The importance of the loops stems from Turing's idea for how to use them to design electrical circuits in the bombe machines. The full plaintext is “Keine Zusaetze Zumvorberiqt” which is German for “No additions to Preliminary Report.” Example 4.5.2. The ciphertext in the top row is paired with the plaintext in the bottom row. In it, he describes how to use a matched matched plaintext/ciphertext pairing from a crib to find cycles of letters. This was known affectionately at Bletchley Park as the “Prof's Book”, but if more officially known as Turing's “Treatise on the Enigma”. Zusand ost waertiger kanal (situation eastern channel)įeuere brannten wie befohlen (beacons lit as ordered)įinding the location of cribs in the ciphertext was made easier by the property that the Enigma machine will never encrypt a letter as itself.įor an example, we will turn to a training manual for the Enigma machine that Turing wrote for use at Bletchley Park. Wetter fuer die nacht (weather for the night) According to Copelands's “The Essential Turing” other routine phrases that occurred often and became cribs include: One main type of message that helped for cribs was weather reports that were sent at the same time everyday. Early decryption work allowed cryptanalysts to learn patterns in German messages that allowed them to discover many useful cribs. In order to use a crib for decrypting ciphertext, it is necessary to know an exact word or phrase that will occur in the plaintext. This required designing more elaborate machines, called bombes after the Polish bomba, to check all the possible initial settings very quickly. Fortunately, Alan Turing, Gordon Welchman and others were working on a new idea to use cribs to make deductions about inital settings. The Zygalski sheets became ineffective in May of 1940 when the repeated encryption of the message key was discontinued. The codebreakers at Bletchley Park relied heavily on the Polish work, especially the deduction of the wirings of the rotors and the Zygalski sheets. The Polish work to break Enigma messages relied heavily on the repeated encryption of the message key and developing some electro-mechanical devices to help check the many potential initial settings. Alan Turing will be honored on the British fifty pound note ![]() Probability, Permutations and Combinationsįigure 4.5.1.
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