The invention relates to a method for authenticating a smart card (SIM) in
a messaging network, preferably a GSM network, wherein an optionally
secret algorithm and a secret key are stored in a smart card (SIM),
whereby for authentication the network or a network component first
transfers a random number to the smart card, a response signal is
generated in the smart card by means of the algorithm, the random number
and the secret key, said signal being transmitted to the network or
network component in order to check the authenticity of the card there.
According to the invention both the secret key and the random number
transferred by the network are split into at least two parts to form the
authentication message, one part of the transferred random number and one
or more parts of the secret key being encrypted by means of a one- or
multistep, preferably symmetrical calculation algorithm. To output an
authentication response, a selectable part of the encryption result is
transferred to the network.