H.235 says, "The session key encryption algorithm is the same as the negotiated media encryption algorithm." At the very least, this of course means that one uses the same cipher; however, there are some important details that this statement overlooks, i.e., mode, IV, salt, and padding. I assume that the guiding principle is to treat the ASN.1 encoded value as much like an RTP payload as possible.
Mode Does one use the same mode as media and therefore the same block processing for each successive block in the encoded ASN.1 value?
IV For H.235v3, one clearly uses the IV specified in the new type, NewKeySyncMaterial.paramS. However, what does one do for v2? There doesn't seem to be any way to specify the IV for session-key encryption. I suppose one could derive it from the session key similar to how SRTP derives stuff from the master key. For example, IV = session key. Maybe this should go in the IG.
Salt This is a tough one. One could have used the same salt being signaled for media, but it is of course encrypted! Therefore, if the mode for encrypting media is EOFB, which uses a salt value, and one is supposed to use the same "encryption algorithm" for encrypting the session key as encrypting media, what salt value does one use? An implied salt value of all zeros would work, but this is not written down anywhere.
Padding This is the toughest to translate from media to session-key encryption. This is the overall H.235 statement as to what kind of padding to perform on media: "Two methods are available to handle packets whose payload is not a multiple of blocks: 1) Ciphertext Stealing for ECB and CBC; Zero pad for CFB and EOFB. 2) Padding in the manner prescribed by [RTP] (Section 5.1)." Since a session key isn't an RTP packet, we can eliminate #2. Therefore, #1 says to use zero padding for EOFB and ciphertext stealing for CBC. Is that how we handle padding when encrypting the session key?
Paul
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