Below are given example of ciphertext, from Affine cipher. In each case, the tas

Question

Below are given example of ciphertext, from Affine cipher. In each case, the task is to determine the plaintext and key. Give a clearly written description of the steps you followed to decrypt each ciphertext. This should include all frequency analysis and computations you performed. If you write a C/C++ program to help you, hand in the source code as well.


Affine cipher:

KQERE JEBCP PCJCR KIEAC UZBKR VPKRB CIBQC ARBJC VFCUP
KRIOF KPACU ZQEPB KRXPE IIEAB DKPBC PFCDC CAFIE ABDKP
BCPFE QPKAZ BKRHA IBKAP CCIBU RCCDK DCCJC IDFUI XPAFF
ERBIC ZDFKA BICBB ENEFC UPJCV KABPC YDCCD PKBCO CPERK
IVKSC PICBR KIJPK ABI

Explanation / Answer

You could always go for trial error to decrypt this but it would be very tiring and time taking... :) You could always find a software to decrypt this but again that is not advisable... A nice thing would be to try to decrypt this and who knows it maybe a lot of fun.. So the affine cipher is a form of Substitution cipher where on letter is substituted by other(has one-one correspondence) The form for encryption is (ax+b)mod(26) where a and b are constants a can take values {1,3,5,7,9,11,15,17,19,21,23,25} and b can take values {0,1,2,..,25} So that makes 312 combinations... The basic aim here is to find a and b so that we can decrypt the message For decryption we use a^(-1)(x-b)mod(26) First do the frequency analysis of the crypted text... Then hypothesize that the highest frequency letter obtained corresponds to "e" and the second highest corresponds to "t" find the values a and b then decode if u get a meaningful sentence well and good otherwise we have to go for a different hypothesis... you can get the letter frequencies in english from http://en.wikipedia.org/wiki/Letter_frequency and you can refer a book Cryptography by Douglas Stinson...

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