ZMAC+ – An Efficient Variable-output-length Variant of ZMAC

There is an ongoing trend in the symmetric-key cryptographic community to construct highly secure modes and message authentication codes based on tweakable block ciphers (TBCs). Recent constructions, such as Cogliati et al.’s HaT or Iwata et al.’s ZMAC, employ both the n-bit plaintext and the t-bit tweak simultaneously for higher performance. This work revisits ZMAC, and proposes a simpler alternative finalization based on HaT. As a result, we propose HtTBC, and call its instantiation with ZHash as a hash function ZMAC+. Compared to HaT, ZMAC+ (1) requires only a single key and a single primitive. Compared to ZMAC, our construction (2) allows variable, per-query parametrizable output lengths. Moreover, ZMAC+ (3) avoids the complex finalization of ZMAC and (4) improves the security bound from Ο(σ2/2n+min(n,t)) to Ο(q/2n + q(q + σ)/2n+min(n,t)) while retaining a practical tweak space

ZMAC: A Fast Tweakable Block Cipher Mode for Highly Secure Message Authentication

We propose a new mode of operation called ZMAC allowing to construct a (stateless and deterministic) message authentication code (MAC) from a tweakable block cipher (TBC). When using a TBC with $n$-bit blocks and $t$-bit tweaks, our construction provides security (as a variable-input-length PRF) beyond the birthday bound with respect to the block-length $n$ and allows to process $n+t$ bits of inputs per TBC call. In comparison, previous TBC-based modes such as PMAC1, the TBC-based generalization of the seminal PMAC mode (Black and Rogaway, EUROCRYPT 2002) or PMAC_TBC1k (Naito, ProvSec 2015) only process $n$ bits of input per TBC call. Since an $n$-bit block, $t$-bit tweak TBC can process at most $n+t$ bits of input per call, the efficiency of our construction is essentially optimal, while achieving beyond-birthday-bound security. The ZMAC mode is fully parallelizable and can be directly instantiated with several concrete TBC proposals, such as Deoxys and SKINNY. We also use ZMAC to construct a stateless and deterministic Authenticated Encryption scheme called ZAE which is very efficient and secure beyond the birthday bound

Cryptanalysis of PMACx, PMAC2x, and SIVx

At CT-RSA 2017, List and Nandi proposed two variable input length pseudorandom functions (VI-PRFs) called PMACx and PMAC2x, and a deterministic authenticated encryption scheme called SIVx. These schemes use a tweakable block cipher (TBC) as the underlying primitive, and are provably secure up to the query complexity of 2n, where n denotes the block length of the TBC. In this paper, we falsify the provable security claims by presenting concrete attacks. We show that with the query complexity of O(2n/2), i.e., with the birthday complexity, PMACx, PMAC2x, and SIVx are all insecure