A user terminal (1) such as a mobile telephone generates a code in response to a request signal. This code is transmitted from the user terminal to a service terminal for decrypting and/or verification. The code is unique to the user. A request signal may be transmitted from the user terminal to a code generator (7), which generates a code (C') which is transmitted to the service terminal where it is decrypted into code (C). The codes (C,C') may be generated separately from the user and the code generator. Transmission may be via a wireless interface (5) or by scanning a bar-code etc. displayed on the user terminal screen.
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