gnupg/doc/HACKING

# HACKING                                                       -*- org -*-
#+TITLE: A Hacker's Guide to GnuPG
#+TEXT: Some notes on GnuPG internals
#+STARTUP: showall
#+OPTIONS: ^:{}

* How to contribute

  The following stuff explains some basic procedures you need to
  follow if you want to contribute code or documentation.

** No more ChangeLog files

Do not modify any of the ChangeLog files in GnuPG.  Starting on
December 1st, 2011 we put change information only in the GIT commit
log, and generate a top-level ChangeLog file from logs at "make dist"
time.  As such, there are strict requirements on the form of the
commit log messages.  The old ChangeLog files have all be renamed to
ChangeLog-2011

** Commit log requirements

Your commit log should always start with a one-line summary, the
second line should be blank, and the remaining lines are usually
ChangeLog-style entries for all affected files.  However, it's fine
--- even recommended --- to write a few lines of prose describing the
change, when the summary and ChangeLog entries don't give enough of
the big picture.  Omit the leading TABs that you are seeing in a
"real" ChangeLog file, but keep the maximum line length at 72 or
smaller, so that the generated ChangeLog lines, each with its leading
TAB, will not exceed 80 columns.  If you want to add text which shall
not be copied to the ChangeLog, separate it by a line consisting of
two dashes at the begin of a line.

The one-line summary usually starts with a keyword to identify the
mainly affected subsystem.  If more than one keyword is required the
are delimited by a comma (e.g. =scd,w32:=). Commonly found keywords
are

 - agent   :: The gpg-agent component
 - ssh     :: The ssh-agent part of the agent
 - common  :: Code in common
 - iobuf   :: The IOBUF system in common
 - gpg     :: The gpg or gpgv components
 - gpgsm   :: The gpgsm component
 - scd     :: The scdaemon component
 - ccid    :: The CCID driver in scdaemon
 - dirmngr :: The dirmngr component
 - wks     :: The web key service tools
 - tools   :: Other code in tools
 - w32     :: Windows related code
 - po      :: Translations
 - build   :: Changes to the build system
 - speedo  :: Speedo build system specific changes
 - doc     :: Documentation changes
 - indent  :: Indentation and similar changes

Typo fixes and documentation updates don't need a ChangeLog entry;
thus you would use a commit message like

#+begin_example
Fix typo in a comment

--
#+end_example

The marker line here is important; without it the first line would
appear in the ChangeLog.

If you exceptionally need to have longer lines in a commit log you may
do this after this scissor line:
#+begin_example
# ------------------------ >8 ------------------------
#+end_example
(hash, blank, 24 dashes, blank, scissor, blank, 24 dashes).
Note that such a comment will be removed if the git commit option
=--cleanup=scissor= is used.

** License policy

  GnuPG is licensed under the GPLv3+ with some files under a mixed
  LGPLv3+/GPLv2+ license.  It is thus important, that all contributed
  code allows for an update of the license; for example we can't
  accept code under the GPLv2(only).

  GnuPG used to have a strict policy of requiring copyright
  assignments to the FSF.  To avoid this major organizational overhead
  and to allow inclusion of code, not copyrighted by the FSF, this
  policy has been relaxed on 2013-03-29.  It is now also possible to
  contribute code by asserting that the contribution is in accordance
  to the "Libgcrypt Developer's Certificate of Origin" as found in the
  file "DCO".  (Except for a slight wording change, this DCO is
  identical to the one used by the Linux kernel.)

  If you want to contribute code or documentation to GnuPG and you
  didn't sign a copyright assignment with the FSF in the past, you
  need to take these simple steps:

  - Decide which mail address you want to use.  Please have your real
    name in the address and not a pseudonym.  Anonymous contributions
    can only be done if you find a proxy who certifies for you.

  - If your employer or school might claim ownership of code written
    by you; you need to talk to them to make sure that you have the
    right to contribute under the DCO.

  - Send an OpenPGP signed mail to the gnupg-devel@gnupg.org mailing
    list from your mail address.  Include a copy of the DCO as found
    in the official master branch.  Insert your name and email address
    into the DCO in the same way you want to use it later.  Example:

      Signed-off-by: Joe R. Hacker <joe@example.org>

    (If you really need it, you may perform simple transformations of
    the mail address: Replacing "@" by " at " or "." by " dot ".)

