Ok – here is MD5 authentication: The PhD Course: SIP REGISTER is authenticated via MD5 authentication.  Everything you need to know is sent in the ‘Authorization’ line except the SIP Method which is at the top of the packet and in this case REGISTER – it could also be INVITE for example – and the user’s password.  That’s the key – that password. Here’s a sample REGISTER  Authorization string: Authorization: Digest username="agrabah_aladdin", realm="jnctn.net", nonce="5d02c49e00007aeb4d90b8fe974cf38a6d6a5b7515c24c19", uri="sip:agrabah.onsip.com", algorithm=MD5, qop=auth, cnonce="59f4a2601ec0874", nc=00000001, response="bb9ee2c392839058a01abcfa8192db47"   You create an A1 string for the user.  This string remains the same for the lifetime of the user’s password.  For this example we will assume Aladdin’s password is…