Cryptography - Algorithms

In the examples described in the "Mathematics" article, once the algorithm had been chosen, the two correspondents agreed on the key K, which they kept secret. The process was then symmetrical: each could send and receive messages confidentially. Such algorithms are said to be symmetrical or secret-key.

The 1970s saw the emergence of a new type of algorithm known as a public-key or asymmetric algorithm. In our formalism, these correspond to a situation where the data given by E_K is not practically sufficient (in a sense yet to be precisely defined, but let's say using existing computational means) to find D_K. In this case, the process is no longer symmetrical; the owner of E_K is able to send messages to the owner of D_K, who will be the only one able to read them. There is then no reason to keep the E_K application secret; it is published as a public key. Anyone can send messages confidentially to the owner of D_K, the latter application or what is needed to build it being called the secret key. In the rest of this text, we'll describe some examples that will help clarify the notion of a public-key algorithm.

The "Cryptography" article is the subject of two booklets:

AF 172 Mathematics

AF 173 Algorithms

The subjects are not independent of each other.

Readers will need to refer to the other booklet often enough.