## Cryptology ePrint Archive: Report 2018/204

Short Non-Malleable Codes from Related-Key Secure Block Ciphers

Serge Fehr and Pierre Karpman and Bart Mennink

Abstract: 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 simply encodes a message $m$ 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 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 $|m| + 2\tau$ (where $\tau$ is the security parameter, e.g., 128 bits), without significant security penalty.

Category / Keywords: secret-key cryptography / Non-malleable code, split-state tampering model, related-key security, block cipher

Original Publication (in the same form): IACR-FSE-2018

Date: received 20 Feb 2018, last revised 21 Feb 2018

Contact author: pierre karpman at univ-grenoble-alpes fr

Available format(s): PDF | BibTeX Citation

Short URL: ia.cr/2018/204

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