Efficient encryption algorithm for encrypting large byte array

For my recent project, so far I have been using SDES for encrypting any binary file. Undoubtedly, the algorithm is simple to implement; but at the same time it is not suitable for encrypting large files as it takes plaintext of size 1 byte as input i.e Encrypts 1 byte at a time. Any suggestions on more efficient encryption algorithm is appreciated.

Below is the simple implementation of SDES on image files:

• This segment of code encrypts the entire data bytes including the header. You can save the data bytes in form of ASCII or UTF-8 in a file and transmit over some insecure channel. Upon decryption and reconverting to an image file you can use magic number programming.

      Conversion.Convert cvt = new Conversion.Convert();
      Console.WriteLine("Please enter the RGB/GRAY/BINARY image path: ");
      string path = Console.ReadLine();
      byte []arrayPT = Conversion.Convert.ImageToBinary(path); // Get the binary data from the image
      byte []arrayCT = new byte[arrayPT.Length];
      int i = 0;
      foreach (byte element in arrayPT)
      {
          arrayCT[i] = ob.Encrypt(element);
          Console.Write("{0}", arrayCT[i]); //Convert the contents of arrayCT[] into char and save into a file
          i++;
      }
  

• Use this approach to encrypt only the color information i.e. pixel values.

      SDES ob = new SDES(key);
      Bitmap img = new Bitmap(imgpath);
      Color pixelColor, pixels;
      //byte[] buff = new byte[3 * img.Height * img.Width];
      for (int i = 0; i < img.Width; i++)
      {
          for (int j = 0; j < img.Height; j++)
          {
              pixels = img.GetPixel(i, j);
              pixelColor = Color.FromArgb(ob.Encrypt(pixels.R), ob.Encrypt(pixels.G), ob.Encrypt(pixels.B));
              img.SetPixel(i, j, pixelColor);
          }
      }
      img.Save(newEncryptedImg);