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POHow to calculate the modular multiplicative inverse for the Affine Cipher
 

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  1. POHow to calculate the modular multiplicative inverse for the Affine Cipher
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    <p>I am trying to create a small software that does the Affine Cipher, which means that <strong>K1</strong> and the amount of letters in the alphabet (using <strong>m</strong> for this number) must be coprime, that is <code>gcd(k1, m) == 1</code>.</p> <p>Basically it's like this:</p> <p>I have a plaintext: <strong>hey</strong></p> <p>I have K1: <strong>7</strong></p> <p>I have K2: <strong>5</strong></p> <p>Plaintext in numerical format is: <strong>8 5 25</strong></p> <p>8 - from h (the position in the alphabet) and ** 5 25** goes the same for <strong>e</strong> and <strong>y</strong></p> <p>Encrypted: <strong>7 13 18</strong></p> <p>Which is the formula:</p> <p><strong>k1 * 8 + k2 mod 27</strong> = 7</p> <p><strong>k1 * 5 + k2 mod 27</strong> = 13</p> <p><strong>k1 * 25 + k2 mod 27</strong> = 18</p> <p>I have a function that crypts this but I don't know how to decrypt.</p> <p>For example I have 7 for h. I want to get the number 8 back again, knowing 7, k1 and k2.</p> <p>Do you guys have any ideas ?</p> <p>Some function where you input k1, k2, result (7 for example, for h), and it gives me back 8, but I really don't know how to reverse this.</p> <p>The function for encryption is this:</p> <pre><code>public List&lt;int&gt; get_crypted_char(string[] strr) { List&lt;int&gt; l = new List&lt;int&gt;(); int i; for (i = 0; i &lt; strr.Length; i++) { int ch = int.Parse(strr[i]); int numberback = k1 * ch + 5; numberback = (numberback % 27); l.Add(numberback); } return l; } </code></pre> <p>Where: string[] strr is a string that contains the plaintext. <strong>Function example:</strong> get_crypted_char({"e","c","b"})</p> <p>The result would be a list like this {"5","3","2"}</p> <p><strong>UPDATE:</strong> Here is a link from wikipedia about this encryption, and also decryption, but ... I don't really understand "how to" <a href="http://en.wikipedia.org/wiki/Affine_cipher" rel="nofollow">http://en.wikipedia.org/wiki/Affine_cipher</a></p>
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    1. USDr. belisarius
    1. USicebox19
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    1. COYour question is not clear IMO..
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      1. USSoner Gönül
    2. CODo you have the encryption code? I think it's easier to read.
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      1. USStefan
    3. COThis has been asked and answered in the past: http://stackoverflow.com/a/10133236/146205. There is no real way of getting that back, as `8 mod 3` and `5 mod 3` both equal `2`, for example.
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      1. USJensen
 

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