Question

Describe a technique that can be used to break a columnar transposition cipher (besides brute force)?

Answer 1

GIVEN THAT:

TRANSPOSITION CIPHER:

1.  is a method of encryption by which the positions held by units of plaintext are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext.

2. The order of the units is changed. Mathematically a bijective function is used on the characters' positions to encrypt and an inverse function to decrypt.

COLUMNAR TRANSPOSITION:

A. In a columnar transposition, the message is written out in rows of a fixed length, and then read out again column by column, and the columns are chosen in some scrambled order.

B. Both the width of the rows and the permutation of the columns are usually defined by a keyword.

For example, the keyword ZEBRAS is of length 6 (so the rows are of length 6), and the permutation is defined by the alphabetical order of the letters in the keyword.

In this case, the order would be "6 3 2 4 1 5".

6 3 2 4 1 5

W E A R E D

I S C O V E

R E D F L E

E A T O N C

E Q K J E U

Providing five nulls(QKJEU), these letters can be randomly selected as they just fill out the incomplete columns and are not part of the message.

The ciphertext is then read off as:

EVLNE ACDTK ESEAQ ROFOJ DEECU WIREE

In the irregular case, the columns are not completed by nulls:

6 3 2 4 1 5

W E A R E D

I S C O V E

R E D F L E

E A T O N C

E

This results in the following ciphertext:

EVLNA CDTES EAROF ODEEC WIREE

To decipher it, the recipient has to work out the column lengths by dividing the message length by the key length.

Then he can write the message out in columns again, then re-order the columns by reforming the key word.

In a variation, the message is blocked into segments that are the key length long and to each segment the same permutation is applied.

This is equivalent to a columnar transposition where the read-out is by rows instead of columns.

Columnar transposition continued to be used for serious purposes as a component of more complex ciphers at least into the 1950s.