SIP Registration Protocol

I hope this is the right place to ask and someone can help me cos I'm having a hell of a time figuring out how to construct my message headers for registration.

I understand the basics of the challenge response protocol, but I cant find any concrete examples on how to create the 'response' value in the Authorization header thats sent back to the server.

Here's an example of the header I'm getting from the server:

Code:

WWW-Authenticate: Digest realm="mksip", nonce="26496944cae8e5b4d55f3a3e5e61d702", opaque="", stale=FALSE, algorithm=MD5

And this is what my X-ten is sending back:

Code:

Authorization: Digest username="3002",realm="mksip",nonce="26496944cae8e5b4d55f3a3e5e61d702",response="4cf01c45c416ae4a5e7eacc2c69fe2ff", uri="sip:192.168.1.5",algorithm=MD5

I'm trying to build my own simple client, but I cant figure out for the life of me how to construct the response value in the response my x-ten is sending.

Everywhere I read it seems to be saying that SIP uses HTTP Digest Authorization, but when I try to duplicate it with my known values, I get a completly different value to what is above.

Could anyone please explain the algorithm behind creating this response, and what values do I need to put into it?

Cheers,

Conor