Towards Tight Security Bounds for OMAC, XCBC and TMAC

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Conference Article

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Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)


OMAC — a single-keyed variant of CBC-MAC by Iwata and Kurosawa — is a widely used and standardized (NIST FIPS 800-38B, ISO/IEC 29167-10:2017) message authentication code (MAC) algorithm. The best security bound for OMAC is due to Nandi who proved that OMAC ’s pseudorandom function (PRF) advantage is upper bounded by O(q2ℓ/ 2 n), where n, q, and ℓ, denote the block size of the underlying block cipher, the number of queries, and the maximum permissible query length (in terms of n-bit blocks), respectively. In contrast, there is no attack with matching lower bound. Indeed, the best known attack on OMAC is the folklore birthday attack achieving a lower bound of Ω(q2/ 2 n). In this work, we close this gap for a large range of message lengths. Specifically, we show that OMAC ’s PRF security is upper bounded by O(q2/ 2 n+ qℓ2/ 2 n). In practical terms, this means that for a 128-bit block cipher, and message lengths up to 64 GB, OMAC can process up to 2 64 messages before rekeying (same as the birthday bound). In comparison, the previous bound only allows 2 48 messages. As a side-effect of our proof technique, we also derive similar tight security bounds for XCBC (by Black and Rogaway) and TMAC (by Kurosawa and Iwata). As a direct consequence of this work, we have established tight security bounds (in a wide range of ℓ ) for all the CBC-MAC variants, except for the original CBC-MAC.

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