Ciphers, also called encryption algorithms, are systems for encrypting and decrypting data. A cipher converts the original message, called plaintext, into ciphertext using a key to determine how it is done.
Ciphers are generally categorized according to how they work and by how their key is used for encryption and decryption. Block ciphers accumulate symbols in a message of a fixed size (the block), and stream ciphers work on a continuous stream of symbols. When a cipher uses the same key for encryption and decryption, they are known as symmetric key algorithms or ciphers. Asymmetric key algorithms or ciphers use a different key for encryption/decryption.
Ciphers can be complex algorithms or simple ones. A common cipher, ROT13 (or ROT-13), is a basic letter substitution cipher, shorthand for “rotate by 13 places” in the alphabet. In a message, ROT13 replaces each letter of the alphabet with the letter that is thirteen places ahead of it.
"The government of ancient Rome was among the first civilizations to use ciphers to transmit sensitive information such as military conversations."