The Committing Security of MACs with Applications to Generic Composition

Message Authentication Codes (MACs) are ubiquitous primitives deployed in multiple flavors through standards such as HMAC, CMAC, GMAC, LightMAC, and many others. Its versatility makes it an essential building block in applications necessitating message authentication and integrity checks, in authentication protocols, authenticated encryption schemes, or as a pseudorandom or key derivation function. Its usage in this variety of settings makes it susceptible to a broad range of attack scenarios. The latest attack trends leverage a lack of commitment or context-discovery security in AEAD schemes and these attacks are mainly due to the weakness in the underlying MAC part. However, these new attack models have been scarcely analyzed for MACs themselves. This paper provides a thorough treatment of MACs committing and context-discovery security. We reveal that commitment and context-discovery security of MACs have their own interest by highlighting real-world vulnerable scenarios. We formalize the required security notions for MACs, and analyze the security of standardized MACs for these notions. Additionally, as a constructive application, we analyze generic AEAD composition and provide simple and efficient ways to build committing and context-discovery secure AEADs.