Only Silent Phone’s end users are involved in the key negotiation, and CALEA does not apply to end users.
Our architecture likely renders that question moot. The Communications Assistance for Law Enforcement Act applies in the US to the PSTN phone companies and VoIP service providers, such as Vonage. CALEA imposes requirements on VoIP service providers to give law enforcement access to whatever they have at the service provider, which would be only encrypted voice packets. ZRTP does all its key management in a peer-to-peer manner, so the service provider does not have access to any of the keys. Only the end users are involved in the key negotiation, and CALEA does not apply to end users.
Here is the operative language from CALEA itself:
47 U.S.C. 1002(b)(3): ENCRYPTION - A telecommunications carrier shall not be responsible for decrypting, or ensuring the government’s ability to decrypt, any communication encrypted by a subscriber or customer, unless the encryption was provided by the carrier and the carrier possesses the information necessary to decrypt the communication. [emphasis added]
Also, from the CALEA legislative history :
Finally, telecommunications carriers have no responsibility to decrypt encrypted communications that are the subject of court-ordered wiretaps, unless the carrier provided the encryption and can decrypt it. This obligation is consistent with the obligation to furnish all necessary assistance under 18 U.S.C. Section 2518(4). Nothing in this paragraph would prohibit a carrier from deploying an encryption service for which it does not retain the ability to decrypt communications for law enforcement access. [...] Nothing in the bill is intended to limit or otherwise prevent the use of any type of encryption within the United States. Nor does the Committee intend this bill to be in any way a precursor to any kind of ban or limitation on encryption technology. To the contrary, section 2602 protects the right to use encryption.