adam@8: datatype association_type = HMAC_SHA1 | HMAC_SHA256
adam@8: datatype association_session_type = NoEncryption | DH_SHA1 | DH_SHA256
adam@8: 
adam@8: val authenticate : association_type -> association_session_type -> string -> transaction string
adam@4: (* Doesn't return normally if everything goes as planned.
adam@4:  * Instead, the user is redirected to his OP to authenticate there. *)