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Format-preserving encryption
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<p>In <a href="/facts/Cryptography/e94PEVau">cryptography</a>, format-preserving encryption (FPE), refers to encrypting in such a way that the output (the <a href="/facts/Ciphertext/luNTs4sY">ciphertext</a>) is in the same format as the input (the <a href="/facts/Plaintext/tWnBDCgE">plaintext</a>). The meaning of "format" varies. Typically only finite sets of characters are used; numeric, alphabetic or alphanumeric. For example: </p> <ul><li>Encrypting a 16-digit credit card number so that the ciphertext is another 16-digit number.</li> <li>Encrypting an English word so that the ciphertext is another English word.</li> <li>Encrypting an <i>n</i>-bit number so that the ciphertext is another <i>n</i>-bit number (this is the definition of an <i>n</i>-bit block cipher).</li></ul> <p>For such finite domains, and for the purposes of the discussion below, the cipher is equivalent to a permutation of <i>N</i> integers {0, ... , <i>N</i>−1} where <i>N</i> is the size of the domain. </p>