Importance

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Abstract

This invention concerns a prepayment metering system using a card on which information may be stored and from which stored information may be erased. In one form, a number (A) recorded on the card is read at the meter and stored (30) in preparation for the next card insertion. To obtain a further credit at the meter, the card is taken to a terminal station (12) at which the number (A) recorded on the card is read and is subjected to an encryption process under the control of a key and the resulting number (A') is recorded on the card in place of the previous number (A). On the next insertion of the card at the meter (21), the number (A') is read from the card and subjected to a decryption step (22) using the same key, the result of the decryption being compared with the number stored in the register (30) and in the event of a match a credit is issued. In another embodiment, the terminal generates a random number which is subjected to encryption and both the random number and the result of the encryption are recorded on the card and subsequently used at the meter.

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First Claim