Banburism

As banburismus one is kryptanalytisches procedure referred that in World War II by the British Code Breakers (code breakers) in English Bletchley Park was used in the secret communications of the German Navy break .

history

The German Navy used special models of the Enigma rotor key machine for secret communication , namely the Enigma-M3 (with three rollers ) and, especially for encrypted radio communication between the commander of the submarines (BdU) and the German submarines operating in the Atlantic. Boot , the Enigma-M4 (with four reels). In contrast to the other parts of the Wehrmacht , i.e. the army and air force , the navy also used a sophisticated spell key agreement . In contrast to what is customary with the army and air force, she did not leave it to the user to turn the Enigma reels into a "random" starting position at the beginning of a spell (see also: Using the Enigma ), but instead wrote a special spell key procedure using Double-letter swap boards before. This measure strengthened the cryptographic security of the German machine and made it difficult for the British to decipher it .

Despite everything, this method also had a serious disadvantage, which the British recognized and exploited. With the exception of the individual starting position of the Enigma rollers, the other partial keys (roller position, connector and ring position) were identical for all sayings of a day. The only difference was the initial position of the rollers. Due to the many possibilities for the roller position (26³ or 17,576 for the M3 or 26 ⁴ or 456,976 for the M4) it practically never happened that two sayings happened to have the same starting position one day, but it could happen that in the course of one Spell (during which the reels continue to spin) a second spell reaches the same position that is the starting position for another.

From this point on, the two sayings are "in phase" or in depth , as the British code breakers called it. Similar to using the coincidence index evaluated or the Chi-Test exploited, then join the two-phase ciphertext parts significantly more likely coincidences than it at random texts is the case or not-phase ciphertext. This means a simultaneous occurrence of identical secret letters in both texts (in the jargon of the codebreaker called clicks ). For random texts from the 26 letters of the Latin alphabet (as used in the Enigma), a coincidence occurs on average at one of 26 characters. That's about 3.8% of the time. As the British recognized, the number of clicks increases noticeably to 5% to 6% if two Enigma ciphertexts are aligned in phase.

Banbury Sheets

Part of a Banbury Sheet ("Banbury sheet") as it was used in Bletchley Park during World War II.

In order to make the tedious manual work of aligning two Enigma radio messages in the correct phase easier, the British cryptanalysts, above all Alan Turing , devised long strips of paper (around 25 cm high and several meters long) into which the individual letters of the respective ciphertext were shaped holes punched in. Many (several hundred) columns with the letters of the alphabet were printed on the strips, from top A to bottom Z (see picture). In the top line, the columns were also numbered according to the position of the letter in the ciphertext.

If you put two such strips (for two different ciphertexts) on top of each other on a light table and then shifted one against the other to the right, you could quickly deduce the number of coincidences from the number of holes on top of each other and the light shining through there. In this way, two sayings can be aligned in phase relatively easily, quickly and reliably. For the evaluation, Turing and his colleague Irving John Good devised a mathematical evaluation measure called ban . For practical work, a tenth or a twentieth of a bans proved to be particularly convenient, as annoying decimal places could be avoided, which simplified the calculations and accelerated the evaluation. Which was abbreviated Deziban with db (not to be confused with the abbreviation dB for decibels ) or hdb for "half Deziban" (English. Helped deciban ).

The paper strips were made in the English town of Banbury , just under 50 km west of Bletchley , and therefore called Banbury sheets ("Banbury sheets") or short as banburies (singular: banbury ). The naming "ban", which can be seen as the forerunner of today's bit , was also inspired by the name of the city. The work of punching holes has been described as to banbury . The cryptanalytic process implemented in this way was consequently named Banburism .

literature

• Friedrich L. Bauer : Deciphered Secrets. Methods and maxims of cryptology. 3rd, revised and expanded edition. Springer, Berlin et al. 2000, ISBN 3-540-67931-6 .
• IJ Good : Enigma and fish . In Francis Harry Hinsley, Alan Stripp: Codebreakers - The inside story of Bletchley Park . Oxford University Press, Reading, Berkshire 1993. ISBN 0-19-280132-5 .
• Hugh Sebag-Montefiore : Enigma - The battle for the code . Cassell Military Paperbacks, London 2004. ISBN 0-304-36662-5 .

Web links

• All You Ever Wanted to Know About Banburismus but were Afraid to Ask . Internet Archive Link, as the original page is no longer available.
• Banburismus and the Brain (English). Retrieved November 17, 2015.
• The Banburismus procedure (English). Retrieved November 17, 2015.

Individual evidence

1. ↑ Tony Sale: The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 27. Accessed: Nov. 17, 2015. PDF; 0.4 MB
2. ↑ Tony Sale: The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 16. Accessed: Nov. 17, 2015. PDF; 0.4 MB
3. ↑ Tony Sale: The Bletchley Park 1944 Cryptographic Dictionary . Publication, Bletchley Park, 2001, p. 5. Accessed: Nov. 17, 2015. PDF; 0.4 MB