Classical Transposition Ciphers

1. Classical Transposition Ciphers Day 13

2. Objectives Students will be able to… • …understand what transposition ciphers are and how they are implemented. • …create a transposition cipher of their own.

3. Transposition ciphers • Rearrange the existing letters in the plaintext.

4. Rail-Fence cipher

5. 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.

6. Permutation: Example d = _________ f = ____________________ Plaintext ________________________________________ Ciphertext ________________________________________

7. Column permutation cipher • Select keyword • Determines the number of columns • Determines the order of the columns

8. Column permutation cipher: Example

9. 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

10. 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.

11. 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)

12. 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.

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