NationMaster - Encyclopedia: Code (cryptography)

In the context of cryptography, a code is a method used to transform a message into an obscured form, 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 or words matched with a codeword. Messages in code are sometimes termed codetext. Cryptography - Wikipedia, the free encyclopedia /**/ @import /skins-1. ... Message in its most general meaning is the object of communication. ... Categories: Cryptography stubs | Cryptography ...

Terms like code and in code are often used to refer to any form of encryption. However, there is a major distinction between codes and ciphers in technical work; it is the scope of the transformation involved. Codes work at the level of meaning; that is, words or phrases are converted into something else. Ciphers work at the level of individual letters, or small groups of letters, or even, in modern ciphers, with individual bits. 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. Ciphers are more convenient than codes in some situations, there being no need for a codebook. In cryptography, encryption is the process of obscuring information to make it unreadable without special knowledge. ... This article is about algorithms for encryption and decryption. ...

Codes on the other hand, were long believed to be more secure than ciphers, there being (if the compiler of the codebook 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), ciphers have come to dominate cryptography.

One- and two-part codes

Codes are usually defined by "codebooks", which are dictionaries of codegroups listed with their corresponding plaintext. Codes originally seem to have had the codegroups assigned in 'plaintext order'. For example, in a code using numeric code groups, a plaintext word starting with "a" would have a low-value group, while one starting with "z" would have a high-value group. The same codebook could be used to "encode" a plaintext message into a coded message or "codetext", and "decode" a codetext back into plaintext message.

However, such "one-part" codes had a certain predictability that made it easier for others to notice patterns and "crack" or "break" the message, revealing the plaintext or part of it. In order to make life more difficult for codebreakers, codemakers then designed codes with no predictable relationship between the codegroups and the ordering of the matching plaintext. This meant that two codebooks were now required, one to look up plaintext to find codegroups for encoding, the other to look up codegroups to find plaintext for decoding. Students of foreign languages work much the same way; for, say, a Frenchman studying English, there is need of both an English-French and a French-English dictionary. Such "two-part" codes required more effort to develop, and twice as much effort to distribute and discard safely, but they were harder to break. Cryptanalysis (from the Greek kryptÃ³s, hidden, and analÃ½ein, to loosen or to untie) is the study of methods for obtaining the meaning of encrypted information without access to the secret information which is normally required to do so. ...

One time code

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. One time codes are often designed to be included in what would appear to be an innocent conversation. Done properly they are almost impossible to detect, though a trained analyst monitoring the communications 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 example is the telegram sent to U.S. President Harry Truman, then in Potsdam to meet with Stalin, informing Truman of the first successful test of an atomic bomb. On April 18, 1775, Paul Revere, William Dawes, and Samuel Prescott kept watch in Boston for the approach of British troops the day before the Battle of Lexington and Concord at the outset of the American Revolution. ... Tora! Tora! Tora! (ãƒˆãƒ©ãƒ»ãƒˆãƒ©ãƒ»ãƒˆãƒ©ï¼) is a 1970 film that dramatizes the Japanese attack on Pearl Harbor and the series of American blunders that aggravated its effectiveness. ... Combatants United States of America Empire of Japan Commanders Husband Kimmel (USN) Walter Short (USA) Chuichi Nagumo (IJN) Strength 8 battleships, 8 cruisers, 29 destroyers, 9 submarines, ~50 other ships, ~390 planes 6 aircraft carriers, 2 battleships, 3 cruisers, 9 destroyers, 8 tankers, 23 fleet submarines, 5 midget submarines, 441... World War II was a truly global conflict with many facets: immense human suffering, fierce indoctrination, and the use of new, extremely devastating weapons such as the atom bomb. ... This article is an overview article about the Crown chartered British Broadcasting Corporation formed in 1927. ... For the victim of Mt. ... (This article is about the German city of Potsdam. ... Iosif (usually anglicized as Joseph) Vissarionovich Stalin (Russian: Иосиф Виссарионович Сталин), original name Ioseb Jughashvili (Georgian: იოსებ ჯუღაშვილი; see Other names section) (December 21, 1879[1] – March 5, 1953) was a Bolshevik revolutionary and leader of the Soviet Union. ... An early stage in the Trinity fireball. ... The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 km (11 mi) above the epicenter. ...

See also one time pad Leslie Groves Leslie Richard Groves (August 17, 1896 - July 13, 1970) was a member of the United States Army who oversaw the construction of the Pentagon and the primary military leader in charge of the Manhattan Project to develop the atomic bomb during World War II. Born in Albany, New... In cryptography, the one-time pad (OTP), is a theoretically unbreakable method of encryption where the plaintext is combined with a random pad the same length as the plaintext. ...

Cryptanalysis of codes

While solving, say, a monoalphabetic substitution cipher is easy, solving even a simple code is difficult. Decrypting a coded message is a little like trying to translate a document written in an alien language, with the task basically amounting to building up a "dictionary" of the codegroups and the plaintext words they represent. In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the units may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. ...

One fingerhold on a simple code is the fact that some words are more common than others, such as "the" or "a" in English. In telegraphic messages, the codegroup for "STOP" (end of sentence) is usually very common. This helps define the structure of the message in terms of sentences, if not their meaning.

Further progress can be made against a code by collecting many messages encrypted with the same code and then using information from other sources

For example, a particular codegroup found almost exclusively in messages from a particular army and nowhere else might very well indicate the commander of that army. A codegroup that appears in messages preceding an attack on a particular location may very well stand for that location. Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. ...

Of course, cribs are an immediate giveaway to the definitions of codegroups. As codegroups are determined, they can gradually build up a critical mass, with more and more codegroups revealed from context and educated guesswork. One-part codes are more vulnerable to such educated guesswork than two-part codes, since if the codenumber "26839" of a one-part code is determined to stand for "bulldozer", then the lower codenumber "17598" will likely stand for a plaintext word that starts with "a" or "b". In cryptanalysis, a crib is a sample of known plaintext; the term originated at Bletchley Park, the British codebreaking operation during World War II (WWII). ...

Various tricks can be used to "plant" or "sow" information into a coded message, for example by executing a raid at a particular time and location against an enemy, and then examining code messages sent after the raid. Coding errors are a particularly useful fingerhold into a code; people reliably make errors, sometimes disastrous ones. Of course, planting data and exploiting errors works against ciphers as well.

Constructing a new code is like building a new language and writing a dictionary for it, which was an especially big job before computers. If a code is compromised, the entire task must be done all over again, and that means a lot of work for both cryptographers and the code users. In practice, when codes were in widespread use, they were usually changed on a periodic basis to frustrate codebreakers.

Once codes have been created, codebook distribution is logistically clumsy, and increases chances the code will be compromised. There is a saying that two people can keep a secret if one of them is dead, and though that may be something of an exaggeration, a secret becomes harder to keep if it is shared among several people. Codes can be reasonably secure if they are only used by a few people, but if whole armies use the same codebook, keeping them secure becomes much more difficult.

In contrast, the security of ciphers is, as mentioned earlier, generally dependent on protecting the cipher keys. Cipher keys can be stolen and people can betray them, but they are much easier to change and distribute.

Superencipherment

In more recent practice it became typical to encipher a message after first encoding it, so as to provide greater security. With a numerical code, this was commonly done with an "additive" - simply a long key number which was digit-by-digit added to the code groups, modulo 10. Unlike the codebooks, additives would be changed frequently.

One might wonder why a code would be used if it had to be enciphered to provide security? As well as providing security, a well designed code can also compress the message, and provide some degree of automatic error correction. In computer science, data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than a more obvious representation would use, through use of specific encoding schemes. ... In information theory and coding, an error-correcting code or ECC is a code in which each data signal conforms to specific rules of construction so that departures from this construction in the received signal can generally be automatically detected and corrected. ...

References

Clifford A. Pickover is a writer in the fields of science, mathematics, and science fiction. ...

Contents

One- and two-part codes GA_googleFillSlot("encyclopedia_square");

One time code

Cryptanalysis of codes

Superencipherment

References

See also

One- and two-part codes