IACR News item: 06 May 2015
Philipp Jovanovic, Samuel Neves
ePrint Report(OSGP). The authenticated encryption (AE) scheme deployed by OSGP is a
non-standard composition of RC4 and a home-brewed MAC, the ``OMA digest\'\'.
We present several practical key-recovery attacks against the OMA digest. The
first and basic variant can achieve this with a mere $13$ queries to an OMA
digest oracle and negligible time complexity. A more sophisticated version
breaks the OMA digest with only $4$ queries and a time complexity of about
$2^{25}$ simple operations. A different approach only requires one arbitrary
valid plaintext-tag pair, and recovers the key in an average of $144$
\\emph{message verification} queries, or one ciphertext-tag pair and $168$
\\emph{ciphertext verification} queries.
Since the encryption key is derived from the key used by the OMA digest, our
attacks break both confidentiality and authenticity of OSGP.
Additional news items may be found on the IACR news page.