The Power of Verification Queries in Message Authentication and Authenticated Encryption

This paper points out that, contrary to popular belief, allowing a message authentication adversary multiple verification attempts towards forgery is not equivalent to allowing it a single one, so that the notion of security that most message authentication schemes are proven to meet does not guarantee their security in practice. We then show, however, that the equivalence does hold for strong unforgeability. Based on this we recover security of popular classes of message authentication schemes such as MACs (including HMAC and PRF-based MACs) and CWschemes. Furthermore, in many cases we do so with a tight security reduction, so that in the end the news we bring is surprisingly positive given the initial negative result. Finally, we show analogous results for authenticated encryption.