1. Day 13 Classical Transposition Ciphers

2. Objectives Students will be able to…  …understand what transposition ciphers are and how they are implemented.  …encrypt and decrypt using classic transposition ciphers.  …create and use transposition ciphers of their own.

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

4. Rail-Fence cipher This is an example tiep hssnxml iaae tiephssnxmliaae

5. Permutation cipher  Key (d, f)  d is a fixed length  f is a permutation function  Example if d = 4 then f could be (4, 2, 1, 3)  Break the plaintext up into blocks of length d.  Shuffle each block according to d.

6. Permutation: Example d = _________ f = ____________________ Plaintext ________________________________________ Ciphertext ________________________________________ 4 This is an example (4, 1, 2, 3) →this isan exam plex sthi nisa mexa xple→STHINISAMEXAXPLE

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

8. Column permutation cipher: Example 12345 thisi sanex ample ofaco lumnp ermut ation ciphe rxxxx Plaintext: This is an example of a column permutation cipher. Key: break Key: (2,5,3,1,4) HAMFURTIXIXEOPT NEXINPAMMIPXTS AOLEACRSELCNUO HX 1 t s a o l e a c r 2 h a m f u r t i x 3 i n p a m m i p x 4 s e l c n u o h x 5 i x e o p t n e x

9. Column permutation cipher: cryptanalysis  Determine possible underlying rectangles. How?  Discover which of the possible rectangles is correct (Note: vowels account for about 40% of characters.)  Determine the column order. How?

10. Column permutation cipher: cryptanalysis – centiban weight  Which columns are matched?  U.S. government studied a collection of 5000 digrams  Calculated a value called a centiban weight, for each digram  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.

12. 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)

13. 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.

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

15. Turning grille thisan exampl eofatu rningg rillec ipherx Plaintext: This an example of a turning grille cipher Ciphertext: HSNEMTIIEAXLFRGGLXIPANLCIHR TAEOUNREP

16. Transposition Assignment  Parts of this assignment are independent and parts require a partner.

17. Review  What is the principle behind transposition ciphers?  Classically what was an important skill for breaking a transposition cipher?  How can mathematics be used to break a column transposition cipher.