Cryptology ePrint Archive: Report 2021/1166

Fine-tuning the ISO/IEC Standard LightMAC

Soumya Chattopadhyay and Ashwin Jha and Mridul Nandi

Abstract: LightMAC, by Luykx et al., is a block cipher based message authentication code (MAC). The simplicity of design and low overhead allows it to have very compact implementations. As a result, it has been recently chosen as an ISO/IEC standard MAC for lightweight applications. LightMAC has been shown to achieve query length independent security bound of $O(q^2/2^n)$ when instantiated with two independently keyed $n$-bit block ciphers, where $q$ denotes the number of MAC queries and the query-length is upper bounded by $(n-s)2^s$ bits for a fixed counter size $s$. In this paper, we aim to minimize the number of block cipher keys in LightMAC. First, we show that the original LightMAC instantiated with a single block cipher key, referred as 1k-LightMAC, achieves security bound of $O(q^2/2^n)$ while the query-length is at least $(n-s)$ bits and at most $(n-s)\min\{2^{n/4},2^s\}$ bits. Second, we show that a minor variant of 1k-LightMAC, dubbed as LightMAC-ds, achieves security bound of $O(q^2/2^n)$ while query-length is upper bounded by $(n-s)2^{s-1}$ bits. Of independent interest, our security proof of 1k-LightMAC employs a novel sampling approach, called the reset-sampling, as a subroutine within the H-coefficient proof setup.

Category / Keywords: secret-key cryptography / LightMAC, MAC, PRF, single-key, lightweight, ISO/IEC standard

Original Publication (with minor differences): IACR-ASIACRYPT-2021

Date: received 12 Sep 2021, last revised 7 Oct 2021

Contact author: s c 2357 at gmail com, ashwin jha1991 at gmail com, mridul nandi at gmail com

Available format(s): PDF | BibTeX Citation

Note: Two missing sub-cases included in the analysis.

Version: 20211007:173135 (All versions of this report)

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