The term cipher refers to encrypted messages and can be a verb or noun. It originated from the Arabic word “sifr” meaning zero. Ciphertext is the encrypted message, while plaintext is the unencrypted message. A secret key is used to encrypt and decrypt messages, and there are different types of cipher systems, including Private Key Encryption and Public-Key Encryption. Ciphers are used for private communication and message authentication.
The American English term cipher and the primarily British English variant, cipher, are used loosely to refer to any encrypted or encrypted message, and more particularly to refer to messages encrypted using a secret key. The word entered English through Old French and Medieval Latin, but originates in the Arabic word which is transliterated into English as sifr, which is a variant of a word meaning “to be empty” and referring to the zero, which derives 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 to the particular cryptographic system used or the key used in that system or 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 refers to what is more specifically called ciphertext. This is in contrast to plaintext, which refers to normal, unprocessed material. The plaintext is what you have both before encryption and after decryption, with the ciphertext temporarily replacing the plaintext during the time it is being protected.
In case the name code is used to refer to the secret key, it refers to a value that is or can be used to encrypt a plain text message. A symmetric key is used to both encrypt and decrypt ciphertext. When asymmetric keys are used, there is one key to encrypt the data and a separate key to decrypt the data.
This brings us to the meaning of cipher where it refers to the encryption and decryption system. 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 dealing with plaintext. The famous cipher used by Julius Caesar and often alluded to was a simple substitution cipher. Ciphers are also classified as block ciphers or stream ciphers.
Ciphers are used to facilitate private communications of many types. Ciphers can be used by a government, a spy, a company or a terrorist. Ciphers are used on the Internet for e-mail and credit card transactions, for example. In addition to making messages unreadable by those for whom they were not intended, ciphers also aid in message authentication, assuring the recipient that the message is from the sender it purports to be.
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