Improved security for OCB3

Abstract

OCB3 is the current version of the OCB authenticated encryption mode which is selected for the third round in CAESAR. So far the integrity analysis has been limited to an adversary making a single forging attempt. A simple extension for the best known bound establishes integrity security as long as the total number of query blocks (including encryptions and forging attempts) does not exceed the birthday-bound. In this paper we show an improved bound for integrity of OCB3 in terms of the number of blocks in the forging attempt. In particular we show that when the number of encryption query blocks is not more than birthday-bound (an assumption without which the privacy guarantee of OCB3 disappears), even an adversary making forging attempts with the number of blocks in the order of 2 n/ ℓMAX (n being the block-size and ℓMAX being the length of the longest block) may fail to break the integrity of OCB3.