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The operation of a cipher usually depends on a piece of auxiliary information, called a [[key (cryptography)|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 language|Arabic]] word for zero: ''{{IPA|ṣ}}ifr'' 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 Mediaval 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 versus codes ==
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