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CopyrigU Ο 1*74, A m trie tn TtUthtnt tad Tciempk Company. PrimUd m U£.A. We consider α new kind of coding problem, which has applications in a variety of situations. A message χ is to be encoded using a key m to form an encrypted message y = Φ (x, m), which is then supplied to a user G. G knows m and so can calculate x. It is desired to choose Φ(·,·) so as to protect G against B, who knows x, y, and Φ(·,·) (but not m); Β may substitute a false message y for y. It is shown that if the key can take Κ values, then an optimal strategy for Β secures him a probability of an undetected substitutionà K~*. Several encoding functions Φ(·,·) are given, some of which achieve this bound.

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