Copyright (C) 2017, 2018, 2019, 2020  Heiko Stamer <HeikoStamer@gmx.net>
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts. A copy of the license is included in the section entitled
  "APPENDIX: GNU Free Documentation License".

Introduction
------------
  Distributed Privacy Guard (DKGPG) implements Distributed Key Generation (DKG)
  and Threshold Cryptography for OpenPGP. The generated public keys are
  compatible with the standard and thus can be used by any RFC4880-compliant
  application (e.g. GnuPG). The main purposes of this software are distributing
  power among multiple parties, eliminating single points of failure, and
  increasing the difficulty of side-channel attacks on private key material.

  Using well-established multi-party protocols a shared private key and a
  common public key (DSA/ElGamal) are generated. Then further interactive
  protocols perform the private operations like decryption of messages and
  signing of files or keys, provided that a previously defined threshold of
  parties/devices take part in the distributed computation. For completeness
  DKGPG contains programs for public-key encryption and signature verification.

  The security is based on the well-established cryptographic assumption that
  computing discrete logarithms in large prime order subgroups of Z_p (finite
  field) is hard. The current implementation is still experimental and should
  NOT be used for production, because it has been not yet reviewed by an
  independend third party. Some details about design criteria can be found in
  these presentation slides: http://www.nongnu.org/libtmcg/dg81_slides.pdf

  DKGPG is 'Free Software' according to the definition of the Free Software
  Foundation. The source code is released under the GNU General Public License.

Overview
--------
  Distributed Privacy Guard consists of a bunch of command-line programs: 
  The program dkg-generate provides an implementation of distributed key share
  generation for DSA/DSS and the ElGamal cryptosystem. The generation and the
  refresh (the latter is done by a separated program called dkg-refresh) needs
  a lot of strong quality randomness (see e.g. D. Eastlake, J. Schiller, and
  S. Crocker: Randomness Requirements for Security, Network Working Group,
  Request for Comments: 4086, June 2005) obtained by DKGPG from the GNU Crypto
  Library (libgcrypt), that means, the execution will slow down or even fail on
  systems (e.g. headless servers) if only low entropy sources for /dev/random
  (i.e. the source of strong randomness for libgcrypt) are available. The random
  number generation, input/output operations, arithmetic functions, OpenPGP
  support, and some high-level protocols are provided by another library of the
  author, i.e., Toolbox for Mental Card Games (LibTMCG). If this LibTMCG is
  built with optional support of Botan (another C++ crypto library), then the
  aquired random numbers incorporate randomness from Botan::AutoSeeded_RNG as
  an additional source to rely not only on libgcrypt's RNG implementation.

  Due to the interactiveness of the protocols a lot of messages resp. data have
  to be exchanged between participating parties in a secure way. We employ
  GNUnet, and in particular its mesh routed CADET service, to establish private
  point-to-point and separated broadcast channels for this message exchange.
  A so-called reliable broadcast protocol (RBC) from LibTMCG is used as a weak
  mechanism to achieve some validity, consistency, and totality in a most
  likely asynchronous communication environment. However, there are limitations
  (termination is not guaranteed) and predefined timeout values. Timing problems
  may be recognized by looking for error messages with prefix "RBC" on STDERR
  output of the interactive programs. In such cases the predefined timeout
  value (default: five minutes) can be adjusted by using option "-W".

  The program dkg-encrypt encrypts a text message for a given public key in
  OpenPGP format. It uses AES256 as fixed symmetric cipher and most of the
  public-key algorithms defined by OpenPGP, even AEAD algorithms from draft
  RFC 4880bis, if they are supported by libgcrypt. Of course, a similar result
  can be achieved by any OpenPGP-compatible implementation --- the program is
  only included for completeness. Conversely, a corresponding threshold
  decryption protocol (see Ronald Cramer, Rosario Gennaro, and Berry
  Schoenmakers: A Secure and Optimally Efficient Multi-Authority Election
  Scheme, Advances in Cryptology – EUROCRYPT ’97, LNCS 1233, pp. 103–118, 1997)
  has been implemented by the program dkg-decrypt. It provides an interactive
  version and a non-interactive version for computing, verifying, and combining
  the required decryption shares from participating players. Finally, if
  everything is sane, it outputs the decrypted message. The included OpenPGP
  signatures can be verified by using the option "-k" with a keyring that
  contains the required public keys.

