, user ID packets, and signature certificates.

Here is an example of an ordered collection of packets on a ring:

--------------------------------------------------------------------
  Public key packet
      Keyring trust packet for preceding key
    User ID packet for preceding key
        Keyring trust packet for preceding user ID/key association
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate

  Public key packet
      Keyring trust packet for preceding key
    User ID packet for preceding key
        Keyring trust packet for preceding user ID/key association
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate
    User ID packet for preceding key
        Keyring trust packet for preceding user ID/key association
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate

  Public key packet
      Keyring trust packet for preceding key
    Compromise certificate for preceding key
    User ID packet for preceding key
        Keyring trust packet for preceding user ID/key association
      Signature certificate to bind preceding User ID and key pkt
          Keyring trust packet for preceding signature certificate
--------------------------------------------------------------------