Hybrid cryptography

Principle

The hybrid cryptography calls upon the two big families of cryptographic systems: the asymmetrical Cryptography and the symmetrical Cryptography. The software like PGP and GnuPG rests on this concept which makes it possible to combine the advantages of the two systems.

Asymmetrical cryptography is intrinsically slow from complex calculations which are associated there whereas symmetrical cryptography shines by its speed. However, the latter suffers from a low register Lacune, one must transmit the key S in a protected way (on an authenticated channel). To mitigate this defect, one resorts to the asymmetrical cryptography which works with a pair of keys: the private key and the public key.

Coding

The majority of the systems Hybride S proceed in the following way. A random key is generated for the symmetrical algorithm (3DES, IDEA, AES and well of others still), this key generally makes between 128 and 512 bits according to the algorithms. The symmetrical encryption algorithm is then used to quantify the message. In the case of a Coding per block S, one must use a mode of operation such as for example CBC, that makes it possible to quantify a message of size higher than that of a block. The random key as for it, is seen quantified thanks to the public key of the recipient, it is here that intervenes asymmetrical cryptography (RSA or Diffie-Hellman). As the key is short, this figuring takes little time. To quantify the whole of the message with an asymmetrical algorithm would be much heavier, this is why one prefers to pass by a symmetrical algorithm. It is then enough to send the coded message with the algorithm symmetrical and accompanied by the corresponding quantified key. The recipient deciphers the asymmetrical key with his private key and via a symmetrical deciphering, the message finds.

Authentification

It is very current to add authentifications and signatures to the messages sent. One uses for that of the functions of chopping (MD5, SHA-1 or of the codes authentificateurs like HMAC).

See too

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