Abstract. Löndahl and Johansson proposed last year a variant of the McEliece cryptosystem which replaces Goppa codes by convolutional codes. This modification is supposed to make structural attacks more difficult since the public generator matrix of this scheme contains large parts which are generated completely at random. They proposed two schemes of this kind, one of them consists in taking a Goppa code and extending it by adding a generator matrix of a time varying convolutional code. We show here that this scheme can be successfully attacked by looking for low-weight codewords in the public code of this scheme and using it to unravel the convolutional part. It remains to break the Goppa part of this scheme which can be done in less than a day of computation in the case at hand.
International audienceCFS is the first practical code-based signature scheme. In the present paper, we present the initial scheme and its evolutions, the attacks it had to face and the countermeasures applied. We compare the different algorithmic choices involved during the implementation of the scheme and aim to provide guidelines to this task. We will show that all things considered the system remains practical. Finally, we present a state-of-the-art software implementation of the signing primitive to prove our claim. For eighty bits of security our implementation produces a signature in 1.3 seconds on a single core of Intel Xeon W3670 at 3.20 GHz. Moreover the computation is easy to distribute and we can take full profit of multi-core processors reducing the signature time to a fraction of second in software
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