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<title><string language="fre"><![CDATA[5.1. Code-Based Digital Signatures]]></string></title>
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<string language="fre"><![CDATA[Welcome to the last
week of this MOOC on code-based cryptography. This week, we will be
discussing other cryptographic constructions
relying on coding theory. We have seen how to do
public key encryption and now we will see other kind of constructions. This first sequence we'll
be having a look at digital signatures and how to design
code-based digital signatures. So, what is digital signature? A digital signature is meant
to replace a paper signature on a digital document.
Exactly like a paper signature, only one person should be
able to create a digital signature which ties the
identity of the signer to the document. And like a normal
signature, everyone should be able to verify that the signature is
correct and so repudiation is impossible. This is kind of the
opposite of what we do with an encryption operation in
a public key cryptosystem where everyone can encrypt
and only one person can decrypt. That is why most digital
signatures use a decryption operation in the process. So, how do we
implement a digital signature? The first operation is
usually to hash the document into something of fixed size
which is a ciphertext. Why do we do this? For two
reasons: one is that we want to be able to sign
documents of arbitrary length, which means that we need, at
some point, to go from a large document to a small ciphertext. And then, we want to tie the
hash ciphertext to the document. So, we need something which
is a one way, for which it is difficult to build
collisions and everything which is exactly the
properties of hash function. Once we have this hash in the
ciphertext space, we simply decrypt it. This operation requires the
knowledge of the secret key as you have seen in
the previous sessions. And that is why only the
legitimate signer can perform this operation and can
compute the signature. Once you have this signature
in the plaintext space, you simply append it to the
document and that is it. For the verification, the
verifier just does the opposite operation: first, takes
the signature outside of the document, encrypts it
using the public key which is something that everyone can do.]]></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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<date><dateTime>2015-05-05</dateTime></date>
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<string language="fre"><![CDATA[Droits réservés à l'éditeur et aux auteurs. 
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