Achieving Shannon Capacity for Computationally Bounded Errors

We study error correction in a computationally bounded world, where errors are introduced by an arbitrary polynomial-time adversarial channel. Recent works construct seeded codes in this model, where the encoding and decoding procedure share a public random seed. They achieve significantly better tradeoffs between rate and error tolerance than what is possible information theoretically for unique decoding, essentially matching the parameters of the best known efficiently list-decodable codes. Over the binary alphabet, however, this is still well short of the optimal Shannon capacity with rate R≈1-H₂(p) for a p<1/4 fraction of errors. Even heuristic constructions meeting this target were not previously known. We make progress towards this goal. Secret-Key Codes. We first study secret-key codes, where the encoder and decoder share a secret key hidden from the adversarial channel. Lipton (STACS ’94) constructed one-time secure secret-key codes achieving Shannon capacity in this setting, but it was unknown whether one can get CPA (resp. CCA) security where the adversary may query an encoding oracle (resp. also a decoding oracle). We construct CCA-secure secret-key codes achieving Shannon capacity via pseudorandom codes (PRCs).Seeded Codes (Heuristic). We can heuristically upgrade the resulting secret-key codes to seeded codes by publishing an obfuscation of the encoding/decoding procedures with a hard-coded secret key as a seed. Security holds in the ideal obfuscation model.Public-key Codes. We also consider public-key codes, where the decoder has a secret key and the encoder has the corresponding public key. We construct such CPA-secure public-key codes achieving Shannon capacity with unique decoding for p<1/4 errors, and list decoding all the way to p<1/2 errors, assuming PRCs and the subexponential security of standard crypto assumptions (e.g., LWE or DDH or QR or DCR). We also show how to get CCA security in the random oracle model. Secret-Key Codes. We first study secret-key codes, where the encoder and decoder share a secret key hidden from the adversarial channel. Lipton (STACS ’94) constructed one-time secure secret-key codes achieving Shannon capacity in this setting, but it was unknown whether one can get CPA (resp. CCA) security where the adversary may query an encoding oracle (resp. also a decoding oracle). We construct CCA-secure secret-key codes achieving Shannon capacity via pseudorandom codes (PRCs). Seeded Codes (Heuristic). We can heuristically upgrade the resulting secret-key codes to seeded codes by publishing an obfuscation of the encoding/decoding procedures with a hard-coded secret key as a seed. Security holds in the ideal obfuscation model. Public-key Codes. We also consider public-key codes, where the decoder has a secret key and the encoder has the corresponding public key. We construct such CPA-secure public-key codes achieving Shannon capacity with unique decoding for p<1/4 errors, and list decoding all the way to p<1/2 errors, assuming PRCs and the subexponential security of standard crypto assumptions (e.g., LWE or DDH or QR or DCR). We also show how to get CCA security in the random oracle model. Our secret-key and public-key codes meeting Shannon capacity are also simultaneously pseudorandom codes.