[Serusers] SER Gateway Authentication

Greetings, I'm looking for a way to authenticate SER to an Asterisk box securely, digest would be the best but whatever is available I'll take. I cannot seem to find anything about this at voip-info, and I've had a solid google through the mailing archives. First of all, is it at all possible to authenticate with a gateway that wants to place a call through SER and vice-versa for that matter? If so, how could I achieve this? Eventually, I'd like to have hardware gateways such as cisco to coexist with SER, is it possible to authenticate those? Can SER initiate the authentication or does the Asterisk/Gateway have to initiate it? If it is also possible I would truly appreciate some examples of both how to achieve this in the ser.conf as well as in asterisk configs. Thankyou.