Encryption can be used to ensure secrecy, but other techniques are still needed to make communications secure, particularly to verify the integrity and authenticity of a message; for example, a Message authentication code (MAC) or digital signatures. Another consideration is protection against Traffic analysis.
The original information is known as Plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it; it should resemble random gibberish to those not intended to read it.
The operation of a cipher usually depends on a piece of auxiliary information, called a key or, in traditional NSA parlance, a cryptovariable. The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without the same key, it should be difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext.
"Cipher" is alternatively spelled "cypher"; similarly "ciphertext" and "cyphertext", and so forth. The word descends from the Arabic word for zero: ṣifr or صِفْر, like (the Italian) zero (which remained in use for 0, the crucial innovation in positional Arabic versus Roman numerals) but soon was used for any decimal digit, even any number. While it may have come to mean encoding because that often involved numbers, a theory says conservative catholic opponents of the Arabic (heathen) numerals equated it with any 'dark secret'.
Historically, cryptography was split into a dichotomy of codes and ciphers, and coding had its own terminology, analogous to that for ciphers: "encoding, codetext, decoding" and so on. However, codes have a variety of drawbacks, including susceptibility to Cryptanalysis and the difficulty of managing a cumbersome Codebook. Because of this, codes have fallen into disuse in modern cryptography, and ciphers are the dominant technique.
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include substitution ciphers and transposition ciphers. During the early 20th century, more sophisticated machines for encryption were used, rotor machines, which were more complex than previous schemes.
Encryption methods can be divided into symmetric key algorithms and asymmetric key algorithms. In a Symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption. In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables him to perform decryption.
Symmetric key ciphers can be distinguished into two types, depending on whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).