Known-plaintext attack
Encyclopedia
The known-plaintext attack (KPA) is an attack model
Attack model
Attack models or attack types specify how much information a cryptanalyst has access to when cracking an encrypted message...

 for cryptanalysis
Cryptanalysis
Cryptanalysis is the study of methods for obtaining the meaning of encrypted information, without access to the secret information that is normally required to do so. Typically, this involves knowing how the system works and finding a secret key...

 where the attacker has samples of both the plaintext
Plaintext
In cryptography, plaintext is information a sender wishes to transmit to a receiver. Cleartext is often used as a synonym. Before the computer era, plaintext most commonly meant message text in the language of the communicating parties....

 (called a crib), and its encrypted
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

 version (ciphertext
Ciphertext
In cryptography, ciphertext 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 that is unreadable by a human or computer without the proper cipher...

). These can be used to reveal further secret information such as secret keys and code books. The term "crib" originated at Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...

, the British World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 decryption operation.

History

The usage "crib" was adapted from a slang
Slang
Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 term referring to cheating—thus, "I cribbed my answer from your test paper." A "crib" originally was a literal or interlinear translation
Translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. Whereas interpreting undoubtedly antedates writing, translation began only after the appearance of written literature; there exist partial translations of the Sumerian Epic of...

 of a foreign-language text — usually a Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 or Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 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
Ciphertext
In cryptography, ciphertext 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 that is unreadable by a human or computer without the proper cipher...

, 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 machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...

, 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 trench operators, on the other hand, were less careful. The Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...

 team would guess some of the plaintext based upon when the message was sent. 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, and knowing the local weather conditions helped Bletchley Park guess other parts of the plaintext as well. Another example was an officer in the Africa Corps who helped greatly by constantly sending: “Nothing to report.” Other operators too would send standard salutations or introductions.

At Bletchley Park in World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, 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
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 to “seed” a particular area in the North Sea with mines
Naval mine
A naval mine is a self-contained explosive device placed in water to destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel...

 (a process that came to be known as gardening
Gardening (cryptanalysis)
In cryptanalysis, gardening was a term used at Bletchley Park, England, during World War II for schemes to entice the Germans to include known plaintext, which the British called "cribs," in their encrypted messages...

, 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.

When a captured German revealed under interrogation, that Enigma operators had been instructed to encode numbers by spelling them out. Alan Turing
Alan Turing
Alan Mathison Turing, OBE, FRS , was an English mathematician, logician, cryptanalyst, and computer scientist. He was highly influential in the development of computer science, providing a formalisation of the concepts of "algorithm" and "computation" with the Turing machine, which played a...

 reviewed decrypted messages, and determined that the number “eins” ("1") appeared in 90% of messages. 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 "ANX" — German for "To," followed by "X" as a spacer.)

Classical cipher
Classical cipher
A cipher is a means of concealing a message, where letters of the message are substituted or transposed for other letters, letter pairs, and sometimes for many letters. In cryptography, a classical cipher is a type of cipher that was used historically but now has fallen, for the most part, into...

s are typically vulnerable to known-plaintext attack. For example, a Caesar cipher
Caesar cipher
In cryptography, a Caesar cipher, also known as a Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number...

 can be solved using a single letter of corresponding plaintext and ciphertext to decrypt entirely. A general monoalphabetic substitution cipher needs several character pairs and some guessing if there are fewer than 26 distinct pairs.

Present day

Modern ciphers such as Advanced Encryption Standard
Advanced Encryption Standard
Advanced Encryption Standard is a specification for the encryption of electronic data. It has been adopted by the U.S. government and is now used worldwide. It supersedes DES...

 are not currently susceptible to known-plaintext attacks.

Encrypted file archives such as ZIP are prone to this attack. For example, an attacker with an encrypted ZIP file needs only (part of) one unencrypted file from the archive which forms the "known-plaintext". Then using some publicly available software they can quickly calculate the key required to decrypt the entire archive. To obtain this unencrypted file the attacker could search the website for a suitable file, find it from another archive they can open, or manually try to reconstruct a plaintext file armed with the knowledge of the filename from the encrypted archive.

See also

  • Chosen-plaintext attack
    Chosen-plaintext attack
    A chosen-plaintext attack is an attack model for cryptanalysis which presumes that the attacker has the capability to choose arbitrary plaintexts to be encrypted and obtain the corresponding ciphertexts. The goal of the attack is to gain some further information which reduces the security of the...

  • Ciphertext-only attack
    Ciphertext-only attack
    In cryptography, a ciphertext-only attack or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts....

  • Cryptanalysis of the Enigma
    Cryptanalysis of the Enigma
    Cryptanalysis of the Enigma enabled the western Allies in World War II to read substantial amounts of secret Morse-coded radio communications of the Axis powers that had been enciphered using Enigma machines. This yielded military intelligence which, along with that from other decrypted Axis radio...

  • Polish Cipher Bureau
  • PC Bruno
    PC Bruno
    PC Bruno was a Polish-French intelligence station that operated outside Paris during World War II, from October 1939 until June 9, 1940. It decrypted German ciphers, most notably messages enciphered on the Enigma machine.-History:...

  • Cadix
    Cadix
    Cadix was the codename of a World War II clandestine Polish-French intelligence center that operated at Uzès, on the Mediterranean coast in southern, Vichy France, for over two years from September 1940 to November 9, 1942.-History:...

  • Ultra
    Ultra
    Ultra was the designation 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 at Bletchley Park. "Ultra" eventually became the standard...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK