NationMaster - Encyclopedia: Autokey cipher

An autokey cipher is a cipher which incorporates the message (the plaintext) into the key. There are two forms of autokey cipher: key autokey and text autokey ciphers. A key-autokey cipher uses previous members of the keystream to determine the next element in the key stream. A text-autokey uses the previous message text to determine the next element in the keystream. Image File history File links Vigenere square / tabula recta. ... Image File history File links Vigenere square / tabula recta. ... Tabula recta In cryptography, the tabula recta is a square table of alphabets, each one made by shifting the previous one to the left. ... This article is about algorithms for encryption and decryption. ... The plain text term has a different meaning. ... A key is a piece of information that controls the operation of a cryptography algorithm. ...

In modern cryptography, self-synchronizing stream ciphers are autokey ciphers. The operation of A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ...

History

The first autokey cipher was invented by Girolamo Cardano, and, although it contained a weakness that made it easy to break, a number of attempts were made by other cryptographers to produce an autokey system that was not trivial to break; eventually one was invented by Blaise de Vigenère. Vigenère started with a tabula recta, a square with 26 copies of the alphabet, the first line starting with 'A', the next line starting with 'B', etc., like the one on the right. Gerolamo Cardano or Jerome Cardan (September 24, 1501 - September 21, 1576) was a celebrated Renaissance mathematician, physician, astrologer, and gambler. ... Blaise de VigenÃ¨re (April 5, 1523 - 1596) was a French diplomat and cryptographer. ... Tabula recta In cryptography, the tabula recta is a square table of alphabets, each one made by shifting the previous one to the left. ...

In order to encrypt a plaintext, one locates the row with the first letter to be encrypted, and the column with the first letter of the key. The letter where the line and column cross is the ciphertext letter. This continues for all the letters of the message. So far, this is identical to an earlier cipher scheme, which, confusingly, was erroneously attributed to Vigenère: the Vigenère cipher. The VigenÃ¨re cipher is named for Blaise de VigenÃ¨re (pictured), although Giovan Batista Belaso had invented the cipher earlier. ...

Despite this, Vigenère's innovation was in the way the key was generated. He started with a relatively short keyword, and appended the message to it. So if Sneha's keyword was "QUEENLY", and the message was "ATTACK AT DAWN", the key would be "QUEENLYATTACKATDAWN".

This text-autokey cipher was hailed as "le chiffre indéchiffrable", and was indeed undeciphered for over 200 years, until Charles Babbage discovered a means of breaking the cipher. Charles Babbage (26 December 1791 â€“ 18 October 1871) was an English mathematician, analytical philosopher, mechanical engineer and (proto-) computer scientist who originated the idea of a programmable computer. ...

Cryptanalysis

Using an example message "meet at the fountain" encrypted with the keyword "KILT":

We try common words, bigrams, trigrams etc. in all possible positions in the key. For example, "THE": The factual accuracy of this article is disputed. ... Bigrams are groups of two written letters, two syllables, or two words, and are very commonly used as the basis for simple statistical analysis of text; one of the most successful language models for Speech Recognition (Collins, 1996). ... The bagua (Chinese: 八卦; pinyin: ; Wade-Giles: pa kua; literally eight trigrams) is a fundamental philosophical concept in ancient China. ...

We know that a correct plaintext fragment will also appear in the key, shifted right by the length of the keyword. Similarly our guessed key fragment ("THE") will also appear in the plaintext shifted left. So by guessing keyword lengths (probably between 3 and 12) we can reveal more plaintext and key.

We see that a shift of 4 looks good (both of the others have unlikely Qs), so we shift the revealed "ETA" back by 4 into the plaintext:

We have a lot to work with now. The keyword is probably 4 characters long ("..LT"), and we have some of the message:

Because our plaintext guesses have an effect on the key 4 characters to the left, we get feedback on correct/incorrect guesses, so we can quickly fill in the gaps:

The ease of cryptanalysis is thanks to the feedback from the relationship between plaintext and key. A 3-character guess reveals 6 more characters, which then reveal further characters, creating a cascade effect, allowing us to rule out incorrect guesses quickly.

Autokey in modern ciphers

Modern autokey ciphers use very different encryption methods, but they follow the same approach of using either key bytes or plaintext bytes to generate more key bytes. Most modern stream ciphers are based on pseudorandom number generators: the key is used to initialize the generator, and either key bytes or plaintext bytes are fed back into the generator to produce more bytes. The operation of A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ... A pseudorandom number generator (PRNG) is an algorithm that generates a sequence of numbers which are not truly random. ...

Some stream ciphers are said to be "self-synchronizing", because the next key byte usually depends only on the previous N bytes of the message. If a byte in the message is lost or corrupted, therefore, the key-stream will also be corrupted--but only until N bytes have been processed. At that point the keystream goes back to normal, and the rest of the message will decrypt correctly. The operation of A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ...

Categories: Classical ciphers | Stream ciphers In cryptography, a rotor machine is a electro-mechanical device used for encrypting and decrypting secret messages. ... CSP 1600, the replacement stepping unit to adapt the ECM Mark II to CCM The Combined Cipher Machine (CCM) (or Combined Cypher Machine) was a common cipher machine system for securing Allied communications during World War II and for a few years after amongst NATO. The British Typex machine and... The plugboard, keyboard, lamps and finger-wheels of the rotors emerging from the inner lid of a three-rotor German military Enigma machine (version with labels) For other uses, see Enigma. ... The advanced Russian cipher machine Fialka (M-125) has only recently been made known to the public. ... A single-rotor Hebern machine. ... A diagram of a loopback-rotor construction; in this simplified example, two rotors with nine contacts each are used. ... KL-7 on display at USAF Communications Agency museum. ... The Lacida (or LCD) was a rotor cipher machine designed before World War II by the Polish Cipher Bureau for wartime use by Polish higher commands. ... Friedman patented the design of the M-325 in US patent #2,877,565. ... Mercury was a British cipher machine used by the Air Ministry from 1950 until at least the early 1960s. ... The NEMA (NEue MAschine) (new machine), also designated the T-D (Tasten-Druecker-Maschine) (key-stroke machine), was a 10-wheel rotor machine designed by the Swiss Army during World War II. Categories: Rotor machines ... The OMI cryptograph was a seven-rotor machine produced by Italian firm Ottico Meccanica Italiana. ... Portex on display at Bletchley Park Museum, UK. Portex (or BID/50/1) was a British cipher machine. ... SIGABA In the history of cryptography, the ECM Mark II was a rotor machine used by the United States from World War II (WWII) until the 1950s. ... SIGCUM on display at the National Cryptologic Museum. ... A Singlet machine on display at Bletchley Park BID/60 (Singlet) was a British encryption machine used mainly by the British intelligence services from around 1949 or 1950 onwards. ... Typex was based on the commercial Enigma machine, but incorporated a number of additional features to improve the security. ... In cryptography, the ADFGVX cipher was a field cipher used by the German Army during World War I. ADFGVX was in fact an extension of an earlier cipher called ADFGX. Invented by Colonel Fritz Nebel and introduced in March 1918, the cipher was a fractionating transposition cipher which combined a... The Affine cipher is a special case of the more general substitution cipher. ... The Alberti Cipher was the first ever Polyalphabetic Cipher. ... Atbash is a simple substitution cipher for the Hebrew alphabet. ... Bacons cipher or the Baconian cipher is a method of steganography (a method of hiding a secret message as opposed to a true cipher) devised by Francis Bacon. ... In classical cryptography, the bifid cipher is a cipher which combines the Polybius square with transposition, and uses fractionation to achieve diffusion. ... A book cipher is a cipher in which the key is the identity of a book. ... The action of a Caesar cipher is to move each letter a number of places down the alphabet. ... The Four-square cipher is a manual symmetric encryption technique. ... Hills cipher machine, from figure 4 of the patent In classical cryptography, the Hill cipher is a polygraphic substitution cipher based on linear algebra. ... A keyword is a monoalphabetic substitution. ... -1... In classical cryptography, a permutation cipher is a transposition cipher in which the key is a permutation. ... The pigpen cipher uses graphical symbols assigned according to a key similar to the above diagram. ... The Playfair system was invented by Charles Wheatstone, who first described it in 1854. ... A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. ... In cryptography, the Polybius square is a device invented by the Ancient Greek historian and scholar Polybius, described in Hist. ... The Rail Fence Cipher is a form of transposition cipher that gets its name from the way in which it is encoded. ... The Reihenschieber was a hand cipher system used by Germany. ... Reservehandverfahren (RHV) (German: Reserve Hand Procedure) was a German Naval World War II hand-cipher system used as a backup method when no working Enigma machine was available[1]. The cipher had two stages: a transposition followed by bigram substitution. ... ROT13 replaces each letter by its partner 13 characters further along the alphabet. ... In classical cryptography, the runnning key cipher is a type of polyalphabetic substitution cipher in which a text, typically from a book, is used to provide a very long key stream. ... This article is about the encryption device; for the Dune character, see Scytale (Dune). ... The Solitaire cryptographic algorithm was designed by Bruce Schneier for use in Neal Stephensons Cryptonomicon (where it was initially called Pontifex). ... In cryptography, a straddling checkerboard is a device for converting an alphabetic plaintext into digits whilst simultaneously achieving fractionation (a simple form of information diffusion) and homophony (a simple method for suppressing peaks of the frequency distribution). ... 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. ... It has been suggested that the section The code from the article Smitty Harris be merged into this article or section. ... In classical cryptography, a transposition cipher changes one character from the plaintext to another (to decrypt the reverse is done). ... In classical cryptography, the trifid cipher is a cipher invented around 1901 by Felix Delastelle, which extends the concept of the bifid cipher to a third dimension, allowing each symbol to be fractionated into 3 elements instead of two. ... The Two-square cipher is a manual symmetric encryption technique. ... The VIC cipher was a pencil and paper cipher used by the Soviet spy Reino Hayhanen, codenamed VICTOR. It was arguably the most complex hand-operated cipher ever seen. ... The VigenÃ¨re cipher is named for Blaise de VigenÃ¨re (pictured), although Giovan Batista Belaso had invented the cipher earlier. ... In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ... In cryptography, coincidence counting is the technique (invented by William F. Friedman) of putting two texts side-by-side and counting the number of times that a letter appears next to itself in both copies. ... A cryptogram is a short piece of text encrypted with a simple substitution cipher in which each letter is replaced by a different letter. ... In cryptography, the Polybius square is a device invented by the Ancient Greek historian and scholar Polybius, described in Hist. ... This article is about the encryption device; for the Dune character, see Scytale (Dune). ... In cryptography, a straddling checkerboard is a device for converting an alphabetic plaintext into digits whilst simultaneously achieving fractionation (a simple form of information diffusion) and homophony (a simple method for suppressing peaks of the frequency distribution). ... Tabula recta In cryptography, the tabula recta is a square table of alphabets, each one made by shifting the previous one to the left. ... The German Lorenz cipher machine, used in World War II for encryption of very high-level general staff messages Cryptography (or cryptology; derived from Greek ÎºÏÏ…Ï€Ï„ÏŒÏ‚ kryptÃ³s hidden, and Î³ÏÎ¬Ï†ÎµÎ¹Î½ grÃ¡fein to write) is the study of message secrecy. ... The history of cryptography dates back thousands of years. ... 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. ... This article is intended to be an analytic glossary, or alternatively, an organized collection of annotated pointers. ... Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related cryptographic keys for both decryption and encryption. ... Encryption Decryption In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. ... The operation of A5/1, a LFSR-based stream cipher used to encrypt mobile phone conversations. ... Public key cryptography is a form of cryptography which generally allows users to communicate securely without having prior access to a shared secret key. ... In cryptography, a cryptographic hash function is a hash function with certain additional security properties to make it suitable for use as a primitive in various information security applications, such as authentication and message integrity. ... A cryptographic message authentication code (MAC) is a short piece of information used to authenticate a message. ... A cryptographically secure pseudo-random number generator (CSPRNG) is a pseudo-random number generator (PRNG) with properties that make it suitable for use in cryptography. ...