Abstract: A method for encrypting and decrypting data with a session key is proposed. The data is exchanged between a first data processing device and a second data processing device via a communications link that is equipped with a communications server. The method exhibits process steps concerning the generation of a permanent common start value and the provision of a formation rule for the session key, process steps concerning the generation of a session key for the encryption of the data, process steps concerning the encryption of data of the first data processing device using the session key, process steps concerning the generation of a session key for decryption of the data and process steps concerning the decryption of the data.