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Double Transposition Cipher

Tools to decrypt/encrypt with double transposition cipher, as its names indicates, it uses two transposition ciphers, with one or two keys, and applied on lines or columns or both.

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Double Transposition Cipher -

Tag(s) : Cryptography,Transposition Cipher

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Double Transposition Cipher

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Tools to decrypt/encrypt with double transposition cipher, as its names indicates, it uses two transposition ciphers, with one or two keys, and applied on lines or columns or both.

Answers to Questions

How to encrypt using Double Transposition cipher?

Encryption uses twice a transposition cipher, usually the first transposition is in columns, and the second in lines.

One wants to crypt DCODE with twice the key KEY

\KEY
1DCO
2DEX

The grid is filled with X and permutated a first time

\EKY
1CDO
2EDX

The intermediate ciphered message is CDOEDX. One makes a second transposition, but with lines

\12
KCD
EOE
YDX

The grid becomes, after the second permutationhref

\12
EOE
KCD
YDX

The ciphered text is then obtained by reading the grid in lines, from left to right and from top to bottom.

The ciphered message is then OECDDX

How to decrypt Double Transposition cipher?

Decryption requires to know both keys and the kind of transposition used for each (line or column)

Let the crypted message be OECDDX, transposed with 1 column transposition and 1 line transposition with two identical keys : KEY.

The crypted message gets two reversed transposition, in the opposite order of the original order, to get back the plain text.

Si the grid is

\12
EOE
KCD
YDX

It becomes, avec a first reversed permutationhref in lines

\12
KCD
EOE
YDX

The intermediate message is then CDOECX, it gets another reversed permutationhref, in column

\EKY
1CDO
2EDX

The grid becomes

\KEY
1DCO
2DEX

The original plain text is DCODEX.

How to recognize Double Transposition ciphered text?

Frequency analysishref should be normal and the ciphered message has an index of coincidencehref similar to the language of the plain text. Its number of letters is not a prime number.

How to decipher Double Transposition without keys?

It is possible to find the key length by analyzing the prime decomposition of the text length.

By writing the text in the grid, one can permute the columns in order to find words on each lines, beginning with common bigrams (such as TH, HE, IN, etc.).

To find the second permutationhref, it is easier, because lines are already in plain text.

What are the variants of the Double Transposition cipher?

It is possible to shuffle lines or columns in the desired order.

When Double Transposition have been invented?

No date of known author.

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Source code

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Questions / Comments


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