ZRTP is a key agreement protocol that performs a Diffie-Hellman key
   exchange during call setup in the media path and is transported over
   the same port as the Real-time Transport Protocol (RTP) [RFC3550]
   media stream which has been established using a signaling protocol
   such as Session Initiation Protocol (SIP) [RFC3261].  This generates
   a shared secret, which is then used to generate keys and salt for a
   Secure RTP (SRTP) [RFC3711] session.  ZRTP borrows ideas from
   [PGPfone].  A reference implementation of ZRTP is available in
   [Zfone].

   The ZRTP protocol has some nice cryptographic features lacking in
   many other approaches to media session encryption.  Although it uses
   a public key algorithm, it does not rely on a public key
   infrastructure (PKI).  In fact, it does not use persistent public
   keys at all.  It uses ephemeral Diffie-Hellman (DH) with hash
   commitment and allows the detection of man-in-the-middle (MiTM)
   attacks by displaying a short authentication string (SAS) for the
   users to read and verbally compare over the phone.

ret = recv(sd, buffer, sizeof(buffer), MSG_DONTWAIT);
if (ret > 0) {
  received = ret;
  bzrtp_processMessage(ctx, self_ssrc, buffer, received);
}

bzrtp_iterate(ctx, self_ssrc, now);

|        Commit (Bob's ZID, options, hash value) F5 |
|<--------------------------------------------------|
| F6 DHPart1 (pvr, shared secret hashes)            |
|-------------------------------------------------->|
|            DHPart2 (pvi, shared secret hashes) F7 |
|<--------------------------------------------------|

   The use of hash commitment in the DH exchange constrains the attacker
   to only one guess to generate the correct Short Authentication String
   (SAS) (Section 7) in his attack, which means the SAS can be quite
   short.  A 16-bit SAS, for example, provides the attacker only one
   chance out of 65536 of not being detected.  Without this hash
   commitment feature, a MiTM attacker would acquire both the pvi and
   pvr public values from the two parties before having to choose his
   own two DH public values for his MiTM attack.  He could then use that
   information to quickly perform a bunch of trial DH calculations for
   both sides until he finds two with a matching SAS.  To raise the cost
   of this birthday attack, the SAS would have to be much longer.  The
   Short Authentication String would have to become a Long
   Authentication String, which would be unacceptable to the user.  A
   hash commitment precludes this attack by forcing the MiTM to choose
   his own two DH public values before learning the public values of
   either of the two parties.

| * SAS(Mallory, Bob) = SAS(Alice, Mallory)       * |
| * Both parties will confirm they share the same * |
| * SAS value                                     * |

| * Note that SRTP material (Alice, Mallory) will * |
| * not match SRTP material (Mallory, Bob)        * |

| * However, Mallory will be able to handle SRTP  * |
| * flows from both Bob and Alice, giving         * |
| * interception and tampering capabilities.      * |