PHP 自訂加密解密類別：維吉尼亞密表變體實作

演算法原理

本類別實作一種以密碼字串產生動態替換表的對稱加密演算法，屬於維吉尼亞密碼 的變體。核心思想：以密碼每字元在基礎字母表中的位置為起點，產生一組位移後的替換字母表，加密時逐字元查表替換，解密則反向查表。密碼長度決定替換表組數，加密過程循環使用。

字母表定義

$ralphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890 !,.:;?~@#$%^&*()_+-=][}{/><'\"";
$alphabet = $ralphabet . $ralphabet; // 雙倍長度，方便 substr 截取位移後的完整表

核心類別程式碼

class Crypto {
    function encrypt($password, $strtoencrypt) {
        global $ralphabet, $alphabet;
        $pos_alpha_ary = [];
        $len_pwd = strlen($password);
        
        // 依密碼字元產生替換表陣列
        for ($i = 0; $i < $len_pwd; $i++) {
            $cur = substr($password, $i, 1);
            $pos_alpha_ary[] = substr(strstr($alphabet, $cur), 0, strlen($ralphabet));
        }

        $encrypted = '';
        $n = 0;
        $c = strlen($strtoencrypt);
        while ($n < $c) {
            $char = substr($strtoencrypt, $n, 1);
            $pos = strpos($ralphabet, $char);
            if ($pos !== false) {
                $encrypted .= substr($pos_alpha_ary[$n % $len_pwd], $pos, 1);
            } else {
                $encrypted .= $char; // 不在字母表中的字元保持原樣
            }
            $n++;
        }
        return $encrypted;
    }

    function decrypt($password, $strtodecrypt) {
        global $ralphabet, $alphabet;
        $pos_alpha_ary = [];
        $len_pwd = strlen($password);
        
        for ($i = 0; $i < $len_pwd; $i++) {
            $cur = substr($password, $i, 1);
            $pos_alpha_ary[] = substr(strstr($alphabet, $cur), 0, strlen($ralphabet));
        }

        $decrypted = '';
        $n = 0;
        $c = strlen($strtodecrypt);
        while ($n < $c) {
            $char = substr($strtodecrypt, $n, 1);
            $table = $pos_alpha_ary[$n % $len_pwd];
            $pos = strpos($table, $char);
            if ($pos !== false) {
                $decrypted .= substr($ralphabet, $pos, 1);
            } else {
                $decrypted .= $char;
            }
            $n++;
        }
        return $decrypted;
    }

    // 顯示密表（除錯/教學用）
    function cryption_table($password) {
        global $ralphabet, $alphabet;
        $pos_alpha_ary = [];
        for ($i = 0; $i < strlen($password); $i++) {
            $cur = substr($password, $i, 1);
            $pos_alpha_ary[] = substr(strstr($alphabet, $cur), 0, strlen($ralphabet));
        }
        echo "";
        echo "";
        for ($j = 0; $j < strlen($ralphabet); $j++) echo "";
        echo "";
        for ($i = 0; $i < count($pos_alpha_ary); $i++) {
            echo "";
            for ($k = 0; $k < strlen($pos_alpha_ary[$i]); $k++) {
                echo "";
            }
            echo "";
        }
        echo "
" . substr($ralphabet, $j, 1) . "
" . ($i+1) . "|
" . substr($pos_alpha_ary[$i], $k, 1) . "
";
    }
}

使用範例

$ct = new Crypto();
$password = "secret";
$plain = "Hello World 123";

$encrypted = $ct->encrypt($password, $plain);
echo "加密：" . $encrypted . "
";

$decrypted = $ct->decrypt($password, $encrypted);
echo "解密：" . $decrypted . "
";

// 驗證
var_dump($plain === $decrypted); // bool(true)

// 顯示密表
$ct->cryption_table($password);

安全性分析與限制

現代替代方案

1. OpenSSL 對稱加密 (AES-256-CBC/GCM)：

   function aes_encrypt($data, $key, $iv) {
       return openssl_encrypt($data, 'AES-256-GCM', $key, OPENSSL_RAW_DATA, $iv, $tag);
   }
   function aes_decrypt($data, $key, $iv, $tag) {
       return openssl_decrypt($data, 'AES-256-GCM', $key, OPENSSL_RAW_DATA, $iv, $tag);
   }
   

2. Sodium (libsodium) - PHP 7.2+：

   $key = sodium_crypto_secretbox_keygen();
   $nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);
   $cipher = sodium_crypto_secretbox($message, $nonce, $key);
   $plain = sodium_crypto_secretbox_open($cipher, $nonce, $key);
   

3. 密碼衍生金鑰 (PBKDF2/Argon2)：使用者密碼轉加密金鑰

教學價值

雖不安全，但展示了：替換表生成、模組化循環使用金鑰、字元級對映、對稱加密結構。適合作為密碼學入門教材，理解「為什麼不能自己造加密」的反面教材。