  - That's it.  From now on you only need to add a "Signed-off-by:"
    line with your name and mail address to the commit message.  It is
    recommended to send the patches using a PGP/MIME signed mail.

** Coding standards

  Please follow the GNU coding standards.  If you are in doubt consult
  the existing code as an example.  Do no re-indent code without a
  need.  If you really need to do it, use a separate commit for such a
  change.

  - Only certain C99 features may be used (see below); in general
    stick to C90.
  - Please do not use C++ =//= style comments.
  - Try to fit lines into 80 columns.
  - Ignore signed/unsigned pointer mismatches
  - No arithmetic on void pointers; cast to char* first.
  - We use our own printf style functions like =es_printf=, and
    =gpgrt_asprintf= (or the =es_asprintf= macro) which implement most
    C99 features with the exception of =wchar_t= (which should anyway
    not be used).  Please use them always and do not resort to those
    provided by libc.  The rationale for using them is that we know
    that the format specifiers work on all platforms and that we do
    not need to chase platform dependent bugs.  Note also that in
    gnupg asprintf is a macro already evaluating to gpgrt_asprintf.
  - It is common to have a label named "leave" for a function's
    cleanup and return code.  This helps with freeing memory and is a
    convenient location to set a breakpoint for debugging.
  - Always use xfree() instead of free().  If it is not easy to see
    that the freed variable is not anymore used, explicitly set the
    variable to NULL.
  - Init function local variables only if needed so that the compiler
    can do a better job in detecting uninitialized variables which may
    indicate a problem with the code.
  - Never init static or file local variables to 0 to make sure they
    end up in BSS.
  - Use --enable-maintainer-mode with configure.

** Variable names

  Follow the GNU standards.  Here are some conventions you may want to
  stick to (do not rename existing "wrong" uses without a goog
  reason).

  - err :: This conveys an error code of type =gpg_error_t= which is
           compatible to an =int=.  To compare such a variable to a
           GPG_ERR_ constant, it is necessary to map the value like
           this: =gpg_err_code(err)=.
  - ec  :: This is used for a gpg-error code which has no source part
           (=gpg_err_code_t=) and will eventually be used as input to
           =gpg_err_make=.
  - rc  :: Used for all kind of other errors; for example system
           calls.  The value is not compatible with gpg-error.


*** C99 language features

  In GnuPG 2.x, but *not in 1.4* and not in most libraries, a limited
  set of C99 features may be used:

  - Variadic macros:
    : #define foo(a,...)  bar(a, __VA_ARGS__)

  - The predefined macro =__func__=:
    : log_debug ("%s: Problem with foo\n", __func__);

  - Variable declaration inside a for():
    : for (int i = 0; i < 5; ++)
    :   bar (i);

  Although we usually make use of the =u16=, =u32=, and =u64= types,
  it is also possible to include =<stdint.h>= and use =int16_t=, =int32_t=, =int64_t=, =uint16_t=, =uint32_t=, and =uint64_t=.  But do
  not use =int8_t= or =uint8_t=.

** Commit log keywords

  - GnuPG-bug-id :: Values are comma or space delimited bug numbers
                    from bug.gnupg.org pertaining to this commit.
  - Debian-bug-id :: Same as above but from the Debian bug tracker.
  - CVE-id :: CVE id number pertaining to this commit.
  - Regression-due-to :: Commit id of the regression fixed by this commit.
  - Fixes-commit :: Commit id this commit fixes.
  - Reported-by :: Value is a name or mail address of a bug reporte.
  - Suggested-by :: Value is a name or mail address of someone how
                    suggested this change.
  - Co-authored-by :: Name or mail address of a co-author
  - Some-comments-by :: Name or mail address of the author of
                        additional comments (commit log or code).
  - Proofread-by :: Sometimes used by translation commits.
  - Signed-off-by :: Name or mail address of the developer

* Windows
** How to build an installer for Windows

   Your best bet is to use a decent Debian System for development.
   You need to install a long list of tools for building.  This list
   still needs to be compiled.  However, the build process will stop
   if a tool is missing.  GNU make is required (on non GNU systems
   often installed as "gmake").  The installer requires a couple of
   extra software to be available either as tarballs or as local git
   repositories.  In case this file here is part of a gnupg-w32-2.*.xz
   complete tarball as distributed from the same place as a binary
   installer, all such tarballs are already included.

