In cryptography, encryption is the process of obscuring information to make it unreadable without special knowledge. Encryption has been used to protect communications for centuries, but only organizations and individuals with an extraordinary need for secrecy had made use of it. In the mid-1970s, strong encryption emerged from the sole preserve of secretive government agencies into the public domain and is now used in protecting widely-used systems, such as Internet e-commerce, mobile telephone networks, and bank automatic teller machines.

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.

In cryptography, a cipher (or cipher) is an algorithm for performing encryption and decryption — a series of well-defined steps that can be followed as a procedure. An alternative term is encipherment. In most cases, that procedure is varied depending on a key which changes the detailed operation of the algorithm. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes, which operated by substituting according to a large codebook.

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 knowledge of the key, it should be difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext.