what SRTP crypto algorithm used?  
Author Message
Kevin Packard





PostPosted: ISV Open Discussions, what SRTP crypto algorithm used? Top

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



Microsoft ISV Community Center Forums3  
 
 
MS ISV Buddy Team





PostPosted: ISV Open Discussions, what SRTP crypto algorithm used? Top

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)



 
 
Kevin Packard





PostPosted: ISV Open Discussions, what SRTP crypto algorithm used? Top

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