Known Plaintext
   HOME

TheInfoList



OR:

The known-plaintext attack (KPA) is an
attack model In cryptanalysis, attack models or attack types are a classification of cryptographic attacks specifying the kind of access a Cryptanalysis, cryptanalyst has to a system under attack when attempting to "break" an Encryption, encrypted message (also ...
for
cryptanalysis Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic sec ...
where the attacker has access to both the
plaintext In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted. Overview With the advent of comp ...
(called a crib), and its
encrypted In cryptography, encryption is the process of encoding information. This process converts the original representation of the information, known as plaintext, into an alternative form known as ciphertext. Ideally, only authorized parties can decip ...
version (
ciphertext In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...
). These can be used to reveal further secret information such as secret keys and
code book A codebook is a type of document used for gathering and storing cryptography codes. Originally codebooks were often literally , but today codebook is a byword for the complete record of a series of codes, regardless of physical format. Crypto ...
s. The term "crib" originated at
Bletchley Park Bletchley Park is an English country house and estate in Bletchley, Milton Keynes ( Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following ...
, the British
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
decryption operation, where it was defined as:


History

The usage "crib" was adapted from a
slang Slang is vocabulary (words, phrases, and linguistic usages) of an informal register, common in spoken conversation but avoided in formal writing. It also sometimes refers to the language generally exclusive to the members of particular in-gro ...
term referring to cheating (e.g., "I cribbed my answer from your test paper"). A "crib" originally was a literal or interlinear
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
of a foreign-language text—usually a
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
or
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
text—that students might be assigned to translate from the original language. The idea behind a crib is that cryptologists were looking at incomprehensible
ciphertext In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...
, but if they had a clue about some word or phrase that might be expected to be in the ciphertext, they would have a "wedge," a test to break into it. If their otherwise random attacks on the cipher managed to sometimes produce those words or (preferably) phrases, they would know they might be on the right track. When those words or phrases appeared, they would feed the settings they had used to reveal them back into the whole encrypted message to good effect. In the case of
Enigma Enigma may refer to: *Riddle, someone or something that is mysterious or puzzling Biology *ENIGMA, a class of gene in the LIM domain Computing and technology * Enigma (company), a New York-based data-technology startup * Enigma machine, a family ...
, the German High Command was very meticulous about the overall security of the Enigma system and understood the possible problem of cribs. The day-to-day operators, on the other hand, were less careful. The
Bletchley Park Bletchley Park is an English country house and estate in Bletchley, Milton Keynes ( Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following ...
team would guess some of the plaintext based upon when the message was sent, and by recognizing routine operational messages. For instance, a daily weather report was transmitted by the Germans at the same time every day. Due to the regimented style of military reports, it would contain the word ''Wetter'' (German for "weather") at the same location in every message. (Knowing the local weather conditions helped Bletchley Park guess other parts of the plaintext as well.) Other operators, too, would send standard salutations or introductions. An officer stationed in the
Qattara Depression The Qattara Depression ( ar, منخفض القطارة, Munḫafaḍ al-Qaṭṭārah) is a depression in northwestern Egypt, specifically in the Matruh Governorate. The depression is part of the Western Desert of Egypt. The Qattara Depressi ...
consistently reported that he had nothing to report. ''"Heil Hitler,"'' occurring at the end of a message, is another well-known example. At Bletchley Park in
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, strenuous efforts were made to use (and even force the Germans to produce) messages with known plaintext. For example, when cribs were lacking, Bletchley Park would sometimes ask the
Royal Air Force The Royal Air Force (RAF) is the United Kingdom's air and space force. It was formed towards the end of the First World War on 1 April 1918, becoming the first independent air force in the world, by regrouping the Royal Flying Corps (RFC) and ...
to "seed" a particular area in the
North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
with mines (a process that came to be known as
gardening Gardening is the practice of growing and cultivating plants as part of horticulture. In gardens, ornamental plants are often grown for their flowers, foliage, or overall appearance; useful plants, such as root vegetables, leaf vegetables, fruits ...
, by obvious reference). The Enigma messages that were soon sent out would most likely contain the name of the area or the harbour threatened by the mines. The Germans themselves could be very accommodating in this regard. Whenever any of the turned German Double-Cross agents sent a message (written by the British) to their respective handlers, they frequently obligingly re-encrypted the message word for word on Enigma for onward transmission to Berlin. When a captured German revealed under interrogation that Enigma operators had been instructed to encode numbers by spelling them out,
Alan Turing Alan Mathison Turing (; 23 June 1912 – 7 June 1954) was an English mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist. Turing was highly influential in the development of theoretical com ...
reviewed decrypted messages and determined that the number "''eins''" ("one") was the most common string in the plaintext (
Benford's law Benford's law, also known as the Newcomb–Benford law, the law of anomalous numbers, or the first-digit law, is an observation that in many real-life sets of numerical data, the leading digit is likely to be small.Arno Berger and Theodore ...
). He automated the crib process, creating the ''Eins Catalogue'', which assumed that "''eins''" was encoded at all positions in the plaintext. The catalogue included every possible position of the various rotors, starting positions, and keysettings of the Enigma. The Polish Cipher Bureau had likewise exploited "cribs" in the "ANX method" before World War II (the Germans' use of "AN", German for "to", followed by "X" as a spacer to form the text "ANX").
Marian Rejewski Marian Adam Rejewski (; 16 August 1905 – 13 February 1980) was a Polish mathematician and cryptologist who in late 1932 reconstructed the sight-unseen German military Enigma cipher machine, aided by limited documents obtained by French mili ...
, "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys, and of German Efforts to Frustrate Those Methods," Appendix C to
Władysław Kozaczuk Władysław Kozaczuk (23 December 1923 – 26 September 2003) was a Polish Army colonel and a military and intelligence historian. Life Born in the village of Babiki near Sokółka, Kozaczuk joined the army in 1944, during World War II, at Bia ...
, ''Enigma'', 1984, pp. 243–44.
The United States and Britain used
one-time tape In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. In this technique, a plaintext is paired with a rand ...
systems, such as the 5-UCO, for their most sensitive traffic. These devices were immune to known-plaintext attack; however, they were point-to-point links and required massive supplies of one time tapes. Networked cipher machines were considered vulnerable to cribs, and various techniques were used to disguise the beginning and ends of a message, including cutting messages in half and sending the second part first and adding nonsense padding at both ends. The latter practice resulted in an infamous incident during World War II when the nonsense padding added to a message sent to American admiral
William Halsey Jr. William Frederick "Bull" Halsey Jr. (October 30, 1882 – August 16, 1959) was an American Navy admiral during World War II. He is one of four officers to have attained the rank of five-star fleet admiral of the United States Navy, the others ...
, "
the world wonders "The world wonders" is a phrase which rose to notoriety following its use during World War II when it appeared as part of a decoded message sent by Fleet Admiral Chester Nimitz, Commander in Chief, U.S. Pacific Fleet, to Admiral William Halsey ...
", was not nonsensical enough and was interpreted as part of the actual message, leading Halsey to change his plans. The
KL-7 The TSEC/KL-7, also known as Adonis was an off-line non-reciprocal rotor encryption machine.
, introduced in the mid-1950s, was the first U.S. cipher machine that was considered safe against known-plaintext attack.A History of U.S. Communications Security; the David G. Boak Lectures
National Security Agency (NSA), Volumes I, 1973, partially released 2008, additional portions declassified October 14, 2015, Quote: The KL-7 "was our first machine designed to serve very large nets which could stand matched plain and cipher text. For the first time, the man in the cryptocenter could take a message and simply type it into the machine as written, without changing the spacing between words, or cutting the message in half and sending the last part first. and without having to paraphrase the message text before it was released."
Classical cipher In cryptography, a classical cipher is a type of cipher that was used historically but for the most part, has fallen into disuse. In contrast to modern cryptographic algorithms, most classical ciphers can be practically computed and solved by hand. ...
s are typically vulnerable to known-plaintext attack. For example, a Caesar cipher can be solved using a single letter of corresponding plaintext and ciphertext to decrypt entirely. A general
monoalphabetic substitution cipher In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters (the most common), pairs of letters, tri ...
needs several character pairs and some guessing if there are fewer than 26 distinct pairs.


See also

*
Cadix ''Cadix'' was a World War II clandestine intelligence center at Uzès, in southern France, from September 1940 to 9 November 1942. During this period southern France was under the control of Vichy France and not occupied by Nazi Germany. At ''Cad ...
*
Cryptanalysis of the Enigma Cryptanalysis of the Enigma ciphering system enabled the western Allies in World War II to read substantial amounts of Morse-coded radio communications of the Axis powers that had been enciphered using Enigma machines. This yielded military in ...
* Kiss (cryptanalysis) *
PC Bruno ''PC Bruno'' was a Polish–French–Spanish signals–intelligence station near Paris during World War II, from October 1939 until June 1940. Its function was decryption of cipher messages, most notably German messages enciphered on the Enigma ...
*
Polish Cipher Bureau The Cipher Bureau, in Polish language, Polish: ''Biuro Szyfrów'' (), was the interwar Polish General Staff's Second Department of Polish General Staff, Second Department's unit charged with SIGINT and both cryptography (the ''use'' of ciphers an ...
*
Ultra adopted by British military intelligence in June 1941 for wartime signals intelligence obtained by breaking high-level encrypted enemy radio and teleprinter communications at the Government Code and Cypher School (GC&CS) at Bletchley Park. '' ...


Notes


References

*
Władysław Kozaczuk Władysław Kozaczuk (23 December 1923 – 26 September 2003) was a Polish Army colonel and a military and intelligence historian. Life Born in the village of Babiki near Sokółka, Kozaczuk joined the army in 1944, during World War II, at Bia ...
, ''Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War Two'', edited and translated by
Christopher Kasparek Christopher Kasparek (born 1945) is a Scottish-born writer of Polish descent who has translated works by numerous authors, including Ignacy Krasicki, Bolesław Prus, Florian Znaniecki, Władysław Tatarkiewicz, Marian Rejewski, and Władysław K ...
, Frederick, MD, University Publications of America, 1984, . *
Marian Rejewski Marian Adam Rejewski (; 16 August 1905 – 13 February 1980) was a Polish mathematician and cryptologist who in late 1932 reconstructed the sight-unseen German military Enigma cipher machine, aided by limited documents obtained by French mili ...
, "Summary of Our Methods for Reconstructing ENIGMA and Reconstructing Daily Keys, and of German Efforts to Frustrate Those Methods," Appendix C to
Władysław Kozaczuk Władysław Kozaczuk (23 December 1923 – 26 September 2003) was a Polish Army colonel and a military and intelligence historian. Life Born in the village of Babiki near Sokółka, Kozaczuk joined the army in 1944, during World War II, at Bia ...
, ''Enigma'', 1984, pp. 241–45. * * * {{DEFAULTSORT:Known-Plaintext Attack Cryptographic attacks Enigma machine