The American English term cipher and the chiefly British English variant, cipher, are used loosely to refer to any coded or encrypted message and more particularly to refer to messages encrypted using a secret key. The word came into English through Old French and Medieval Latin, but originates in the Arabic word which is transliterated in English as sifr, which is a variant of a word meaning “to be empty,” and referring to the zero, which comes from the same Arabic root. In the realm of cryptography—hidden writing, cipher can be a verb meaning “to encrypt a text” or a noun, referring either to the particular cryptographic system used or to the key used in that system or to the text transmitted by that system. In other words, a cipher is three different things.
When the noun cipher is used to refer to a message that has been encrypted, it is referring to what is more specifically called ciphertext. This stands in contrast to plaintext, which refers to the regular, untreated material. Plaintext is what one has both before encryption and after decryption, with ciphertext temporarily replacing the plaintext during the time that it is being protected.
In the case in which the noun cipher is used to refer to the secret key, it is referring a value that is or can be used to encrypt a plaintext message. A symmetric key is used both to encrypt and decrypt the ciphertext. When asymmetric keys are used, there is a key to encrypt the data and a separate key to decrypt the data.
This brings us to the meaning of cipher in which the system of encryption and decryption is referred to. The system that uses a symmetric key is called Private-Key Encryption. The system that uses asymmetric keys is called Public-Key Encryption. Other categorizations of cipher systems include transposition ciphers and substitution ciphers, which describe different ways of treating the plaintext. The famous cipher used by Julius Caesar and often alluded to was a simple substitution cipher. Ciphers are also categorized as block ciphers or streaming ciphers.
Ciphers are used to facilitate private communications of many types. Ciphers may be used by a government, a spy, a business, or a terrorist. Ciphers are used on the Internet for email and credit card transactions, for example. In addition to making messages unreadable by those for whom they were not intended, ciphers also assist in the authentication of messages, assuring the recipient that the message is from the sender it purports to be from.