Equipping branded goods with RFID tags is an effective measure to fight the growing black market of counterfeit products. Asymmetric cryptography is the technology of choice to achieve strong authentication but suffers from its ample demand of area and power resources. The GPS authentication scheme showed that a coupon-based approach can cope with the limited resources of passive RFID tags. This article extends the idea of coupons by recalculating coupons during the idle time of tags when they are powered but do not actively communicate. This approach relaxes latency requirements and allows to implement GPS hardware using only 800 gate equivalents plus storage for 560 bytes. In the average case it has the same performance as the classical coupon-based approach but does not suffer its susceptibility to denial-of-service attacks.
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