In accordance with an example embodiment, there is provided an apparatus, such as a user equipment, configured to receive, from a communication network, an authentication request which comprises a nonce and a received sequence number, check, whether the received sequence number is advanced with respect to a first sequence number, the first sequence number being from a most recent previous authentication request handled by the apparatus, check, responsive to the received sequence number not being advanced with respect the first sequence number, whether the nonce is identical to one from among plural stored nonces, and send, responsive to the nonce being identical to the one stored nonce, a response to the authentication request which comprises as a synchronization failure token a dummy value which is not derived from the first sequence number.