Cryptography (from Ancient Greek words kryptós and graphein, which together mean "secret writing") is the set of theory and techniques used to guarantee a secure communication between sender and receiver. A communication is secure if there is no risk for the message to reach an unwanted receiver. Cryptography originates in the military, from the need to secretly communicate with allies. Coding the message, in such a way that it is no longer understandable for who does not have the key to decrypt it, solves the problem. In cryptography literature, the encrypted message is called ciphertext. Coding and decoding are the operations to obtain the cyphertext from the plaintext and vice versa.