Optimal Error Correction for Computationally Bounded Noise

For adversarial but computationally bounded models of error, we construct appealingly simple and efficient cryptographic encoding and unique decoding schemes whose error-correction capability is much greater than classically possible. In particular: 1) For binary alphabets, we construct positive-rate coding schemes that are uniquely decodable under a 1/2 - γ error rate for any constant γ > 0. 2) For large alphabets, we construct coding schemes that are uniquely decodable under a 1 - R error rate for any information rate R > 0. Our results for large alphabets are actually optimal, since the "computationally bounded but adversarial channel" can simulate the behavior of the q-ary symmetric channel, where q denotes the size of the alphabet, the capacity of which is known to be upper-bounded by 1 - R. Our results hold under minimal assumptions on the communication infrastructure, namely: 1) we allow the channel to be more powerful than the receiver and 2) we only assume that some information about the sender-a public key-is known. (In particular, we do not require any shared secret key or joint local state between sender and receivers).