Classical Transposition Ciphers

Day 13

Objectives

Students will be able to…• …understand what transposition

ciphers are and how they are implemented.

• …create a transposition cipher of their own.

Transposition ciphers• Rearrange the existing letters in the

plaintext.

Rail-Fence cipher

Permutation cipher• Key (d, f)• d is a fixed length• f is a permutation function• Break the plaintext up into blocks of

length d.• Shuffle each block according to d.

Permutation: Example

d = _________

f = ____________________

Plaintext

________________________________________

Ciphertext

________________________________________

Column permutation cipher

• Select keyword– Determines the number of columns– Determines the order of the columns

Column permutation cipher: Example

Column permutation cipher: cryptanalysis

• Determine possible underlying rectangles

• Discover which of the possible rectangles is correct (Note: vowels account for about 40% of characters.)

• Determine the column order

Column permutation cipher: cryptanalysis – centiban weight• U.S. government studied a collection of

5000 digrams• Calculated a value called a centiban

weight• Columns that produced a sum total of

centiban values from the digrams that are greater then other columns have a higher probability of being the correct column.

Column Cipher: Example

NETEF LTDSR TSSTF MDCET DRHXS WHOHO EEADU OUUFI RRRRS NEROT CFIEM EDSHA RTCPJ AOEGE WNLHO EPMWA WERUV AAINA TSDDS OEOAC EHNTL HFLAU RAEEN OTOTS SOSYS TNNCG EMETT YDYRR NEOOE RESTH INR

(Spillman, 2005, p. 91)

Double-Transposition Cipher

• Perform a column transposition cipher twice. You can use the same keyword.

• Makes it more difficult for cryptanalysis.

• Can be broken when you have multiple ciphertext documents.

History of Transposition Ciphers• Used by Greeks• Used by the U.S. during the Civil war• Used by the Germans in WWI (Turning

grille)