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Transposition (one time pad (problems (perfectly random one time pad, key…
Transposition
one time pad
described in 1882
used by germans in 1923
security proved by claude shannon
key as long as message
reciever has no more info than before recieving the cipher
problems
perfectly random one time pad
key must be as long as pad
cannot be reused
no authentication
Cannot brute force!
automated solver
scan dictionairy for all word combinations
record the number of times E maps to A,B,C
pick the most popular mappings and replace throughout the text
omptimisations
rank words by popularity
XOR ciphers
A XOR 0 = A
A xor A = 0
(A XOR B) XOR C = A XOR (B XOR C)
(B XOR A) XOR A = B XOR 0 = B
good encryption
plaintext and password are about the same length
password is not reused
password cannot be guessed
cracking
vigenere
index of coincidence
find key length
Ca XOR Cb = A XOR B
crib dragging (pattern matching)
guess a word that might appear in one of the messages
convert to hex
xor with combined ciphers to reveal words
what is it
moving position of plaintext letters
hard to spot patters unless transposition method is known
columnar
put each letter into a cell of a grid
read down
solving
try all grid sizes
bifid cipher
cryptanalysis
Step 2: determine the square with trial and error in a similar way to PlayFair
Step 1: Determine the period
polybius square of the alphabet
each letter described by its coordinates
fractionation
diffusion so that one letter does not map directly to another text letter
cipher text letter could depend on more than one letter
anagramming
take pieces of cipher and slide them together