When we started the IETF process to get an RFC for the ZRTP protocol, we filed an IPR statement with the IETF, promising to grant a free patent license for US Patent number 7,730,309. The actual license grant appears below, which we derived from the guidelines that the IETF publishes in the document "Guidelines for Free Standards in the IETF". We also have a PGP-signed copy of this license, and uploaded a copy as an IPR statement on the IETF web site. Note that this license provides an incentive for implementors to properly implement ZRTP's back-door-resistant features.
At this time, Philip Zimmermann (together with his assignee, "PRZ") believes US Patent Number 7730309 ("0309") contains claims which may be necessary and essential to implementations of the IETF RFC based on the ZRTP specification draft-zimmermann-avt-zrtp.
Subject to the terms and conditions of this License, PRZ hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this License) patent license to 0309 to the extent that 0309 is infringed by Your implementation (in whole or in part) of this specification or its successor documents published by the IETF.
If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the implementation of the specification constitutes direct or contributory patent infringement, then any patent license for the specification granted to You under this License shall terminate as of the date such litigation is filed.
If You assert, in litigation (including a cross claim or counter claim in a lawsuit) or otherwise (including in a licensing discussion) any patent rights against either Philip Zimmermann or his assignee, then any patent license for the specification granted to You under this License shall terminate immediately.
If the "ZRTP Disclosure Flag" in the ZRTP specification is applicable to Your implementation, and You knowingly fail to properly implement it in a truthful manner, and You are informed of this fact and fail to remedy it within 90 days, then any patent license for the specification granted to You under this License shall terminate.