Introductory lecture on classical and quantum cryptography given by Sébastien Tanzilli at both the Spring School of Physics organized by Médéric Argentina in Saint Étienne de Tinée, from the 20th to the 22nd of March 2010, and the "Fête de la Science" at the University of Nice in October 2010.
After a review of ancient cryptography methods, the usual methods, i.e., public and secret key ciphering, are presented/
In the framework of secret key ciphering, the question of how to share or to distribute the key between distant partners is discussed. It is shown that quantum key distribution, widely quantum cryptography, answers the question in a very efficient manner, and permits reaching unprecedented levels of security.
Note the presentation below is in French.