Transposition Cipher

Question 1

How to encrypt using a Transposition cipher?

Answer

Transposition cipher encryption consists of writing the plaint text in an array/a grid and then arranging the columns of this array according to a defined permutation. If there are empty boxes, fill them with a neutral letter (such as X or another letter more frequent).

The permutation key is a series of numbers (which can be generated from a word) that indicates the order in which the columns are arranged.

Example: The word KEY is used to generate the permutation 2,1,3:

	Before alphabetical sorting	After alphabetical sorting
Keyword	K,E,Y	E,K,Y
Order of columns	1,2,3	2,1,3

Example: Encrypt DCODE with the key KEY (permutation 2,1,3).

Columns	1,2,3	Columns sorted	2,1,3
Plaintext	D,C,O	Ciphertext	C,D,O
	D,E,X		E,D,X

The encrypted message consists of the table after swapping the columns in the order of the key.

Example: The encrypted message is CDOEDX

Some variants read the table in column and not by row, in this case the message encrypted with a column reading would be CEDDOX

Question 2

How to decrypt with a transposition cipher?

Answer

Transposition cipher decryption is identical to encryption except that the order of the columns is changed.

Example: A permutation 2,1,3 has been used to get the message CDOEDX (read by row):

Columns	2,1,3	Sorted columns	1,2,3
Ciphertext	C,D,O	Plaintext	D,C,O
	E,D,X		D,E,X

Example: The plain text is DCODEX.

If the message was read in columns, first write the table by columns

Example: A permutation 2,1,3 has been used to get the message CEDDOX (read by column):

Columns	2,1,3	Sorted columns	1,2,3
Ciphertext	C,D,O	Plaintext	D,C,O
	E,D,X		D,E,X

Example: The plain text is DCODEX.

Question 3

How to recognize a transposition ciphertext?

Answer

The message consists of the letters of the original message but in a different order.

The index of coincidence is identical to that of the one of the language of the plaintext.

Question 4

How to decipher a transposition cipher without key?

Answer

It is possible to test all the permutations if the key is not too long, but the most effective method is to have or try to guess a word from the plain text and to deduce the permutations of the columns.

If the encrypted message is composed of very few words (1, 2 or 3) then an anagram solver can make it possible to find them.

Question 5

What are the variants of the transposition cipher?

Answer

The transposition cipher is, along with the substitution cipher, one of the most used bricks for more elaborate ciphers. There are dozens of ciphers that use it like ADFGVX, Amsco, Double Transposition, Redefence, etc.

Question 6

Why completing the empty cells of the transposition table?

Answer

Empty boxes if they are not handled can, in some cases, introduce a decryption error on the last letters of the message.

Example: DCODE encrypted with 3,1,2 becomes ODC-DE (read online with - the empty cell) and during decryption, ODCDE decrypted with 3,1,2 becomes DCOED (line reading)

The same problem appears for column reading, but the error is then even more important (it can appear from the beginning of the message).

Transposition Cipher

Transposition Decoder

Transposition key

Writing/Reading direction

Transposition Encoder

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