An authentication method including operation of a personal token, a personal token for a terminal in a communication network, an authentication server, and a computer program for an authentication server. The secure server producing derived key material on the basis of a random and a secret key (K), said personal token including program instructions for re-computing the derived key material (Ck, Ik) on the basis of the received random and the secret key (K) as stored in the personal token. The personal token includes program instructions for using a re-computed part of the derived key material in order to interpret the received additional data.