   Cd to the GnuPG source directory and use one of one of these
   command:

   - If sources are included (gnupg-w32-*.tar.xz)

     make -f build-aux/speedo.mk WHAT=this installer

   - To build from tarballs

     make -f build-aux/speedo.mk WHAT=release TARBALLS=TARDIR installer

   - To build from local GIT repos

     make -f build-aux/speedo.mk WHAT=git TARBALLS=TARDIR installer

   Note that also you need to supply tarballs with supporting
   libraries even if you build from git.  The makefile expects only
   the core GnuPG software to be available as local GIT repositories.
   speedo.mk has the versions of the tarballs and the branch names of
   the git repositories.  In case of problems, don't hesitate to ask
   on the gnupg-devel mailing for help.

* Debug hints

  See the manual for some hints.

* Standards
** RFCs

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 (obsolete)

2144  The CAST-128 Encryption Algorithm.

2279  UTF-8, a transformation format of ISO 10646.

2440  OpenPGP (obsolete).

3156  MIME Security with Pretty Good Privacy (PGP).

4880  Current OpenPGP specification.

6337  Elliptic Curve Cryptography (ECC) in OpenPGP

* Various information

** Directory Layout

  - ./	      :: Readme, configure
  - ./agent   :: Gpg-agent and related tools
  - ./doc     :: Documentation
  - ./g10     :: Gpg program here called gpg2
  - ./sm      :: Gpgsm program
  - ./jnlib   :: Not used (formerly used utility functions)
  - ./common  :: Utility functions
  - ./kbx     :: Keybox library
  - ./scd     :: Smartcard daemon
  - ./scripts :: Scripts needed by configure and others
  - ./dirmngr :: The directory manager

** Detailed Roadmap

  This list of files is not up to date!

  - g10/gpg.c :: Main module with option parsing and all the stuff you
                 have to do on startup.  Also has the exit handler and
                 some helper functions.

  - 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/keyring.c  :: Keyring access functions
  - g10/keydb.h    ::

  - g10/keyid.c	  :: Helper functions to get the keyid, fingerprint etc.

  - g10/trustdb.c :: Web-of-Trust computations
  - g10/trustdb.h ::
  - g10/tdbdump.c :: Export/import/list the trustdb.gpg
  - g10/tdbio.c   :: I/O handling for the trustdb.gpg
  - g10/tdbio.h   ::

  - 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_t 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/keyserver.h :: Keyserver access dispatcher.
  - g10/packet.h   :: Definition of OpenPGP structures.
  - g10/passphrase.c :: Passphrase handling code

  - g10/pubkey-enc.c :: Process a public key encoded packet.
  - g10/seckey-cert.c :: Not anymore used
  - g10/seskey.c     :: Make sesssion keys etc.
  - g10/import.c     :: Import keys into our key storage.
  - g10/export.c     :: Export keys to the OpenPGP format.
  - g10/sign.c       :: Create signature and optionally encrypt.
  - g10/plaintext.c  :: Process plaintext packets.
  - g10/decrypt-data.c :: Decrypt an encrypted data packet
  - g10/encrypt.c    :: Main encryption driver
  - g10/revoke.c     :: Create recovation certificates.
  - g10/keylist.c    :: Print information about OpenPGP keys
  - g10/sig-check.c  :: Check a signature
  - g10/helptext.c   :: Show online help texts
  - g10/verify.c     :: Verify signed data.
  - g10/decrypt.c    :: Decrypt and verify data.
  - g10/keyedit.c    :: Edit properties of a key.
  - g10/dearmor.c    :: Armor utility.
  - g10/keygen.c     :: Generate a key pair

** Memory allocation

Use only the functions:

 - xmalloc
 - xmalloc_secure
 - xtrymalloc
 - xtrymalloc_secure
 - xcalloc
 - xcalloc_secure
 - xtrycalloc
 - xtrycalloc_secure
 - xrealloc
 - xtryrealloc
 - xstrdup
 - xtrystrdup
 - xfree


The *secure versions allocate memory in the secure memory.  That is,
swapping out of this memory is avoided and is gets overwritten on
free.  Use this for passphrases, session keys and other sensitive
material.  This memory set aside for secure memory is linited to a few
k.  In general the function don't print a memeory message and
terminate the process if there is not enough memory available.  The
"try" versions of the functions return NULL instead.

** Logging

 TODO

** 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
starting 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.