The Ceasar Cipher produced the message bc hwdscwbu Determine

The Ceasar Cipher produced the message bc hwdscwbu. Determine the plaintext.

Solution

ANSWER IS:no tipeoing

because you don\'t know the right key (how much to shift), first you have to find the right key .

For cracking the encryption, we could iterate over all opportunities and as our alphabet uses just 26 latin letters, we would obtain the decrypted string in at most 25 tries, which is quite trivial

For cracking the whole thing, an algorithm must simply find the smallest distance beween the encrypted and every decrypted string. I\'ve written a decrypter to crack any Caesar cipher and to obtain the used key by simply guessing the right answer. First we will implement an algorithm to encrypt a string using Caesar to get a perfect initial situation for our furthermore cracking attempt.. This is relatively easy to implement, we run through a given string and replace each letter by the ($char + $n) % 26. place on it:

<?php

function caesar($str, $n) {

   $ret = \"\";
   for($i = 0, $l = strlen($str); $i < $l; ++$i) {
       $c = ord($str[$i]);
       if (97 <= $c && $c < 123) {
           $ret.= chr(($c + $n + 7) % 26 + 97);
       } else if(65 <= $c && $c < 91) {
           $ret.= chr(($c + $n + 13) % 26 + 65);
       } else {
           $ret.= $str[$i];
       }
   }
   return $ret;
}

?>
In our cracking algorithm we run over the encrypted string and produce an array with a simple frequency statistic. Then we compare the resulting table with a table of frequencies of every single letter of our alphabet. The last step is simply finding the smallest distance between every occurrences:

<?php

function crack_caesar($str) {

   $max = 0;

   $weight = array(
       6.51, 1.89, 3.06, 5.08, 17.4,
       1.66, 3.01, 4.76, 7.55, 0.27,
       1.21, 3.44, 2.53, 9.78, 2.51,
       0.29, 0.02, 7.00, 7.27, 6.15,
       4.35, 0.67, 1.89, 0.03, 0.04, 1.13);

   $c = $s = array(
       0, 0, 0, 0, 0,
       0, 0, 0, 0, 0,
       0, 0, 0, 0, 0,
       0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0);

   for($i = 0, $l = strlen($str); $i < $l; ++$i) {
       $x = (ord($str[$i]) | 32) - 97;
       if (0 <= $x && $x < 26) {
           ++$c[$x];
       }
   }

   for ($off = 0; $off < 26; ++$off) {

       for ($i = 0; $i < 26; ++$i) {
           if ($max < ($s[$off]+= 0.01 * $c[$i] * $weight[($i + $off) % 26])) {
               $max = $s[$off];
           }
       }
   }
   return (26 - array_search($max, $s)) % 26;
}

?>