Cryptography

The cryptography is one of the disciplines of the Cryptologie attempting to protect from the messages (ensuring Confidentialité, authenticity and integrity) by often helping of secret or key .

It is used since the Antiquité, but some of its most important methods, like the asymmetrical Cryptographie, have only a few tens of years of existence.

Vocabulary

Because of the use of Anglicism S then of creation of the television channels known as “encrypted”, a great confusion reigns concerning the various terms of cryptography:
  • Coding: transformation using a key of coding of a message into light into an incomprehensible message if one does not have a Clé of deciphering (in English encryption );
  • figure: in the past secret code, by extension the algorithm used for coding;
  • Cryptogram: coded message;
  • To decipher: to find the clear message corresponding to a coded message without having the key of deciphering (term that do not have the english-speaking, who them “break” secret codes);
  • cryptography: étymologiquement “secret writing”, become by extension the study of this art (thus today science aiming at creating cryptograms, i.e. to quantify);
  • Cryptanalyse : science analyzing the cryptograms in order to decipher them;
  • Cryptology: science gathering cryptography and the cryptanalyse.

It thus appears that put at the glance couples it to quantify/decipher and of the direction of the word “to decipher”, the term “crypter” does not have a raison d'être (the French Academy precise that the word is to banish and this one does not appear in its dictionary), in any case not in the direction where one finds it used in general.

Algorithms and protocols

Weak encryption algorithms (easily breakable)

The first algorithms used for the Chiffrement of information were rather rudimentary as a whole. They consisted in particular with the replacement of characters by others. The confidentiality of the encryption algorithm was thus the angular stone of this system to avoid a fast decoding.

Examples of weak encryption algorithms:

  • ROT13 (rotation of 13 characters, without key);
  • Figure of César (shift of three letters in the alphabet).
  • Figure of Vigenère (introduced the concept of key)

Algorithms of symmetrical cryptography (with secret key)

See also: symmetrical Cryptography

The symmetrical encryption algorithms are based on same a key to quantify and decipher a message. The problem of this technique is that the key, which must remain completely confidential, must be transmitted to the correspondent in a sure way.

Some symmetrical encryption algorithms very much used:

  • Figure of Vernam (only offering an absolute theoretical safety, provided that the key has at least the same length as the message, than it is used only once to quantify and than it is completely random)
  • 3DES
  • AES
  • RC4
  • RC5
  • MISTY1
  • and others (see the more exhaustive list of algorithms of symmetrical cryptography).

Algorithms of asymmetrical cryptography (with public and private key)

See also: asymmetrical Cryptography

To solve the problem of the exchange of keys, asymmetrical cryptography was developed in the Années 1970. It is based on the principle of two keys:

  • public, allowing coding;
  • deprived, allowing the deciphering.
As its name indicates it, the public key is availability of whoever wishes to quantify a message. This last could be deciphered only with the private key, which must be confidential.

Some algorithms of asymmetrical cryptography very much used:

  • RSA ;
  • DSA ;
  • Protocol of exchange of keys Diffie-Hellman;
  • and others; to see this more complete list of algorithms of asymmetrical cryptography.

The principal disadvantage of RSA and other algorithms with public keys is their great slowness compared to the algorithms with secret keys. RSA is for example 1000 times slower than DES. In practice, within the framework of the confidentiality, one makes use of it to quantify a random random number which is used then as secret key for a symmetrical encryption algorithm. It is the principle which use of the software like PGP for example.

Asymmetrical cryptography is also used to ensure the authenticity of a message. The print of the message is quantified using the private key and is united with the message. The recipients decipher then the cryptogram using the public key and find the print normally. That ensures to them that the transmitter is well the author of the message. One speaks then about signature or about sealing.

Functions of chopping

See also: Function of chopping

A function of chopping is a function which converts a great unit into a smaller unit, the print. It is impossible to decipher it to return to the whole of origin, it is thus not a technique of coding.

Some functions of chopping very much used:

  • MD5 ;
  • SHA-1 ;
  • and others; to see this more complete list of algorithms of chopping.

The print of a message generally does not exceed 256 bytes and makes it possible to check its integrity.

The Community

  • Projet NESSIE
  • Concours AES
  • the Cryptologue S are experts in cryptology, they design, analyze and break the algorithms; to see this list of cryptologists

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