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The Double Transposition Cipher
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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.

	NDODR WTRFH ASEER AERMR OFLBE OERSA YEAEI
	HMRAL UTERH MTTYS OSU


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.

Double Transposition Hint
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