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Q: Is Zfone open source?

A: No. The Zfone application is not available under the GPL, the AGPL,
or any other open source license. However, the Zfone libZRTP SDK
library is published under the AGPL open source license. The SDK is
also available under a commercial license, in a dual-licensing model.
See our Licensing Policy page for details.

I'm a firm believer in publishing the source code for cryptographic
software for peer review, to build public confidence that it contains
no back doors, a tradition I started in 1991 with PGP. PGP is a
proprietary product, even though the source code is available for peer
review. Publishing the source code for peer review is not the same as
making it availble under an open source license.

Zfone has several major components, and not all of them are published
under the same licensing terms. The entire body of source code for the
complete Zfone client software is published for peer review purposes.
In addition, the Zfone libZRTP SDK library is published under the AGPL
version 3. However, the rest of the Zfone application remains
proprietary. That's why we show you an End User License Agreement
before you download Zfone.

[2] https://secure.wikimedia.org/wikipedia/en/wiki/ZRTP

Free ZRTP implementation

    * SFLphone has ZRTP support integrated and is available under GNU
General Public License.
    * Twinkle uses GNU ccRTP and GNU ZRTP to implement the ZRTP
support. All these packages are available under the GNU General Public
License.
    * SIP Communicator supports ZRTP through the ZRTP4J lib. Full
support is available in the release candidates and is also scheduled
for inclusion in the final 1.0 release.
    * FreeSWITCH currently has basic support for ZRTP through the libzrtp S=
DK.