When audio/video conferencing between Office Communicator clients, what crypto algorithm is used for the SRTP

I logged the SIP INVTE messages in the LCS and took a look at the SDP. I was hoping to see an "a=crypto..." line, but I see only a d-line in the with a key("d=Base64:...").

I believe that a d-line means that the SRTP crypto algorithm is defined elsewhere. Can anyone tell me what that algorithm is AES DES

Thanks in advance,

Kevin Packard

Hi Kevin,

Here's some info from our support engineer:

Thanks for using Microsoft ISV Community Forum, according to your question, I’ve consulted our team members working on LCS, it seemed that Office Communicator 1.0 doesn’t support SRTP, and it will use the encryption mechanism described in RFC-1890 (http://www.ietf.org/rfc/rfc1890.txt). And SRTP support will be shipped with Office Communicator 2.0 using the AES encryption algorithm.

-brenda (ISV Buddy Team)

I have a followup question. When encrypting RTP, Office Communicator generates SDP with a crypto key, something like this:

k=base64:PlUeTaTiRmnMNdXkxwpSkjy4UvQkUwZGUxqLLrdK60I

This 43 character base64 encoding represents 43*6=258 bits. The trailing 2 bits are always 0, so I'm guessing that this is a 256 bit key.

My question is, what crypto algorithm is used  RFC1890 suggests that when no algorithm is specified, as in this case, that DES-CBC is used. However, DES-CBC
does not use a 256 bit key. So, is the algorithm AES  Or am I way off base 

Thanks in advance,
--
Kevin