Paper 2026/425

Committing Security of BBB Secure MACs

Abstract

Committing security has recently emerged as an essential property for message authentication codes (MACs), driven by applications such as abuse reporting in end-to-end encrypted messaging systems. Traditional notions, such as unforgeability or pseudorandomness, are insufficient in these contexts, prompting the introduction of stronger security notions. Bhaumik et al. (CRYPTO'24) initiated this line of research by formalizing the notions of commitment and context-discovery for MACs and analyzing several standardized birthday-bound secure constructions. We extend this line of work to beyond-birthday-bound (BBB) secure MACs. In particular, we examine the class of constructions identified by Chen et al. (ASIACRYPT'21), which employ two block ciphers and a single block hash function, together with well-studied BBB MACs from the DbHtS paradigm. Our findings depict a heterogeneous picture: while several constructions succumb to simple attacks, others exhibit some resilience, highlighting that committing and context-discovery security for BBB MACs depends strongly on structural design choices.