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Classical Transposition Ciphers. Day 13. 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. Transposition ciphers.
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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.
Transposition ciphers • Rearrange the existing letters in the plaintext.
Rail-Fence cipher This is an example tiephssnxmliaae
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.
Permutation: Example 4 d = _________ f = ____________________ Plaintext ________________________________________ Ciphertext ________________________________________ (4, 1, 2, 3) This is an example → this isan exam plex sthinisamexaxple → sthinisamexaxple
Column permutation cipher • Select keyword • Determines the number of columns • Determines the order of the columns
Column permutation cipher: Example Plaintext: This is an example of a column permutation cipher. Key: break Key: (2,5,3,1,4) HAMFURTIXIXEOPTNEXINPAMMIPXTSAOLEACRSELCNUOHX
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?
Column permutation cipher: cryptanalysis – centiban weight • Which columns are matched? • U.S. government studied a collection of 5000 digrams • Calculated a value called a centibanweight, 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.
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)
Turning grille Plaintext: This an example of a turning grille cipher Ciphertext: HSNEMTIIE AXLFRGGLX IPANLCIHR TAEOUNREP
Transposition Assignment • Parts of this assignment are independent and parts require a partner.
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.