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This paper proposes an authentication protocol between two parties. The protocol uses a shared secret key, representing a large variable-length code and a lookup table representing a permutation. The authenticating parties exchange randomly generated messages. Both challenge and response messages consist of multiple codewords from the code, and these messages are each truncated to a fixed number of symbols. The actual information to be recovered by the respondent are the codeword lengths used in the challenge message. These lengths are re-encoded to different lengths via the lookup table, and randomly chosen codewords with these re-encoded lengths are used in the response.