| 摘要 |
The method is for modifying the data in a system for transactions by card, where the card is with a memory, and a reader can read the card when it is in a determined position with respect to the reader. The memory is read-only, erasable and re-writable, and comprises two counter stages (A,B) for registering the data values with respect to transactions effected by the card. Each transaction involves the modifications of the data value, which is a monotonic function of time. At each transaction, the writing of the data value effects the writing of new data value (Y) in the counter stage (B), and the write operation effects the erasing of old data value (X) registered in the counter stage (A). At the end of the write operation which is correctly effected, the counter stage (B) contains the new data value while the counter stage (A) contains the value zero. The data value registered in one of the counter stages (A,B) is incremented at each write operation. The transaction is not validated if the write operation is not effected correctly because of pulling out the card in the course of the transaction, when none of the two stages registers a value equal to zero. The counter is restored by a rewrite operation when the card is pulled out during the write phase. If the counter stage (B) contains an incorrect value (Y') comprised between the old data value and the new data value, the counter is restored by a rewrite operation followed by a write operation consistent with the writing of the new data value in the counter stage (A) and the erasing the incorrect value in the counter stage (B). If the incorrect data value (Y') is below the old value (X), the counter is restored by a rewrite operation of the old data value in the counter stage (A) and erasing the incorrect data value, followed by a write operation of the new data value (Y) in the counter stage (B) and erasing the old data value in the counter stage (A). The counter is restored by a rewrite operation when the pulling-out takes place between the writing phase and the erasing phase. When the pulling-out takes place during the erasing phase and an incorrect data value (X') is registered in the counter stage (A), the counter is restored by a rewrite operation of the new data value (Y) in the counter stage (B) and erasing the incorrect data value. A system (claimed) for transactions with card implements the method (claimed). The card with memory is a card operated without contact.
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