A server obtains a challenge from another computer system during a negotiation with a client according to a protocol. The server injects the challenge into a message of the protocol to the client. The client uses the challenge in an authentication request. The server submits the authentication request to the other computer system for verification. The other computer system verifies the authentication request using a key registered to the client. The server operates further dependent at least in part on whether verification of the authentication request was successful.