Abstract: Method for encrypting information transferred between two mobile telephones, each having a security module and where a security application contains encryption and decryption algorithms. The security application is stored on an SD card introduced into a card-holder location in the relevant telephone, then transferred from the card to the security module with a session key stored on the card, a key stream forms in the card on the basis of the session key, and during encryption is transferred from the card to the security module where encryption takes place, such encrypted stream of data is transmitted by the telephone, the stream of data is received by a second mobile telephone, decryption takes place with a corresponding key stream in the security module of the second telephone and the corresponding key stream is formed in the SD card of the second telephone on the basis of the session key.