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The History of Cylinder Cipher Originally invented by Thomas Jefferson in 1795. In 1891, Commandant Etienne Bazeries re- invented and improved upon Jefferson’s cipher cylinder for the French Army. The system was used by the U.S. Army from 1923 to 1942 as the M-94. The cylinder cipher was cracked by Germans in 1944.

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Background The Bazeries cylinder consists of 20 or 30 disks, each with a different scramble alphabet on the edge. There is a hole in the center of the discs. This allows the discs to be rotated and stacked on an axle. UDNAJFBOWTGVRSCZQKELMXYIHP 10: XPLTDSRFHENYVUBMCQWAOIKZGJ 9: NOZUTWDCVRJLXKISEFAPMYGHBQ 8: GWTHSPYBXIZULVKMRAFDCEONJQ 7: AMKGHIWPNYCJBFZDRUSLOQXVET 6: IHFRLABEUOTSGJVDKCPMNZQWXY 5: RPLNDVHGFCUKTEBSXQYIZMJWAO 4: BDMAIZVRNSJUWFHTEQGYXPLOCK 3: KPBELNACZDTRXMJQOYHGVSFUWI 2: ZWAXJGDLUBVIQHKYPNTCRMOSFE 1: http://www.apprendre-en- ligne.net/crypto/instruments/chiffrierzylinder.jpg

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Encryption Arrange the discs in any order and remember the order of the discs. This order is the key for decrypting the message. In this example the disc are arranged in the order of 7,9,5,10,1,6,3,8,2,4. This arrangement is the key. W AORPL N DVHGFCUKTEBSXQYIZMJ 4: O YHGVS F UWIKPBELNACZDTRXMJQ 2: N OZUTW D CVRJLXKISEFAPMYGHBQ 8: T EQGYX P LOCKBDMAIZVRNSJUWFH 3: A MKGHI W PNYCJBFZDRUSLOQXVET 6: E ZWAXJ G DLUBVIQHKYPNTCRMOSF 1: R SCZQK E LMXYIHPUDNAJFBOWTGV 10: T SGJVD K CPMNZQWXYIHFRLABEUO 5: E NYVUB M CQWAOIKZGJXPLTDSRFH 9: R AFDCE O NJQGWTHSPYBXIZULVKM 7: Plain textCipher text Then rotate each disc to get plain text in one row. “RETREAT NOW” is produced in the 1 st column. Then select any other column as the cipher text.

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Decryption To decrypt we arrange the disc in the order that is given in the key. Then we rotate the discs to produce the cipher text in one row. The last step is to look at the other rows until you find a row of text that makes sense. Cylinder cipher is very easy to decrypt if the key or the arrangement of the discs is known.

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Cryptanalysis We must know the key to decrypt the cipher text. To find the key you should have a crib, a known piece of plain text. For example, you know that the 1 st block of plaintext is “heilhitler”. The corresponding cipher text is “AZNCZEAPBH”. Now you must create a table of offset values of letter pairs for each disc.

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Cryptanalysis (continued) The number that occurs atleast once in each column is the row offset from the plaintext to the ciphertext. In our case, “14” is the offset. Now we must get rid of all the values in the table except “14”. Then we have to rearrange the rows so that “14" appears in every cell along the table's main diagonal. Main Diagonal The arrangement of the table rows gives the key.

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Bibliography http://www.quadibloc.com/crypto/ro020101.htm http://en.wikipedia.org/wiki/Crib_%28cryptanalysis% 29 http://en.wikipedia.org/wiki/Bazeries_cylinder http://www.smithsrisca.demon.co.uk/crypto- modern.html

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