[Resource Topic] 2017/845: Improved Security for OCB3

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Improved Security for OCB3

Authors: Ritam Bhaumik, Mridul Nandi


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 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 birthdaybound (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 2n=L_MAX (n being the block-size and L_MAX being the length of the longest block) may fail to break the integrity of OCB3.

ePrint: https://eprint.iacr.org/2017/845

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