A California company is selling RFID tags that would take a counterfeiter years to duplicate. Microscopic differences between the tags--a side effect of normal manufacturing techniques--mean that each will yield different answers in a set of test calculations. A forger would have to correctly predict the results of 16 billion billion such calculations to be sure of accurately simulating a single tag. The tags are being marketed as a way for manufacturers to authenticate brand-name luxury goods.

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