Cylinder Cipher

1
Cylinder Cipher
2
The History of Cylinder Cipher

• Originally invented by Thomas Jefferson in 1795.
• In 1891, Commandant Etienne Bazeries re-invented
  and improved upon Jeffersons 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.

3
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.

http//www.apprendre-en-ligne.net/crypto/instrumen
ts/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.

• Then rotate each disc to get plain text in one
  row. RETREAT NOW is produced in the 1st column.
• Then select any other column as the cipher text.

5
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.

6
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 1st 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.

8
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.h
  tml