In communications, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another object or action, not necessarily of the same sort. One reason for this is to enable communication in places where ordinary spoken or written language is difficult or impossible. For example, a cable code replaces words (eg, ship, invoice, ...) into shorter words, allowing the same information to be sent with fewer characters, more quickly, and most important, less expensively. Another example is the use of semaphore flags, where the configuration of flags held by a signaller or the arms of a semaphore tower encodes parts of the message, typically individual letters and numbers. Another person standing a great distance away can interpret the flags and reproduce the words sent.
- For other meanings of the word code see Code (disambiguation)
Cryptography: codes versus ciphers
Codes have long been used in cryptography. Codes are used to transform the content or meaning of a message into something else, preventing those not in on the secret from understanding what is actually transmitted. The usual method is to use a "codebook" with a list of common phrases/words matched with different phrases/words, so that people without the codebook who might intercept the message get nothing but a message referring to something else altogether, or alternatively, complete gibberish. A cable code would be one such.
In general usage, the term code is very often confused with the term cipher, but in cryptography they refer to different concepts. They can be distinguished best by the scope of the differing transformations: A code is a set of rules which result in a change in the representation of meaningful information to some other form -- this need not include secrecy. Most cable codes are examples in which secrecy isn't the purpose. Cable codes were developed to lower transmission costs by reducing message length. Codes work at the level of meaning; that is, words or phrases are converted into something else; for a cable code the something else had fewer letters. But, when the information should not be known to anyone but the intended recipient, the transformation must not be reversible by just anyone. There must be some secret information required to successfully reverse the transformation.
A code which requires such secret knowledge is a cryptographic code, and the secret information and rules to use it is a codebook.
A cipher by contrast, does not work at the level of meaningful information. While a code might transform "attack" into "FRGPL" or "mincemeat pie", a cipher transforms elements below the semantic level, ie, below the level of meaning. The "a" in attack might be converted to "Q", the first "t" to "f", the second "t" to "3", and so on. Cyphers are more convenient than codes in some situations, there being no need for a codebook.
Codes on the other hand, were long believed to be more secure than cyphers, there being (if one's codebook constructor did a good job) no 'pattern of transformation' which can be discovered. With the advent of automatic processors (ie, in recent times the electronic computer), cyphers have come to dominate cryptography.
Codes in communication used for brevity
Code can be used for brevity. When telegraph messages were the state of the art in rapid long distance communication, elaborate commercial codes which encoded complete phrases into single words (commonly five-letter groups) were developed, so that telegraphers became conversant with such "words" as BYOXO ("Are you trying to weasel out of our deal?"), LIOUY ("Why do you not answer my question?"), BMULD ("You're a skunk!"), or AYYLU ("Not clearly coded, repeat more clearly."). Code words were chosen for various reasons: length, pronouncability, etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes, ... Codebooks and codebook publishers proliferated, inlcuding one run as a front for the American Black Chamber run by Herbert Yardley between WWI and WWII. The purpose of most of these codes was to save on cable costs.
In the computer era since World War II, there are also "codes" for data compression, e.g. Huffman coding, which uses short codes for frequent symbols and longer codes for seldom used symbols - the same principle is used in the Morse code. It and the Baudot code which uses the same length representation for all symbols and characters, both go back to telegraph days. The later was a primary ancestor of the ASCII character code widely used in computers.
An example: the ASCII code
Probably the most widely known data communications code (aka character representation) in use today is ASCII. In one or another (somewhat compatible) version, it is used by nearly all personal computers, terminalss, printerss, and other communication equipment. Its original version represents 128 characters with seven-bit binary numbers--that is, as a string of seven 1s and 0s.
In ASCII a lowercase "a" is always 1100001, an uppercase "A" always 1000001, and so on.
Extensions to ASCII have included 8-bit characters (for letters of European languages and such thingsas card suit symbols), and in fullest flowering have included glyphs from essentially all of the world's writing systems (see Unicode and Bob Bemer).
Codes to detect or correct errors (e.g., in storage or transmission)
Codes may also be used to represent data in a way more resistant
to errors in transmission or storage. Such a "code" is
called an Error-correcting code, and works by including carefully crafted redundancy with the stored (or transmitted) data. Examples include Hamming codes, Reed-Solomon, Reed-Muller, Bose-Chaudhuri-Hochquenghem, Turbo, Golay, Goppa, and Gallager Low-density parity-check codes.
Codes and acronyms
Acronyms and abbreviations can be considered codes, and in a sense all languages and writing systems are codes for human thought. Occasionally a code word achieves an independent existence (and meaning) while the original equivalent
phrase is forgotten or at least no longer has the precise meaning attributed to the code word. For example, the number "86" was once used as a code word in restaurants meaning "We're out of the requested item". It is now commonly used to mean the removal or destruction of something. '30' was widely used in journalism to mean "end of story", and it is sometimes used in other contexts to signify "the end".
- See also: Glossary of coding terms.
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