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.