4. Cryptography applications: communication protocols

Cryptography is used today in many applications: in cell phones, on the Internet or for pay TV. In the case of cell phones, cryptography is used to ensure the confidentiality of communications. Indeed, telecommunications law requires operators to guarantee the security of user communications. In the case of cell phones in particular, communications between the phone and the radio station are encrypted. Only secret-key cryptography is used, and the encryption algorithm is a stream algorithm known as A5. On the Internet, cryptography is used to guarantee the confidentiality of certain communications, such as the transmission of a credit card code (SSL protocol), or to ensure the confidentiality, integrity and authentication of the sender in electronic messaging (S/MIME protocol) (see [H 5 330] ). Finally, cryptography...