Mad Cow !
Cryptanalysis of the Substitution Cipher
The basic idea is to examine the text using a knowledge of the statistical
characteristics of the language being used. Some words and letters
will occur more frequently than others. These candidates are replaced
in the text to see if a valid message developed. Here are some common
tests for english text:
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Most frequently occurring letter from highest to lowest - E, T, A, O, N,
I, R, S, D, H, L, C, U, F, P, M, W, Y, B, G, V, K, X, J, Q, Z
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Most common single letter word - I, A
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Most common two letter words - OF, TO, IN, IS, IT, ON, AS, SO, WE, BY
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Most common three letter words - THE, AND, ARE, YOU, CAN, HER, WAS, HAS,
HIM, HIS
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A common four letter word that also begins and ends with the same letter
- THAT
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Most common two letter combinations - TH, HE, AN, IN, ER, RE, ES, ON, TI,
AT
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Most common three letter combinations - THE, AND, HAT, ENT, ION, FOR, TIO,
HAS, TIS
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Most common doubled letters - LL, TT, SS, EE, PP, OO, RR, FF, CC, DD, NN
-
Most common two letter combinations that show up as reversals of themselves
- ER RE, ES SE, AN NA, ON NO, TI IT, EN NE, TO OT, ED DE
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Most common letters beginning a message - T, A, O, W, C, H, I
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Most common letters ending a word - E, T, S, D, N, R, Y
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Most common three letter words containing a double letter - ALL, SEE
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Most common letters following an apostrophe - S, T
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Most common two letter combinations to follow an apostrophe - RE, VE
By counting the total number of occurrences each letter, each pair of letters,
and each trio of letters in the ciphertext, an educated guess can be made
as to what these would represent in the plaintext message. An intelligent
form of trial and error is used reconstruct the plaintext message.
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