by Dinesh Thakur

Cipher textis information that has been changed into secret code for security reasons. A scheme for encoding messages to prevent them being read by unauthorized persons. It may have been enciphered,encrypted,or encoded, but it all means the same thing: it doesn't look like English so no one can use the information unless they have thepasswordto decode it.

The simplest ciphers work by substituting one letter for another as prescribed by a list of substitutions called a KEY, possession of which enables the message to be decoded and read.

For example the key could be a table of random number pairs, or an agreed passage from a published book (where consecutive letters of the enciphered message are replaced by those in the equivalent position in the passage). There are endless possibilities for such ciphers, the study of which falls within the discipline of CRYPTOGRAPHY.

Machines may be devised to generate fresh substitutions for every message, but substitution ciphers still remain vulnerable to the rapid processing power of modern computers, which can try out billions of permutations and exploit unchangeable facts such as the known frequencies of occurrence of the letters (and letter pairs, triples, etc) in texts written in different languages - in English for example, the most common letter will be E. The WWII German Enigma machine was the most famous such machine, and the cracking of its codes played a crucial role in the history of computing.

Modern cryptographers depend instead on encrypting the binary representation of a text, using mathematical algorithms that exploit the difficulty of factoring huge numbers.