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ROT13 Encoding Algorithm

Discussion in 'PHP' started by pradeep, Aug 14, 2007.

  1. pradeep

    pradeep Team Leader

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    Introduction



    ROT13 ("rotate by 13 places", sometimes hyphenated ROT-13) is a simple Caesar cipher used in online forums as a means of hiding spoilers, punchlines, puzzle solutions, and offensive materials from the casual glance. ROT13 has been described as the "Usenet equivalent of a magazine printing the answer to a quiz upside down".

    ROT13 provides no real cryptographic security and is not used for such; in fact it is often used as the canonical example of weak encryption. An additional feature of the cipher is that it is symmetrical; that is, to undo ROT13, the same algorithm is applied, so the same action can be used for encoding and decoding.

    Algorithm



    Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary. A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence reverses: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own inverse.
    In other words, two successive applications of ROT13 restore the original text.

    Implementation



    PHP:
      function rot13$text )
      {
          
    $codec_text '';
          
    $len strlen($text);
          
          for ( 
    $i=0$i $len$i++ )
          {
              
    $k ord($text[$i]);
              
    // 65-77 to 78-90 and 97-109 to 110-122
              
    if ( ($k >= 65 && $k <= 77) || ($k >= 97 && $k <= 109) )
              {
                  
    $codec_text .= chr($k+13);
              }
              
    // 78-90 to 65-77 and 110-122 to 97-109
              
    elseif ( ($k >= 78 && $k <= 90) || ($k >= 110 && $k <= 122) )
              {
                  
    $codec_text .= chr($k-13);
              }
              else
              {
                  
    $codec_text .= chr($k);
              }
          }
          return 
    $codec_text;
      }
      
     
    Last edited: Aug 14, 2007
  2. jwshepherd

    jwshepherd New Member

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    rot 13 is also one of the easiest to crack even without using a script. By Identifying the most used letters and the single characters you can usually decipher say about 40% on just visual inspection. Please do not rely on it to encrpyt or hid secret notes...
     
  3. LenoxFinlay

    LenoxFinlay Banned

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    Applying ROT13 to a piece of text merely requires examining its alphabetic characters and replacing each one by the letter 13 places further along in the alphabet, wrapping back to the beginning if necessary.[2] A becomes N, B becomes O, and so on up to M, which becomes Z, then the sequence reverses: N becomes A, O becomes B, and so on to Z, which becomes M. Only those letters which occur in the English alphabet are affected; numbers, symbols, whitespace, and all other characters are left unchanged. Because there are 26 letters in the English alphabet and 26 = 2 × 13, the ROT13 function is its own inverse
     

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