In the mid-1970s, strong encryption emerged from the sole preserve of secretive government agencies into the public domain, and is now employed 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.

Encryption or software code obfuscation is also used in software copy protection against reverse engineering, unauthorized application analysis, cracks and software piracy used in different encryption or obfuscating software.

CIPHERS

A cipher is an algorithm for performing encryption (and the reverse, decryption) — a series of well defined steps that can be followed as a procedure.  An alternative term is encipherment.

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: cifr 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.

There are also etymological roots to the Middle French word cifre, and the Medieval Latin cifra, both of which are probably originated from the Arabic root.  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'.

CIPHERS VS. CODES

In non-technical usage, a "(secret) code" is the same thing as a cipher.  Within technical discussions, however, they are distinguished into two concepts. Codes work at the level of meaning — that is, words or phrases are converted into something else.  Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits.