Code (cryptography)
In cryptology, a code is a method used to encrypt a message that operates at the level of meaning; that is, words or phrases are converted into something else.
Codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing the cumbersome codebooks, so ciphers are now the dominant technique in modern cryptography.
In contrast, because codes are representational, they are not susceptible to mathematical analysis of the individual codebook elements.
Codes are defined by "codebooks" (physical or notional), which are dictionaries of codegroups listed with their corresponding plaintext.
The Zimmermann Telegram in January 1917 used the German diplomatic "0075" two-part code system which contained upwards of 10,000 phrases and individual words.
[3] A one-time code is a prearranged word, phrase or symbol that is intended to be used only once to convey a simple message, often the signal to execute or abort some plan or confirm that it has succeeded or failed.
Done properly they are almost impossible to detect, though a trained analyst monitoring the communications of someone who has already aroused suspicion might be able to recognize a comment like "Aunt Bertha has gone into labor" as having an ominous meaning.
Famous example of one time codes include: Sometimes messages are not prearranged and rely on shared knowledge hopefully known only to the recipients.
An early use of the term appears to be by George Perrault, a character in the science fiction book Friday[5] by Robert A. Heinlein: Terrorism expert Magnus Ranstorp said that the men who carried out the September 11 attacks on the United States used basic e-mail and what he calls "idiot code" to discuss their plans.
Decrypting a coded message is a little like trying to translate a document written in a foreign language, with the task basically amounting to building up a "dictionary" of the codegroups and the plaintext words they represent.
Constructing a new code is like building a new language and writing a dictionary for it; it was an especially big job before computers.
In practice, when codes were in widespread use, they were usually changed on a periodic basis to frustrate codebreakers, and to limit the useful life of stolen or copied codebooks.
