The procedure Encode takes a string s, and an optional encoding= argument, and returns a string obtained from s by applying the encoding method indicated by the encoding= option.

The procedure Decode takes an encoded string s, and an optional encoding= argument, and returns the string obtained by decoding according to the method indicated in the second argument. It is an approximate inverse of Encode.

The currently supported encodings are:

The default encoding is null, which returns the input unchanged.

The rot13 encoding is a simple Caesar cypher that is applied only to alphabetic characters in the input string. Other characters are not encoded.) It treats the alphabetic characters ("a".."z" and "A".."Z") as integers modulo 26 and shifts each one by 13.

The general  encoding performs a Caesar cypher modulo 256 on the individual bytes in the input string by treating each as an integer in the range  and shifting each by the offset n modulo 256. The parameter n of the encoding must be an integer in the range .

While the general  encoding is applied indiscriminately to all nonzero bytes of a string, the  encoding is generally more suitable for didactic purposes. The parameter n may be any integer in the range , and only alphabetic characters in the text are affected by the encoding. Other characters in the text are passed through to the output unchanged. Upper and lower case letters are encoded independently, each letter being shifted cyclically (modulo 26) within its own case.

The base64 encoding is described in detail in RFC 2045. It is standard encoding for arbitrary data in a subset of the US-ASCII character set so that it can be transmitted via channels that are unable to handle 8-bit character data. It is frequently used in electronic mail (SMTP) and web (HTTP) transactions.

The percent encoding is used to encode URLs. Certain characters in a URL are replaced with a % sign, followed by a two-digit hexadecimal code for the US-ASCII code point of the character.

Information Sciences Institute, "RFC 2045 Internet Message Bodies," ISI Home Page, http://www.isi.edu/in-notes/rfc2045.txt; accessed 28 November 2005.