Security procedure

A method comprising:

• receiving, in an apparatus, from a communication network, an authentication request which comprises a nonce and a received sequence number;
• checking, 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;
• checking, 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
• sending, 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 preconfigured dummy value which is not derived from the first sequence number.

A non-transitory computer readable medium having stored thereon a set of computer readable instructions that, when executed by at least one processor, cause an apparatus to at least perform:

• receive, in an apparatus, 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 preconfigured dummy value which is not derived from the first sequence number.