This
was one of the most secure hand ciphers used in the Second World War. It was
used by both the Allies and the Axis, and served both well. Its main weakness
was that if the attacker intercepted two or more messages of the same length
using the same key, they could be compared by a tedious process known as
"multiple anagramming," finding solutions to both. This weakness was not
important if only one message was sent using each key. It had an additional
problem: Executing it correctly requires considerable care, leading to
difficulties in decryption if an error is made at a sensitive point. In the
United States, information about cryptanalysis of the cipher remained
classified until a few years ago.
Double Transposition consists of two applications of columnar transposition to
a message. The two applications may use the same key for each of the two steps,
or they may use different keys.
Columnar transposition works like this: First pick a keyword, such
as DESCRIBE, then write the message under it in rows:
D E S C R I B E
---------------
Y O U R M O T H
E R W A S A H A
M S T E R A N D
Y O U R F A T H
E R S M E L T O
F E L D E R B E
R R I E S
Now number the letters in the keyword in alphabetical order.
3 4 8 2 7 6 1 5
D E S C R I B E
---------------
Y O U R M O T H
E R W A S A H A
M S T E R A N D
Y O U R F A T H
E R S M E L T O
F E L D E R B E
R R I E S
Then read the cipher off by columns, starting with the lowest-numbered column:
Column 1 is THNTTB, followed by RAERMDE YEMYEFR ORSORER HADHOE OAAALR MSRFEES
UWTUSLI. This completes the first columnar transposition. Next, select and
number a second keyword, and write this intermediate ciphertext under it in
rows:
2 7 1 8 9 5 4 6 3
C O A S T L I N E
-----------------
T H N T T B R A E
R M D E Y E M Y E
F R O R S O R E R
H A D H O E O A A
A L R M S R F E E
S U W T U S L I
Finally, take it off by columns again and put it into five-letter
groups for transmission.
To decrypt a double transposition, construct a block with the right number of
rows under the keyword, blocking off the short columns. Write the cipher in by
columns, and read it out by rows. Lather, rinse, repeat.
