Introduction

A fundamental objective of cryptography is to enable two persons to communicate over an insecure channel (a public channel such as the internet) in such a way that any other person is unable to recover their message (called the plaintext) from what is sent in its place over the channel (the ciphertext). The transformation of the plaintext into the ciphertext is called encryption, or enciphering. Encryption-decryption is the most ancient cryptographic activity (ciphers already existed four centuries b.c.), but its nature has deeply changed with the invention of computers, because the cryptanalysis (the activity of the third person, the eavesdropper, who aims at recovering the message) can use their power.

The encryption algorithm takes as input the plaintext and an encryption key KE, and it outputs the ciphertext. If the encryption key is secret, then we speak of conventional cryptography, of private key cryptography, or of symmetric cryptography. In practice, the principle of conventional cryptography relies on the sharing of a private key between the sender of a message (often called Alice in cryptography) and its receiver (often called Bob). If, on the contrary, the encryption key is public, then we speak of public key cryptography. Public key cryptography appeared in the literature in the late 1970s.