Cryptology ePrint Archive: Report 2018/1087

Breaking the confidentiality of OCB2

Bertram Poettering

Abstract: OCB2 is a widely standardized mode of operation of a blockcipher that aims at providing authenticated encryption. A recent report by Inoue and Minematsu (IACR EPRINT report 2018/1040) indicates that OCB2 does not meet this goal. Concretely, by describing simple forging attacks the authors evidence that the (sub)goal of authenticity is not reached. The report does not question the confidentiality offered by OCB2.

In this note we show how the attacks of Inoue and Minematsu can be extended to also break the confidentiality of OCB2. We do this by constructing both IND-CCA and plaintext recovering adversaries, all of which require minimal resources and achieve overwhelming success rates.

Category / Keywords: secret-key cryptography / OCB2, confidentiality, attack, AEAD

Date: received 8 Nov 2018, last revised 20 Mar 2019

Contact author: bertram poettering at rhul ac uk

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Version: 20190320:142959 (All versions of this report)

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