encryption - C# AES: Encrypt a file causes "Length of the data to encrypt is invalid." error

Question

I have a PDF File.
When I want to encrypt it using codes below the Length of the data to encrypt is invalid. error occurred:

  string inputFile = @"C:sample.pdf";
  string outputFile = @"C:sample_enc.pdf";

  try
  {    
    using (RijndaelManaged aes = new RijndaelManaged())
    {
      byte[] key = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };
      byte[] iv = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 };

      aes.Key = key;
      aes.IV = iv;

      aes.Mode = CipherMode.CFB;
      aes.Padding = PaddingMode.None;
      aes.KeySize = 128;
      aes.BlockSize = 128;

      using (FileStream fsCrypt = new FileStream(outputFile, FileMode.Create))
      {
        using (ICryptoTransform encryptor = aes.CreateEncryptor(key,iv))
        {
          using (CryptoStream cs = new CryptoStream(fsCrypt, encryptor, CryptoStreamMode.Write))
          {
            using (FileStream fsIn = new FileStream(inputFile, FileMode.Open))
            {
              int data;
              while ((data = fsIn.ReadByte()) != -1)
              {
                cs.WriteByte((byte)data);
              }
            }
          }
        }
      }
    }
  }
  catch (Exception ex)
  {
    // Length of the data to encrypt is invalid.
    Console.WriteLine(ex.Message);
  }

With CipherMode.CBC and PaddingMode.PKCS7, I don't have any errors.
But because of my client, I have to encrypt the file using AES/CFB with No Padding.

Any ideas what's happening here?

Answer 1

A block cipher expects input with a length that is a multiple of the block size. With AES, the length of the input must be a multiple of 16.

You must apply some sort of padding to the plaintext so that this requirement is satisfied. PKCS#7 padding is the best choice.

However, on second thought, CFB mode turns a block cipher into a stream cipher. Stream ciphers don't need padding. The .NET implementation seems to be broken in this regard.