adam@8: datatype association_type = HMAC_SHA1 | HMAC_SHA256
adam@8: datatype association_session_type = NoEncryption | DH_SHA1 | DH_SHA256
adam@10: datatype authentication = AuthenticatedAs of string | Canceled | Failure of string
adam@8: 
adam@10: val authenticate : (authentication -> transaction page)
adam@10:                    -> {AssociationType : association_type,
adam@10:                        AssociationSessionType : association_session_type,
adam@10:                        Identifier : string}
adam@10:                    -> transaction string
adam@4: (* Doesn't return normally if everything goes as planned.
adam@10:  * Instead, the user is redirected to his OP to authenticate there.
adam@10:  * Later, the function passed as the first argument should be called with the result. *)