2. Symmetrical cryptography

Symmetric cryptography is widely used, although it suffers from the problems inherent in the fact that sender and receiver must share the same key.

The first argument in favor of symmetrical systems is that they are faster than asymmetrical systems, because they make direct use of the hardwired instructions of commercial processors. In fact, public-key cryptography requires the use of large-number arithmetic, which is not implemented in standard computer processors.

First, we present a few primitives (block ciphers, stream ciphers and hash functions) that can be used to build protocols or mechanisms guaranteeing confidentiality, integrity and authentication of data origin (encryption systems and message authentication codes: MAC).