The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography

An algorithm is an equation or consistent program by which a process is achieved. In mathematics, an algorithm is a complex mathematical equation. In cryptology, an algorithm is the actual process by which the substitution cipher is carried out, changing the characters of the original message into the characters of the encrypted message.

In typical encryption, symmetric keys are employed whereby the encryption key is the same as the decryption key, thus necessitating that it be kept secret. An asymmetric key, however, is one in which there are two keys in the process: a public key and a private key. With an asymmetric key cipher, a public key is used to encrypt a message, available to anyone, but only one person possesses the key by which the message can be deciphered, thus keeping the message private between sender and recipient.

A cipher is a set and established system by which to transform a plaintext message into an encrypted message. The cipher works by substituting each of the characters in the original message—typically individual letters—into different, corresponding characters that the recipient of the encrypted message will be able to interpret and understand.