A Hacker's Guide to GNUPG

A Hacker’s Guide to GNUPG
================================
(Some notes on GNUPG internals.)
===> Under construction

1423 Privacy Enhancement for Internet Electronic Mail:
Part III: Algorithms, Modes, and Identifiers.
1489 Registration of a Cyrillic Character Set.
1750 Randomness Recommendations for Security.
1991 PGP Message Exchange Formats.
2015 MIME Security with Pretty Good Privacy (PGP).
2144 The CAST-128 Encryption Algorithm.
2279 UTF-8, a transformation format of ISO 10646.
2440 OpenPGP.
Debug Flags
-----------
Use the option "--debug n" to output debug information. This option
can be used multiple times, all values are ORed; n maybe prefixed with
0x to use hex-values.
value used for
----- ----------------------------------------------
1 packet reading/writing
2 MPI details
4 ciphers and primes (may reveal sensitive data)
8 iobuf filter functions
16 iobuf stuff
32 memory allocation stuff
64 caching
128 show memory statistics at exit
256 trust verification stuff
Directory Layout
----------------
./ Readme, configure
./scripts Scripts needed by configure and others
./doc Documentation
./util General purpose utility function
./mpi Multi precision integer library
./cipher Cryptographic functions
./g10 GnuPG application
./tools Some helper and demo programs
./keybox The keybox library (under construction)
./gcrypt Stuff needed to build libgcrypt (under construction)
Detailed Roadmap
---------------g10/g10.c
Main module with option parsing and all the stuff you have
to do on startup. Also has the exout handler and some
helper functions.

g10/sign.c Create signature and optionally encrypt
g10/parse-packet.c
g10/build-packet.c
g10/free-packet.c
Parsing and creating of OpenPGP message packets.
g10/getkey.c Key selection code
g10/pkclist.c Build a list of public keys
g10/skclist.c Build a list of secret keys
g10/ringedit.c Keyring I/O
g10/keydb.h
g10/keyid.c Helper functions to get the keyid, fingerprint etc.
g10/trustdb.c
g10/trustdb.h
g10/tdbdump.c
Management of the trustdb.gpg
g10/compress.c Filter to handle compression
g10/filter.h Declarations for all filter functions
g10/delkey.c Delete a key
g10/kbnode.c Helper for the KBNODE linked list
g10/main.h Prototypes and some constants
g10/mainproc.c Message processing
g10/armor.c Ascii armor filter
g10/mdfilter.c Filter to calculate hashs
g10/textfilter.c Filter to handle CR/LF and trailing white space
g10/cipher.c En-/Decryption filter
g10/misc.c Utlity functions
g10/options.h Structure with all the command line options
and related constants
g10/openfile.c Create/Open Files
g10/tdbio.c I/O handling for the trustdb.gpg
g10/tdbio.h
g10/hkp.h Keyserver access
g10/hkp.c
g10/packet.h Defintion of OpenPGP structures.
g10/passphrase.c Passphrase handling code
g10/pubkey-enc.c
g10/seckey-cert.c
g10/seskey.c
g10/import.c
g10/export.c
g10/comment.c
g10/status.c
g10/status.h
g10/sign.c
g10/plaintext.c
g10/encr-data.c
g10/encode.c
g10/revoke.c
g10/keylist.c
g10/sig-check.c
g10/signal.c
g10/helptext.c
g10/verify.c
g10/decrypt.c

g10/keyedit.c
g10/dearmor.c
g10/keygen.c
Memory allocation
-----------------
Use only the functions:
m_alloc()
m_alloc_clear()
m_strdup()
m_free()
If you want to store a passphrase or some other sensitive data you may
want to use m_alloc_secure() instead of m_alloc(), as this puts the data
into a memory region which is protected from swapping (on some platforms).
m_free() works for both. This functions will not return if there is not
enough memory available.
Logging
-------
Option parsing
---------------
GNUPG does not use getopt or GNU getopt but functions of it’s own. See
util/argparse.c for details. The advantage of these functions is that
it is more easy to display and maintain the help texts for the options.
The same option table is also used to parse resource files.
What is an IOBUF
----------------
This is the data structure used for most I/O of gnupg. It is similar
to System V Streams but much simpler. Because OpenPGP messages are nested
in different ways; the use of such a system has big advantages. Here is
an example, how it works: If the parser sees a packet header with a partial
length, it pushes the block_filter onto the IOBUF to handle these partial
length packets: from now on you don’t have to worry about this. When it sees
a compressed packet it pushes the uncompress filter and the next read byte
is one which has already been uncompressed by this filter. Same goes for
enciphered packet, plaintext packets and so on. The file g10/encode.c
might be a good staring point to see how it is used - actually this is
the other way: constructing messages using pushed filters but it may be
easier to understand.
How to use the message digest functions
--------------------------------------cipher/md.c
implements an interface to hash (message digest functions).

a) If you have a common part of data and some variable parts
and you need to hash of the concatenated parts, you can use this:
md = md_open(...)
md_write( md, common_part )
md1 = md_copy( md )
md_write(md1, part1)
md_final(md1);
digest1 = md_read(md1)
md2 = md_copy( md )
md_write(md2, part2)
md_final(md2);
digest2 = md_read(md2)
An example are key signatures; the key packet is the common part
and the user-id packets are the variable parts.
b) If you need a running digest you should use this:
md = md_open(...)
md_write( md, part1 )
digest_of_part1 = md_digest( md );
md_write( md, part2 )
digest_of_part1_cat_part2 = md_digest( md );
....
Both methods may be combined. [Please see the source for the real syntax]
How to use the cipher functions
------------------------------cipher/cipher.c
implements the interface to symmetric encryption functions.
As usual you have a function to open a cipher (which returns a handle to be used
with all other functions), some functions to set the key and other stuff and
a encrypt and decrypt function which does the real work. You probably know
how to work with files - so it should really be easy to work with these
functions. Here is an example:
CIPHER_HANDLE hd;
hd = cipher_open( CIPHER_ALGO_TWOFISH, CIPHER_MODE_CFB, 0 );
if( !hd )
oops( use other function to check for the real error );
rc = cipher_setkey( hd, key256bit, 32 ) )
if( rc )
oops( weak key or something like this );
cipher_setiv( hd, some_IV_or_NULL_for_all_zeroes );
cipher_encrypt( hd, plain, cipher, size );
cipher_close( hd );
How to use the public key functions
----------------------------------cipher/pubkey.c
implements the interface to asymmetric encryption and
signature functions. This is basically the same as with the symmetric
counterparts, but due to their nature it is a little bit more complicated.
[Give an example]

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