classical transposition ciphers day 13. objectives students will be able to… …understand what...
TRANSCRIPT
Objectives
Students will be able to…• …understand what transposition
ciphers are and how they are implemented.
• …create a transposition cipher of their own.
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: 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.