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POVigenère cipher implementation
 

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  1. POVigenère cipher implementation
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    <p>I have to implement a variant of the <a href="http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher" rel="nofollow">Vigenère cipher</a>. I got the encryption part without issues, but I have a bug in the decryption code and I don't understand what I'm doing wrong.</p> <p>The requirements are:</p> <ul> <li><p>the key can only contain <code>A</code> - <code>Z</code> (uppercase)</p></li> <li><p>code values for the key characters are 0 for A, 1 for B, ..., and 25 for Z</p></li> <li><p>do not encode a character if the code is &lt; 32 (preserve control characters)</p></li> <li><p>encrypted character code = original character code + key character code</p></li> <li><p>the final encrypted character must be between 32 and 126, exclusively so if the final encrypted character > 126 it must be brought back into the 32 - 126 range by adding 32 to the value and then subtracting 126</p></li> </ul> <p>The encryption code:</p> <pre><code>// it works ok // I have tested it with some provided strings and the results are as expected public String encrypt(String plainText) { StringBuilder sb = new StringBuilder(); for (int i = 0; i &lt; plainText.length(); i++) { char c = plainText.charAt(i); if (c &gt;= 32) { int keyCharValue = theKey.charAt(i % theKey.length()) - 'A'; c += keyCharValue; if (c &gt; 126) { c = (char) (c + 32 - 126); } } sb.append(c); } return sb.toString(); } </code></pre> <p>The decryption code:</p> <pre><code>// there probably is an off-by-one error somewhere // everything is decrypted ok, except '~' which gets decrypted to ' ' (space) public String decrypt(String cipherText) { StringBuilder sb = new StringBuilder(); for (int i = 0; i &lt; cipherText.length(); i++) { char c = cipherText.charAt(i); if (c &gt;= 32) { int keyCharValue = theKey.charAt(i % theKey.length()) - 'A'; c -= keyCharValue; if (c &lt; 32) { c = (char) (c + 126 - 32); } } sb.append(c); } return sb.toString(); } </code></pre> <p>Example (with key <code>ABCDEFGHIJKLMNOPQRSTUVWXYZ</code>):</p> <ul> <li><p>original <code>~~~~~~~~~~~~~~~~~~~~~~~~~~</code></p></li> <li><p>encrypted <code>~!"#$%&amp;'()*+,-./0123456789</code></p></li> <li><p>decrypted <code>~</code> ('~' followed by spaces)</p></li> </ul> <p>EDIT:</p> <p>Here is the code I use for testing (it tests every character from 0 to 126 repeated as a string):</p> <pre><code>public static void main(String[] args) { int passed = 0; int failed = 0; String key = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; for (int c = 0; c &lt;= 126; c++) { StringBuilder sbString = new StringBuilder(); for (int i = 0; i &lt;= 25; i++) { sbString.append((char) c); } String original = sbString.toString(); Cipher cipher = Cipher(key); String encrypted = cipher.encrypt(original); String decrypted = cipher.decrypt(encrypted); if (!original.equals(decrypted)) { failed++; System.out.println("--FAILED--"); System.out.println(original); System.out.println(encrypted); System.out.println(decrypted); } else { passed++; } } int tests = passed + failed; System.out.println(tests + " tests"); System.out.println("passed: " + passed); System.out.println("failed: " + failed); } </code></pre>
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    1. COwhat was the key?
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      1. USPratik Shelar
    2. CO@Pratik Looks like it's the uppercase alphabet.
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      1. USJoachim Isaksson
    3. COHi I ran the above methods and its working correctly...I am getting the the original message back from decrypt method.
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