A non-malleable code is an unkeyed randomized encoding scheme that offers the strong guarantee that decoding a tampered codeword either results in the original message, or in an unrelated message. We consider the simplest possible construction in the computational split-state model, which encodes a message m simply as k||E_k(m) for a uniformly random key k, where E is a block cipher. This construction is comparable to, but greatly simplifies over, the one of Kiayias et al. (ACM CCS 2016), who eschewed this simple scheme in fear of related-key attacks on E. In this work, we prove this construction to be a strong non-malleable code in the split-state tampering model as long as E is (i) a pseudorandom permutation under leakage and (ii) related-key secure with respect to an arbitrary but fixed key relation. Both properties are believed to hold for ``good'' block ciphers, such as AES-128, making this non-malleable code very efficient with short codewords of length |m| + 2 kappa (where kappa is the security parameter, e.g., 128 bits), without significant security penalty.

Additional Metadata
Journal IACR Transactions on Symmetric Cryptology
Conference FSE 2018
Fehr, S, Karpman, P, & Mennink, B. (2018). Short Non-Malleable Codes from Related-Key Secure Block Ciphers. In IACR Transactions on Symmetric Cryptology (Vol. 2018).