Abstract: In a data communication system a plurality of users are equipped with respective devices for computation and authentication of message authenticators. Each device stores a common cryptographic function; a common secret key and a respective non-secret offset for the key. They are programmed to perform the functions of: (i) computing and outputting an authenticator for an entered message using the cryptographic function and key combined with the respective offset; and (ii) computing an authenticator for an entered message using the cryptographic function and key combined with any entered offset, comparing that authenticator with one received with the message and displaying a "pass" or "fail" decision. The devices are incapable of displaying or otherwise outputting any authenticator computed using any offset other than its respective stored offset, however. In this way it can be ensured that a transmitted authenticator can act as a verifiable "signature" to a message uniquely identifying the sender.

Abstract: A plurality of portable tokens are issued e.g. for authenticating instructions within an electronic banking network. Each token requires entry of a respective password in order to operate and becomes locked if a number of incorrect password entries are made. Each token also stores a respective cryptographic key and a random number generator. In order to unlock a locked token it outputs a random number and also encrypts that number under its respective key but does not output the result of the encryption. The random number is transmitted by the user to a central authority which stores the keys for all tokens. The authority encrypts that number under the respective key and returns the result of the encryption to the user of the token. The returned number is entered into the token and if it agrees with the result of the internal encryption the token is unlocked.