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Math Help - Cracking Affine Ciphers

  1. #1
    Newbie
    Joined
    Oct 2008
    Posts
    15

    Cracking Affine Ciphers

    Hey,
    I have a question to decipher an affine cipher. The cipher text is RBYDKLQTSLQJBYN and I am told that E(A)=L and E(O) = B.

    Cipher
    L -> 12
    B -> 2
    Plain
    A -> 1
    O -> 15

    So basically I set up two equations in Z26:
    <br />
12 = a + b<br />
    <br />
2 = 15a + b<br />
    I take the top and subtact the bottom:

    <br />
10 = -14a<br />
    <br />
10 = 12a<br />

    Now my problem is I cannot find the inverse of 12 in Z26 because they are not relatively prime. I am not sure how to proceed from this point? Can someone point me in right direction? I've tried subtracting the equations in the reverse order, but run into the same relatively prime issue.

    Thanks
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  2. #2
    Member
    Joined
    Apr 2008
    Posts
    204
    A --> 0 not 1 and O --> 14 not 15

    The correspondence between letters in the plaintext and ciphertext spaces and Z_26 is a,A --> 0 all the way up to z,Z --> 25

    This is because it is in mod 26 form, so you can have a 0, but not a 26!

    Hope this helps
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  3. #3
    Member
    Joined
    Apr 2008
    Posts
    204
    sorry, forgot to put that L --> 11 and B --> 1
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