gnupg/doc/DETAILS

    * For packet version 3 we calculate the keyids this way:
	RSA	:= low 64 bits of n
	ELGAMAL := build a v3 pubkey packet (with CTB 0x99) and calculate
		   a rmd160 hash value from it. This is used as the
		   fingerprint and the low 64 bits are the keyid.

    * Revocation certificates consists only of the signature packet;
      "import" knows how to handle this.  The rationale behind it is
      to keep them small.




Layout of the TrustDB
=====================
FIXME: use a directory record as top node instead of the pubkey record

The TrustDB is build from fixed length records, where the first bytes
describes the record type.  All numeric values are stored in network
byte order. The length of each record is 40 bytes. The first record of
the DB is always of type 1 and this is the only record of this type.

Record type 0:
--------------
    Unused record, can be reused for any purpose.

Record type 1:
--------------
    Version information for this TrustDB.  This is always the first
    record of the DB and the onyl one with type 1.
     1 byte value 1
     3 bytes 'gpg'  magic value
     1 byte Version of the TrustDB
     3 byte reserved
     1 u32  locked by (pid) 0 = not locked.
     1 u32  timestamp of trustdb creation
     1 u32  timestamp of last modification
     1 u32  timestamp of last validation
	    (Used to keep track of the time, when this TrustDB was checked
	     against the pubring)
     1 u32  reserved
     1 byte marginals needed
     1 byte completes needed
     1 byte max. cert depth
	    If any of this 3 values are changed, all cache records
	    muts be invalidated.
     9 bytes reserved


Record type 2: (directory record)
--------------
    Informations about a public key certificate.
    These are static values which are never changed without user interaction.

     1 byte value 2
     1 byte   reserved
     8 bytes keyid (We keep it here to speed up searching by keyid)
     1 u32   Local-Id.	This is simply the record number of this record.
     1 u32   pubkey (record number of it)
     1 u32   cache record
     1 u32   sigrecord
     1 byte  No signatures flag  (used to avoid duplicate building).
     13 byte reserved


Record type 3:
--------------
    Informations about a public key certificate.
    These are static values which are never changed without user interaction.

     1 byte value 3
     1 byte   reserved
     1 u32   owner  This is used to bind all records for
	     a given certificate together. It is valid only in this TrustDB
	     and usefull if we have duplicate keyids
	     It points back to the directory node.
     1 byte pubkey algorithm
     1 byte reserved
     20 bytes fingerprint of the public key
     1 byte ownertrust:
     3 byte reserved


Record type 4:	(cache record)
--------------
    Used to bind the trustDB to the concrete instance of keyblock in
    a pubring. This is used to cache informations.

     1 byte   value 4
     1 byte   reserved
     1 u32    Local-Id.
     8 bytes  keyid of the primary key (needed?)
     1 byte   cache-is-valid the following stuff is only
	      valid if this is set.
     1 byte   reserved
     20 bytes rmd160 hash value over the complete keyblock
	      This is used to detect any changes of the keyblock with all
	      CTBs and lengths headers. Calculation is easy if the keyblock
	      is optained from a keyserved: simply create the hash from all
	      received data bytes.

     1 byte   number of untrusted signatures.
     1 byte   number of marginal trusted signatures.
     1 byte   number of fully trusted signatures.
	      (255 is stored for all values greater than 254)
     1 byte   Trustlevel
		0 = undefined (not calculated)
		1 = unknown
		2 = not trusted
		3 = marginally trusted
		4 = fully trusted
		5 = ultimately trusted (have secret key too).

Record type 5  (sigrec)
-------------
    Used to keep track of valid key signatures. Self-signatures are not
    stored.

     1 byte   value 5
     1 byte   reserved
     1 u32    For Local-Id (points back to the directory record)
     1 u32    chain: next sigrec of this owner or 0 to indicate the
	      last sigrec.
     6 times
	1 u32  Local_id of signators pubkey record
	1 byte reserved


Record Type 6 (hash table)
-------------
    Due to the fact that we use the keyid to lookup keys, we can
    implement quick access by some simple hash methods, and avoid
    the overhead gdbm.	A property of keyids is that they can be
    used directly as hash value (They can be considered as strong
    random numbers.
      What we use is a dynamic multilevel architecture, which combines
    Hashtables, record lists, and linked list.

    This record is a hashtable of 256 entries; a special property
    is, that all these records are adjacent stored to make up one
    big table. The hash value is simple the 1st, 2nd, ... byte of
    the keyid (depending on the indirection level).

     1 byte value 5
     1 byte reserved
     n u32  recnum; n depends on th record length:
	    n = (reclen-2)/4  which yields 9 for the current record length
	    of 40 bytes.

    the total number of surch record which makes up the table is:
	 m = (256+n-1) / n
    which is 29 for a record length of 40.

    To look up a key we use its lsb to get the recnum from this
    hashtable and look up this addressed record:
       - If this record is another hashtable, we use 2nd lsb
	 to index this hast table and so on.
       - if this record is of hashlist, we lwalk thru these
	 reclist record until we found one whos hash fields
	 matches the MSB of our keyid, and lookup this record
       - if this record is a dir record, we compare the
	 keyid and if this is correct, we get the keyrecod and compare
	 the fingerprint to decide wether it is the requested key;
	 if this is not the correct dir record, we look at the next
	 dir record which is linked by the link field.

Record type 7  (hash list)
-------------
    see hash table for an explanation.

    1 byte value 6
    1 byte reserved
    1 u32  chain	 next hash list record
    n times		 n = (reclen-6)/5
	1 byte hash
	1 u32  recnum

    For the current record length of 40, n is 6




Packet Headers
===============

GNUPG uses PGP 2 packet headers and also understand OpenPGP packet header.
There is one enhavement used ith the old style packet headers:

   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).

+  This will be changed with another version, where the new meaning of
+  the value 11 (see below) will also take place.
+
+  A value of 11 for other packets enables a special length encoding,
+  which is used in case, where the length of the following packet can
+  not be determined prior to writing the packet; especially this will
+  be used if large amounts of data are processed in filter mode.
+
+  It works like this: After the CTB (with a length field of 11) a
+  marker field is used, which gives the length of the following datablock.
+  This is a simple 2 byte field (MSB first) containig the amount of data
+  following this field, not including this length field. After this datablock
+  another length field follows, which gives the size of the next datablock.
+  A value of 0 indicates the end of the packet. The maximum size of a
+  data block is limited to 65534, thereby reserving a value of 0xffff for
+  future extensions. These length markers must be insereted into the data
+  stream just before writing the data out.
+
+  This 2 byte filed is large enough, because the application must buffer
+  this amount of data to prepend the length marker before writing it out.
+  Data block sizes larger than about 32k doesn't make any sense. Note
+  that this may also be used for compressed data streams, but we must use
+  another packet version to tell the application that it can not assume,
+  that this is the last packet.