RSA authentication with IAX

Hi guys,

I wonder if RSA for authentication in Asterisk (with IAX) uses sign or encryption function.

Is it correct that when I receive a challenge from the server, I’ll use my private key to only decrypt/unsign it with my private key and put it back to the RSA result?

Another relevant question:
Are there any IAX client that have RSA support, that let me try it?

Any help will be appreciated