  Since version 1.0.8 most DKGPG programs support a new option "-y" that turns
  them into a regular OpenPGP implementation operating with any transferable
  private key (ASCII-armored format). For example, this option is available for
  dkg-generate, dkg-decrypt, dkg-sign, dkg-keysign, and dkg-timestamp.

  The programs dkg-keycheck and dkg-keyinfo print some basic information for a
  given OpenPGP public and private key, respectively. However, there is another
  special use case for DKGPG: The option "-m" of dkg-keyinfo migrates a changed
  peer identity, if the new name meets some requirements. Note that only the
  name of the peer is migrated, other parts of the private key material are not
  changed.
  
  The private key shares of all active parties should be changed regularly by
  program dkg-refresh. Each player who does not join this refresh process is
  excluded from any further operation with the shared private key.

  The program dkg-sign creates a detached signature on a binary or a text file,
  if a sufficient threshold of the parties work together in the interactive
  signing protocol. Such a detached signature can be verified with any OpenPGP-
  compatible software or the included programm dkg-verify. Thus DKGPG solves
  the problem of distributed code-signing without secret key splitting (e.g.
  libgfshare) and laborious reconstruction ceremonies (e.g. as used for DNSSEC
  root signing key). The program dkg-keysign generates certification signatures
  in a similar manner for each selected and valid user ID of a given public key.
  User attributes are not supported yet. With the program dkg-adduid an
  additional user ID can be created and with dkg-revuid a present user ID can be
  revoked. The program dkg-addrevoker creates a direct-key signature which
  specifies an external key (given by option "-i") that can act as an additional
  revoker. Last but not least, the program dkg-revoke generates corresponding
  revocation signatures (also known as revocation certificates) for the key.

  Please consult the manual pages for detailed information on usage and options.

  Note that for building GNUnet support of dkg-generate, dkg-adduid, dkg-revuid,
  dkg-refresh, dkg-sign, dkg-keysign, dkg-timestamp, dkg-addrevoker, dkg-revoke,
  and the interactive version of dkg-decrypt the development files and a working
  environment of a very recent version (at least 0.11.x series) must be present.

  However, as simple workaround for systems without GNUnet a TCP/IP-based
  mechanism for message exchange is included (option "-H <hostname>") in each
  program that needs network interaction. To keep confidentiality and integrity
  of the established point-to-point channels some mutually chosen and (manually)
  distributed passwords (option "-P <password list>") are required. The format
  of such a password list is explained in the corresponding manual pages.
  Moreover, with torsocks and some obvious port forwarding rules for a hidden
  service (see manual page of dkg-generarte for the details) this mechanism
  allows running the interactive DKGPG programs over Tor.

Requirements
------------
  You need the following programs, libraries and appropriate header files to
  build (GNU gcc/g++ >= 5.4.x or higher is recommended) DKGPG successfully:

  * Toolbox for Mental Card Games (LibTMCG), version >= 1.4.0
    https://savannah.nongnu.org/projects/libtmcg
  * GNU Multiple Precision Arithmetic Library (libgmp), version >= 6.1.2,
    https://gmplib.org/
  * GNU Crypto Library (libgcrypt), version >= 1.8.0,
    https://gnupg.org/download/index.html
  * GNU Privacy Guard Error Code Library (libgpg-error), version >= 1.27,
    https://gnupg.org/download/index.html
  * zlib Compression Library (libz), version >= 1.2.3
    https://zlib.net/
  * [OPTIONAL] Library for data compression (libbzip2), version >= 1.0.6,
    http://www.bzip.org/
  * [OPTIONAL] GNUnet, version >= 0.11.7
    https://gnunet.org/

  The DKGPG package was tested (++ successful, -- erroneous) on the following
  platforms with the given compilers, respectively:

