Fine-Tuning the ISO/IEC Standard LightMAC

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.

Soumya Chattopadhyay and Mridul Nandi are supported by the project “Study and Analysis of IoT Security” under Government of India at R. C. Bose Centre for Cryptology and Security, Indian Statistical Institute, Kolkata. Ashwin Jha’s work was carried out in the framework of the French-German-Center for Cybersecurity, a collaboration of CISPA and LORIA.