Cryptology ePrint Archive: Report 2018/066

Tweaking Generic OTR to Avoid Forgery Attacks

Hassan Qahur Al Mahri and Leonie Simpson and Harry Bartlett and Ed Dawson and Kenneth Koon-Ho Wong

Abstract: This paper considers the security of the Offset Two-Round (OTR) authenticated encryption mode \cite{cryptoeprint:2013:628} with respect to forgery attacks. The current version of OTR gives a security proof for specific choices of the block size $(n)$ and the primitive polynomial used to construct the finite field $\mathbb{F}_{2^n}$. Although the OTR construction is generic, the security proof is not. For every choice of finite field the distinctness of masking coefficients must be verified to ensure security. In this paper, we show that some primitive polynomials result in collisions among the masking coefficients used in the current instantiation, from which forgeries can be constructed. We propose a new way to instantiate OTR so that the masking coefficients are distinct in every finite field $\mathbb{F}_{2^n}$, thus generalising OTR without reducing the security of OTR.

Category / Keywords: secret-key cryptography / Authenticated encryption, OTR, confidentiality, integrity, forgery attack, tweakable block cipher, symmetric encryption, AEAD

Original Publication (in the same form): International Conference on Applications and Techniques in Information Security, ATIS 2016
DOI:
10.1007/978-981-10-2741-3_4