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+## GNU ZRTP C++
+
+This package provides a library that adds ZRTP support to the GNU
+ccRTP stack. Phil Zimmermann developed ZRTP to allow ad-hoc, easy to
+use key negotiation to setup Secure RTP (SRTP) sessions. GNU ZRTP works
+together with GNU ccRTP (1.5.0 or later) and provides a ZRTP
+implementation that can be directly embedded into client and server
+applications.
+
+The GNU ZRTP implementation is compliant to [RFC 6189][]. Currently GNU ZRTP
+C++ supports the following features:
+
+* multi-stream mode
+* Finite field Diffie-Helman with 2048 and 3072 bit primes
+* Elliptic curve Diffie-Helman with 256 and 384 bit curves
+* AES-128 and AES-256 symmetric cipher
+* Twofish-128 and Twofish-256 bit symmetric ciphers
+* The SRTP authentication methods HMAC-SHA1 with 32 bit and 80 bit length and
+ the Skein MAC with 32 bit and 64 bit length
+* The Short Authentication String (SAS) type with base 32 encoding (4
+ characters)
+
+Enhanced features like PBX SAS relay aka *trusted Man-in-the-Middle* or
+preshared mode are not supported but the GNU ZRTP C++ implementation defines
+the necessary external interfaces and functions for these enhanced features
+(stubs only).
+
+### Interoperability
+During the development of ZRTP and its sister implementation ZRTP4J (the Java
+version of the ZRTP) Phil Zimmermann, his developers, and I worked together to
+make sure Phil's [Zfone][] implementation and the GNU ZRTP implementations can
+work together.
+
+[zfone]: http://zfoneproject.com/index.html
+
+
+### Other implementations based on GNU ZRTP C++
+
+The ZRTP4J implementation is a copycat of the original C++ code. I used the
+same overall class structure and copied a lot of C++ functionality to Java. Of
+course some Java adaptation were done, for example to overcome the problem of
+non-existing pointers :-), thus I use some non-obvious array handling. If you
+are interessted in the Java implementation of ZRTP then you may have a look
+[here][javazrtp]. The Jitsi project uses the Java implementation. Jitsi is a
+powerfull communication client and is definitely worth a [look][jitsi].
+
+To enable C based code to use ZRTP C++ I did a C wrapper that offers the same
+functionality to C based RTP implementations. The first use of the ZRTP C
+wrapper was for the [PJSIP][] library, actually the RTP part of this
+library. The ZRTP handler for PJSIP is [here][pjzrtp]. This port enables PJSIP
+based clients to use ZRTP. One of the first clients that use this feature is
+*[CSipSimple][]*, an very good open source Android SIP client.
+
+[pjsip]: http://www.pjsip.org
+[pjzrtp]: https://github.com/wernerd/ZRTP4PJ
+[javazrtp]: https://github.com/wernerd/ZRTP4J
+[jitsi]: http://www.jitsi.org
+[csipsimple]: http://code.google.com/p/csipsimple
+
+
+### Some notes on GNU ZRTP C++ history
+The first application that demonstrated the embedded ZRTP was Minisp (now
+defunct). Minisip has it's own RTP stack and the very first version of this
+embedded ZRTP implementation worked together with this specific RTP stack.
+
+A few weeks later I implemented the GNU ccRTP glue code and ZRTP became part
+of the official GNU ccRTP project and was named GNU ZRTP C++. The Twinkle
+softphone uses GNU ccRTP and GNU ZRTP C++ since it's 0.8.2 release and Michel
+de Boer, the implementor of Twinkle, created a nice user interface. All
+following versions of Twinkle include GNU ZRTP C++ as well.
+
+
+### License and further information
+Please note, this library is licensed under the GNU GPL, version 3 or
+later, and has been copyright assigned to the Free Software Foundation.
+
+For further information refer to the [ZRTP FAQ][zrtpfaq] and the
+[GNU ZRTP howto][zrtphow]. Both are part of the GNU Telephony wiki and are
+located in its documentation category.
+
+[zrtphow]: http://www.gnutelephony.org/index.php/GNU_ZRTP_How_To
+[zrtpfaq]: http://www.gnutelephony.org/index.php/ZRTP_FAQ
+[rfc 6189]: http://tools.ietf.org/html/rfc6189
+
+## Building GNU ZRTP C++
+Since version 1.6 GNU ZRTP C++ supports the *cmake* based build process
+only. The cmake build process is simpler than the GNU automake/autoconf
+process. To build GNU ZRTP C++ perform the following steps after you unpacked
+the source archive or pulled the source from [Github][]:
+
+ cd <zrtpsrc_dir>
+ mkdir build
+ cd build
+ cmake ..
+ make
+
+Running cmake in a separate `build` directory is the preferred way. Cmake and
+the following `make` generate all files in or below the build directory. Thus
+the base directory and the source directories are not polluted with `*.o`,
+`*.la`, or other files that result from the build process. You may delete the
+build directory and create a new one to start from fresh (this is the ultimate
+`make clean` :-) ) or you may create a second directory to build with
+different settings without mixing the two builds.
+
+[github]: http://github.com/wernerd/ZRTPCPP