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Code

For other meanings of the word code see Code (disambiguation)

Table of contents

1 Cryptography: codes versus ciphers 2 Codes in communication used for brevity 3 An example: the ASCII code 4 Codes to detect or correct errors (e.g., in storage or transmission) 5 Codes and acronyms

Cryptography: codes versus ciphers

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

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

Codes to detect or correct errors (e.g., in storage or transmission)

Codes and acronyms

See also: Glossary of coding terms.

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