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<title><string language="fre"><![CDATA[5.4. Parallel-CFS]]></string></title>
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<string language="fre"><![CDATA[In this session, I will
present a variant of the CFS signature scheme called
parallel-CFS. We start from a simple question: what
happens if you try to use two different hash functions and
compute two different CFS signatures? For the signer, you simply
take twice as much computation because you have to
do two signatures. And then, the signature is
twice longer because you have just to concatenate two signatures. One would assume that for
the attacker it is the same, he simply has to forge two signatures. Well, things are a little
more complicated than that. What happens when you want to do
decoding one out of many twice in a row? So, you start with a set of
N documents and compute the hashes of these documents to
build a list of target syndromes. As we have seen, if N =
2^(mt/3), one solution is found on average. Then, we can
move on to the second hash function and try to do
also decoding one out of many. The only problem is, you
only have one solution with the first hash function. So,
you only have one target document for the second
problem and you cannot do decoding one out of many anymore. In order to be able to do decoding one out of many
twice in a row, you need to start from a much
larger list of syndromes. Then, find a set of
solutions instead of just a single solution and use this set
of solutions to find one solution to both hash
functions at the time. This means that the set of
target syndromes has to be larger and the complexity
of the attack will be larger. We have just seen that
for the attacker, computing syndrome decoding twice in
a row is more complicated. But the same kind of
problem happens to the legitimate signer when using counters.
The first strategy would be first, pick a document D,
use the first hash function to compute a signature, this
will get the value of the counter i; then, use h' to
compute the second signature with a second value
of the counter i'.]]></string></description>
<keyword><string language="fre"><![CDATA[algèbre linéaire]]></string></keyword><keyword><string language="fre"><![CDATA[chiffrement à clé publique]]></string></keyword><keyword><string language="fre"><![CDATA[cryptage des données]]></string></keyword><keyword><string language="fre"><![CDATA[cryptographie]]></string></keyword><keyword><string language="fre"><![CDATA[code correcteur]]></string></keyword><keyword><string language="fre"><![CDATA[algorithmes]]></string></keyword><keyword><string language="fre"><![CDATA[CFS]]></string></keyword><keyword><string language="fre"><![CDATA[Courtois-Finiasz-Sendrier]]></string></keyword>
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<entity><![CDATA[BEGIN:VCARD
VERSION:3.0
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REV:2021-07-06 18:03:09
FN:Irene MARQUEZ-CORBELLA
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<date><dateTime>2015-05-05</dateTime></date>
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<string language="fre"><![CDATA[Droits réservés à l'éditeur et aux auteurs. 
Ces ressources de cours sont, sauf mention contraire, diffusées sous Licence Creative Commons. L’utilisateur doit mentionner le nom de l’auteur, il peut exploiter l’œuvre sauf dans un contexte commercial et il ne peut apporter de modifications à l’œuvre originale.]]></string>
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<string language="fre"><![CDATA[5: Other cryptographic constructions relying on coding theory]]></string>
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