A method, apparatus and computer program in which a cellular terminal: transmits a request that requires authentication procedure triggering to a cellular network and responsively receiving from the cellular network an authentication request message with an indication of a selected cryptographic algorithm from a group of a plurality of cryptographic algorithms; decodes the authentication request message to a decoded authentication request according to the selected cryptographic algorithm and based on a shared secret known by the cellular terminal and a network operator of the cellular terminal; based on the decoded authentication request, the shared secret and the selected cryptographic algorithm, produces and encrypts an authentication response message; and transmits the authentication response message to the cellular network.