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说明：
本文来自《Understanding Cryptography》习题

  • 密文

xultpaajcxitltlxaarpjhtiwtgxktghidhipxciwtvgtpilpit

  • 字频分析
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$str = 'xultpaajcxitltlxaarpjhtiwtgxktghidhipxciwtvgtpilpitghlxiwiwtxgqadds';
 
$result = array_count_values(str_split($str));
asort($result, SORT_NUMERIC);
print_r(array_slice($result, -5));
/*
Array
(
    [p] => 5
    [l] => 5
    [x] => 7
    [i] => 9
    [t] => 10
)
*/$shift = ord('e') - ord('t');  // 最高频字符 t 映射成 e
$begin = ord('a');
$end = ord('z');
 
$arr = [];
for ($i = 0; $i < strlen($str); $i++) {
    $ord = ord($str[$i]) + $shift;
    if ($ord < $begin) {
       $ord += 26;
    }
    if ($ord > $end) {
      $ord -= 26;
    }
    $arr[] = chr($ord);
}
echo implode('', $arr) . "\n";
  • 暴力破解
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$str = 'xultpaajcxitltlxaarpjhtiwtgxktghidhipxciwtvgtpilpitghlxiwiwtxgqadds';
$begin = ord('a');
$end = ord('z');
for ($shift = 1; $shift < 26; $shift++) {
    $arr = [];
    for ($i = 0; $i < strlen($str); $i++) {
        $ord = ord($str[$i]) - $shift;
        if ($ord < $begin) {
           $ord += 26;
        }
        if ($ord > $end) {
          $ord -= 26;
        }
        $arr[] = chr($ord);
    }
    echo implode('', $arr) . "\n";
}

结果：

ifweallunitewewillcausetheriverstostainthegreatwaterswiththeirblood

使用 Google搜索 if we all unite we will cause the rivers to stain the great water switch their blood得到结果：

Tecumseh's Speech to the Osages (Winter 1811-12)

www.historyisaweapon.com/defcon1/tecumosages.html

Tecumseh的简介： https://en.wikipedia.org/wiki/Tecumseh

反抗侵略的北美土著英雄。

Cryptography
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