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202 CHAPTER 10. TRANSPOSITION CIPHERS
keyword bombs gives AKLLT AUTKC TOFNN TYEAO CRFAE. Guessing that this was
a 5×5 square would lead to almost immediate decryption. But if we re-encipher
it with the same keyword we get AATTC LKNAA LTFEF KUOYR TCNOE.
Suppose we try to decrypt this cipher, and for ease we grant ourselves the
knowledge that it is 5 × 5. Arranging it gives
1 2 3 4 5
A L L K T
A K T U C
T N F O N
T A E Y O
C A F R E
There are only 7 vowels out of 25 letters, or 28%, which is a smaller percentage
than normal, but it still stands out that there is only one vowel in each of the
first two rows. Cheating (by looking at the previous paragraph) how will we
guess that AUTKC is the correct ordering of the second row
The purpose of a transposition is to mix-up the position of the letters. A single
columnar transposition cipher can be broken because this mixing is far from
complete. The second enciphering causes many more of the letter connections to
be shredded, making decryption very difficult. If the cryptanalyst has a number
of messages, all enciphered with the same key, and all of exactly the same length,
then, by moving from one message to another and back again, it is possible to
decrypt the whole set. (This is the trick that the brilliant French cryptanalyst
Georges Jean Painvin used to break the German ÜBCHI double transposition
cipher of World War I.) However, even the experts cannot routinely decrypt a
single message carefully enciphered with a double transposition.
10.9 Transposition during the Civil War
The Civil War turned out to be an interesting test of Kerckhoff’s maxim that
the enemy knows the system being used. Not only did the two armies speak
the same language, but most of the trained officers knew each other, having
attended West Point together, and some even had fought together during the
Mexican–American War of 1848. We have already seen that the South put their
trust largely in polyalphabetic ciphers, with rather little success. The North
would turn to transposition.
Anton Stager was born in 1833, in New York, and worked in a printer’s office
and as a bookkeeper before becoming a telegraph operator in Pennsylvania. He
was rapidly promoted and by his early thirties he was the general superintendent
of the Western Union Telegraph Company, head-quartered in Cleveland. After
the outbreak of hostilities in the Civil War, Governor Denison of Ohio gave him
responsibility for all telegraph lines in the Ohio military district, and asked him
to prepare a cipher so that the governors of Ohio, Indiana and Illinois could