文字列をパスワードで暗号化・複号化するサンプルです。

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// using System.Security.Cryptography; // パスワードから共有キーと初期化ベクタを作成 private static void GenerateKeyFromPassword(string password, int keySize, out byte[] key, int blockSize, out byte[] iv) {     byte[] salt = Encoding.UTF8.GetBytes("RandomByte");  // ８バイト以上の任意の文字列を指定     Rfc2898DeriveBytes rdb = new Rfc2898DeriveBytes(password, salt);     rdb.IterationCount = 999;  // 演算反復処理回数     key = rdb.GetBytes(keySize / 8);     iv = rdb.GetBytes(blockSize / 8); } // 文字列を暗号化 private static string EncryptString(string sourceString, string password) {     RijndaelManaged rm = new RijndaelManaged();     byte[] key, iv;     GenerateKeyFromPassword(password, rm.KeySize, out key, rm.BlockSize, out iv);     rm.Key = key;     rm.IV = iv;     byte[] strBytes = Encoding.UTF8.GetBytes(sourceString);     string strRet;     using (ICryptoTransform ict = rm.CreateEncryptor())     {         byte[] encBytes = ict.TransformFinalBlock(strBytes, 0, strBytes.Length);         strRet = Convert.ToBase64String(encBytes);     }     return strRet; } // 暗号化された文字列を復号化 private static string DecryptString(string encryptString, string password) {     RijndaelManaged rm = new RijndaelManaged();     byte[] key, iv;     GenerateKeyFromPassword(password, rm.KeySize, out key, rm.BlockSize, out iv);     rm.Key = key;     rm.IV = iv;     byte[] strBytes = Convert.FromBase64String(encryptString);     string strRet;     using (ICryptoTransform ict = rm.CreateDecryptor())     {         byte[] decBytes = ict.TransformFinalBlock(strBytes, 0, strBytes.Length);         strRet = Encoding.UTF8.GetString(decBytes);     }     return strRet; }