Abstract: When a system detects that a transaction key in the SIM card (18) of a mobile station (36, 38) is non-existent or is no longer valid, the method automatically performs the following steps: generating in the application key server (42) a transaction key; encrypting the transaction key in the application server (42) using a transmission key generated when the SIM card was customized; transmitting the encrypted transition key via the SMS service centre (40) to the mobile station (36, 38); decrypting in the SIM card (18) the encrypted transaction key using the transmission key; and recording the decrypted transaction key in the SIM card storage. Furthermore, the method enables to select among several possible keys one key which corresponds both to a specific application and to a specific service provider.