  ++ GNU/Linux (Gentoo Linux)    x86_64    gcc 10.2.0  (details omitted)
  ++ GNU/Linux (Gentoo Linux)    x86_64  clang 11.0.0  (details omitted)
  ++ GNU/Linux (Debian Linux)    x86_64    gcc  8.3.0  (details omitted)
  ++ GNU/Linux (Trisquel Linux)  x86_64    gcc  4.8.4  (details omitted)
  ++ OpenBSD 6.5                 amd64     gcc  4.2.1  (details omitted)
  ++ FreeBSD 12.1                amd64   clang  8.0.1  (details omitted)

Build & Install
---------------
  You should proceed by executing the following magic commands:

    1. ./configure
    2. make
    3. make install

Bugs, Contribution, GIT
-----------------------
  Please send detailed bug reports to Heiko Stamer <HeikoStamer@gmx.net>,
  or even better, use our bug tracker at:
 
    https://savannah.nongnu.org/bugs/?func=additem&group=dkgpg

  You can get the latest sources from the source repository by using the
  command

    git clone https://git.savannah.gnu.org/git/dkgpg

  The GNU C++ compiler from GCC and the GNU Make program is required.
  Additionally you will need the following free software packages:
  
  * GNU Autoconf, version >= 2.61, https://www.gnu.org/software/autoconf/
  * GNU Automake, version >= 1.12, https://www.gnu.org/software/automake/
  * GNU Libtool, version >= 2.4.2, https://www.gnu.org/software/libtool/

Example: working configuration in .muttrc (replacement of classic pgp)
----------------------------------------------------------------------
[...]
  unset crypt_use_gpgme
  unset pgp_use_gpg_agent
  set pgp_list_pubring_command="~/bin/dkg-keycheck -y -k ~/keyring.asc %r"
  set pgp_self_encrypt=yes
  set pgp_default_key="0x76F73011329D27DB8D7C3F974F584EB8FB2BE14F"
  set pgp_encrypt_only_command="~/bin/dkg-encrypt -k ~/keyring.asc -i %f -r %r"
  set pgp_check_gpg_decrypt_status_fd=no
  set pgp_decryption_okay="^INFO: decrypted message"
  set pgp_decrypt_command="~/bin/dkg-decrypt -E -V -k ~/keyring.asc -y ~/secretkey.asc -i %f"
  set pgp_verify_command="~/bin/dkg-verify -V -k ~/keyring.asc -i %f -s %s"
  set pgp_good_sign="^INFO: Good signature"
  set pgp_sign_command="~/bin/dkg-sign -E -t -V -y ~/secretkey.asc -i %f"
[...]

Copyright and License of this software
--------------------------------------
  (C) 2017, 2018, 2019, 2020  Heiko Stamer <HeikoStamer@gmx.net>
        GNU General Public License version 2,
            or (at your option) any later version published by
            the Free Software Foundation.

  Please have a look at file 'COPYING' and https://www.fsf.org/.

APPENDIX: GNU Free Documentation License
----------------------------------------

                GNU Free Documentation License
                 Version 1.3, 3 November 2008


 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
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10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the
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as a draft) by the Free Software Foundation.  If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy's public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works.  A
public wiki that anybody can edit is an example of such a server.  A
"Massive Multiauthor Collaboration" (or "MMC") contained in the site
means any set of copyrightable works thus published on the MMC site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 
license published by Creative Commons Corporation, a not-for-profit 
corporation with a principal place of business in San Francisco, 
California, as well as future copyleft versions of that license 
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or in 
part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this 
License, and if all works that were first published under this License 
somewhere other than this MMC, and subsequently incorporated in whole or 
in part into the MMC, (1) had no cover texts or invariant sections, and 
(2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.


ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

    Copyright (c)  YEAR  YOUR NAME.
    Permission is granted to copy, distribute and/or modify this document
    under the terms of the GNU Free Documentation License, Version 1.3
    or any later version published by the Free Software Foundation;
    with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
    A copy of the license is included in the section entitled "GNU
    Free Documentation License".

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the "with...Texts." line with this:

    with the Invariant Sections being LIST THEIR TITLES, with the
    Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.