凯撒密码未达到预期运行

Question

我正在研究 Caesar Cipher，但它似乎不起作用。

这是我的代码：

class CaesarCipher {
  const CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  protected $encrypt_sequence = array();
  protected $decrypt_sequence = array();

  public function __construct($seed = 1) {
      $total_chars = strlen(self::CHARS);
      $seed = $seed % $total_chars;
      for ($i = 0; $i < $total_chars; $i++) {
          $src = substr(self::CHARS, $i, 1);
          $dst = substr(self::CHARS, ($i + $seed) % $total_chars, 1);
          $this->encrypt_sequence[$src] = $dst;
          $this->decrypt_sequence[$dst] = $src;
      }
      $this->encrypt_sequence[' '] = ' ';
      $this->decrypt_sequence[' '] = ' ';
  }

  public function encrypt($value) {
      $value = strtoupper($value);
      return str_replace($this->encrypt_sequence, $this->decrypt_sequence, $value);
  }

  public function decrypt($value) {
      $value = strtoupper($value);
      return str_replace($this->decrypt_sequence, $this->encrypt_sequence, $value);
  }

  public function getEncryptSequence() {
      return $this->encrypt_sequence;
  }

  public function getDecryptSequence() {
      return $this->decrypt_sequence;
  }
}

运行它：

$seed = mt_rand(1, 35);
$cipher = new CaesarCipher($seed);
$source = 'THIS IS JUST A TEST WITH 123 NUMBERS';
$encrypted = $cipher->encrypt($source);
$decrypted = $cipher->decrypt($encrypted);

这是我的输出：

CAESAR CIPHER (seed=16)

Source:    THIS IS JUST A TEST WITH 123 NUMBERS
Encrypted: X12W 2W 3YWX U XYWX 02X1 567 7Y6VYVW   ENCRYPTED :)
Decrypted: DHIC IC JECD A DECD GIDH LMN NEMBEBC   DOES NOT MATCH SOURCE :(


Encryption: A>Q B>R C>S D>T E>U F>V G>W H>X I>Y J>Z K>0 L>1 M>2 N>3 O>4 P>5 Q>6 R>7 S>8 T>9 U>A V>B W>C X>D Y>E Z>F 0>G 1>H 2>I 3>J 4>K 5>L 6>M 7>N 8>O 9>P
Decryption: Q>A R>B S>C T>D U>E V>F W>G X>H Y>I Z>J 0>K 1>L 2>M 3>N 4>O 5>P 6>Q 7>R 8>S 9>T A>U B>V C>W D>X E>Y F>Z G>0 H>1 I>2 J>3 K>4 L>5 M>6 N>7 O>8 P>9

谁能告诉我为什么它不起作用？

Answer 1

最明显的问题是 str_replace 手册中的这一部分：

Caution

Replacement order gotcha

Because str_replace() replaces left to right, it might replace a previously inserted value when doing multiple replacements. See also the examples in this document.

您多次替换某些字符，这导致了您得到的结果。

这适用于加密和解密，因此您不能使用 str_replace() 和数组来做您想做的事。

一个可能的解决方案是逐个字符地加密（en decrypt...）您的字符串，这样每个字符只传递一次。

Answer 2

知道了！

public function encrypt($value) {
    $value = strtoupper($value);
    return strtr($value, $this->encrypt_sequence);
}

public function decrypt($value) {
    $value = strtoupper($value);
    return strtr($value, $this->decrypt_sequence);
}

将正确加密和解密值。感谢 jeroen 和 GoogleHireMe :)

Answer 3

这是一种非常低效的方法，每个 str_replace 都会扫描每一对的整个字符串。相反，为什么不扫描输入字符串并在 encrypt/decrypt 时间计算替换。您要求的是提示而不是解决方案...但这是一个示例 encrypt func（不需要构造，尽管可以存储构造种子而不是将其作为参数传递）：

public function encrypt($value,$seed) {
    $n=strlen($value);
    $nchars=strlen(self::CHARS);
    for($i=0;$i<$n;$i++) {
        //This saves you doing the space-space replace, and allows 
        //  any chars not in your ciper to remain unmolested
        $pos=strpos(self::CHARS,$value[$i]);
        if ($pos>=0) $value[$i]=self::CHARS[($pos+$seed)%$nchars];
    }
}

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