Abstract: The present invention relates to a method of changing an identification code of a transaction authorisation medium, wherein the transaction authorisation medium is designed for authorising transactions of a credit parameter. The identification code is registered by the transaction authorisation medium and by a management server. The transaction authorisation medium is operatively connected to a terminal for changing the identification code, and the terminal is operatively and communicatively connected to the management server via a public telecommunication network. The method comprises the steps of: initiating a transaction; storing the changed identification code in the transaction authorisation medium; and transmitting a changed identification code in encrypted form to the management server via the telecommunication network for carrying out the transaction.