第4章　C#でファイルを暗号化する

  private bool FileEncrypt(string FilePath, string Password)
  {
    System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();
    sw.Start();

    int i, len;
    byte[] buffer = new byte[4096];

    //Output file path.
    string OutFilePath = Path.Combine(Path.GetDirectoryName(FilePath), Path.GetFileNameWithoutExtension(FilePath)) + ".enc";

    using (FileStream outfs = new FileStream(OutFilePath, FileMode.Create, FileAccess.Write))
    {
        using (AesManaged aes = new AesManaged())
        {
            aes.BlockSize = 128;              // BlockSize = 16bytes
            aes.KeySize = 128;                // KeySize = 16bytes
            aes.Mode = CipherMode.CBC;        // CBC mode
            aes.Padding = PaddingMode.PKCS7;    // Padding mode is "PKCS7".

            Rfc2898DeriveBytes deriveBytes = new Rfc2898DeriveBytes(Password, 16);
            byte[] salt = new byte[16];
            salt = deriveBytes.Salt；
            byte[] bufferKey = deriveBytes.GetBytes(16);

            /*
            byte[] bufferKey = new byte[16];
            byte[] bufferPassword = Encoding.UTF8.GetBytes(Password);
            for (i = 0; i < bufferKey.Length; i++)
            {
                if (i < bufferPassword.Length)
                {
                    bufferKey[i] = bufferPassword[i];
                }
                else
                {
                    bufferKey[i] = 0;
                }
            */

            aes.Key = bufferKey;
            // IV ( Initilization Vector ) は、AesManagedにつくらせる
            aes.GenerateIV();

            //Encryption interface.
            ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

            using (CryptoStream cse = new CryptoStream(outfs, encryptor, CryptoStreamMode.Write))
            {
              outfs.Write(salt, 0, 16);
              outfs.Write(aes.IV, 0, 16);
                using (DeflateStream ds = new DeflateStream(cse, CompressionMode.Compress)) //圧縮
                {
                    using (FileStream fs = new FileStream(FilePath, FileMode.Open, FileAccess.Read))
                    {
                        while ((len = fs.Read(buffer, 0, 4096)) > 0)
                        {
                            ds.Write(buffer, 0, len);
                        }
                    }
                }
            }
        }
    }
    sw.Stop();
    long resultTime = sw.ElapsedMilliseconds;

    textBox1.AppendText("暗号化成功: " + Path.GetFileName(OutFilePath) + Environment.NewLine);
    textBox1.AppendText("実行時間: " + resultTime.ToString() + "ms");

    return (true);
}

  private bool FileDecrypt(string FilePath, string Password)
  {
      int i, len;
      byte[] buffer = new byte[4096];

      if (String.Compare(Path.GetExtension(FilePath), ".enc", true) != 0)
      {
          //The file are not encrypted file! Decryption failed
          MessageBox.Show("暗号化されたファイルではありません！" + Environment.NewLine + "復号に失敗しました。",
              "Error", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
          return (false); ;
      }

      //Output file path.
      string OutFilePath = Path.Combine(Path.GetDirectoryName(FilePath), Path.GetFileNameWithoutExtension(FilePath)) + ".txt";

      using (FileStream outfs = new FileStream(OutFilePath, FileMode.Create, FileAccess.Write))
      {
          using (FileStream fs = new FileStream(FilePath, FileMode.Open, FileAccess.Read))
          {
              using (AesManaged aes = new AesManaged())
              {
                  aes.BlockSize = 128;              // BlockSize = 16bytes
                  aes.KeySize = 128;                // KeySize = 16bytes
                  aes.Mode = CipherMode.CBC;        // CBC mode
                  aes.Padding = PaddingMode.PKCS7;    // Padding mode is "PKCS7".

                  // salt
                  byte[] salt = new byte[16];
                  fs.Read(salt, 0, 16);

                  // Initilization Vector
                  byte[] iv = new byte[16];
                  fs.Read(iv, 0, 16);
                  aes.IV = iv;

                  /*

                  byte[] bufferKey = new byte[16];
                  byte[] bufferPassword = Encoding.UTF8.GetBytes(Password);
                  for (i = 0; i < bufferKey.Length; i++)
                  {
                      if (i < bufferPassword.Length)
                      {
                          bufferKey[i] = bufferPassword[i];
                      }
                      else
                      {
                          bufferKey[i] = 0;
                      }
                  */
                  Rfc2898DeriveBytes deriveBytes = new Rfc2898DeriveBytes(Password, salt);
                  byte[] bufferKey = deriveBytes.GetBytes(16);
                  aes.Key = bufferKey;

                  //Decryption interface.
                  ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);

                  using (CryptoStream cse = new CryptoStream(fs, decryptor, CryptoStreamMode.Read))
                  {
                      using (DeflateStream ds = new DeflateStream(cse, CompressionMode.Decompress))
                      {
                          while ((len = ds.Read(buffer, 0, 4096)) > 0)
                          {
                              outfs.Write(buffer, 0, len);
                          }
                      }
                  }
              }
          }
      }
      //Decryption succeed.
      textBox1.AppendText("復号成功: " + Path.GetFileName(OutFilePath) + Environment.NewLine);
      return (true);
  }