This paper considers the security of the (OTR) authenticated encryption mode [9] 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 F2n. 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 F2n, thus generalising OTR without reducing the security of OTR.
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