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February 24th, 2009, 07:58 AM   #1
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Affine transformation and RSA cipher

1. If the two most common letters in a long ciphertext, enciphered by a an affine transformation
C is congruent aP + b (mod 26)
are W and B, respectively, then what are the most likely values for a and b?

2. This message
WEZBF TBBNJ THNBT ADZQE TGTYR BZAJN ANOOZ ATWGN ABOVG FNWZV A
was enciphered using an affine transformation
C is congruent to aP + b (mod 26):
Suppose it is know that the most common letters in the plaintext message are A, E, N, and S. Use the
frequency of letters to determine a and b. Then decrypt the message.

3. If the ciphertext message produced by the RSA cipher with key (e; n) = (13; 2747) is
2206 0755 0436 1165 1737
what is the plaintext message?
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February 25th, 2009, 07:47 PM   #2
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Re: Affine transformation and RSA cipher

can anybody help me out with this??
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March 12th, 2009, 03:36 AM   #3
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Re: Affine transformation and RSA cipher

For the first cyphertext:

plaintext=
"this message was enciphered using an affine transformation"
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March 12th, 2009, 04:46 AM   #4
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Re: Affine transformation and RSA cipher

For the second cyphertext:

2747=41*67

phi(2747)=40*66=2640
and 13*2437=1 mod 2640

Decryption key is (2437,2747)

Decryption:

2206^2437 mod 2747=617
0755^2437 mod 2747=404
0436^2437 mod 2747=1908
1165^2437 mod 2747=1306
1737^2437 mod 2747=1823
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