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Improving the Efficiency of a Reaction Attack on the QCMDPC McEliece
Thales BANDIERA PAIVA Routo TERADA
Publication
IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences
Vol.E101A
No.10
pp.16761686 Publication Date: 2018/10/01
Online ISSN: 17451337
DOI: 10.1587/transfun.E101.A.1676
Type of Manuscript: PAPER Category: Cryptography and Information Security Keyword: QCMDPC McEliece, postquantum cryptography, reaction attack,
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Summary:
The QCMDPC McEliece scheme was considered one of the most promising public key encryption schemes for efficient postquantum secure encryption. As a variant of the McEliece scheme, it is based on the syndrome decoding problem, which is a hard problem from Coding Theory. Its key sizes are competitive with the ones of the widely used RSA cryptosystem, and it came with an apparently strong security reduction. For three years, the scheme has not suffered major threats, until the end of 2016, at the Asiacrypt, when Guo, Johansson, and Stankovski presented a reaction attack on the QCMDPC that exploits one aspect that was not considered in the security reduction: the probability of a decoding failure to occur is lower when the secret key and the error used for encryption share certain properties. Recording the decoding failures, the attacker obtains information about the secret key and then use the information gathered to reconstruct the key. Guo et al. presented an algorithm for key reconstruction for which we can point two weaknesses. The first one is that it cannot deal with partial information about the secret key, resulting in the attacker having to send a large number of decoding challenges. The second one is that it does not scale well for higher security levels. To improve the attack, we propose a key reconstruction algorithm that runs faster than Guo's et al. algorithm, even using around 20% less interactions with the secret key holder than used by their algorithm, considering parameters suggested for 80 bits of security. It also has a lower asymptotic complexity which makes it scale much better for higher security parameters. The algorithm can be parallelized straightforwardly, which is not the case for the one by Guo et al.

