Abstract: A system for authenticating a document, D. A trusted party applies an algorithm to D, for example, by treating each byte of data within D as a number, and treating the numbers as inputs to the algorithm. The trusted party obtains a result from the algorithm, termed a Message Authentication Code, MAC. The trusted party gives a courier both (1) D and (2) the MAC, as by loading both into a portable computer carried by the courier. The courier delivers both D and MAC to a recipient, who is equipped with the identical algorithm. The recipient applies the algorithm to D. If the recipient obtains the MAC, the recipient concludes that no tampering of D occurred. The reason is that successful tampering requires the courier to replace MAC with a fabricated MAC(fab). MAC(fab) must possess the characteristic that the algorithm would produce MAC(fab) when applied to the tampered document D.