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Title:
Boosting OMD for Almost Free Authentication of Associated Data
Authors: Reza Reyhanitabar, Serge Vaudenay, Damian Vizár
Abstract:We propose \emph{pure} OMD (p-OMD) as a new variant of the Offset Merkle-Damgård (OMD) authenticated encryption scheme. Our new scheme inherits all desirable security features of OMD while having a more compact structure and providing higher efficiency. The original OMD scheme, as submitted to the CAESAR competition, couples a single pass of a variant of the Merkle-Damgård (MD) iteration with the counter-based XOR MAC algorithm to provide privacy and authenticity. Our improved p-OMD scheme dispenses with the XOR MAC algorithm and is \emph{purely} based on the MD iteration; hence, the name ``pure’’ OMD. To process a message of \ell blocks and associated data of a blocks, OMD needs \ell+a+2 calls to the compression function while p-OMD only requires \max\left\{\ell, a\right\}+2 calls. Therefore, for a typical case where \ell \geq a, p-OMD makes just \ell+2 calls to the compression function; that is, associated data is processed almost freely compared to OMD. We prove the security of p-OMD under the same standard assumption (pseudo-randomness of the compression function) as made in OMD; moreover, the security bound for p-OMD is the same as that of OMD, showing that the modifications made to boost the performance are without any loss of security.
ePrint: https://eprint.iacr.org/2015/302
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