ITHit Webdav Download and decrypt office document

Question

I need to download decrypted file office document in webdav server, because the document in file system are encrypted. I am using ITHit to implement webdav server.. and a little modify the code in order to be able to decrypt the file. However this code doesn't work. The file won't open and said "The file xxx can not be opened". It is only work for unencrypted file. Can anyone help me why this code wont work?

private async Task readInternalAsync(Stream output, long startIndex, long count)
{
  var buffer = new byte[bufSize];
  var document = this.documentManagement.GetDocumentById("1");

  using (var fileStream = fileInfo.Open(FileMode.Open, FileAccess.Read, FileShare.Read))
  {
       try
       {
           if (document.EncryptionInfo.IsEncrypted)
           {
                // byte array after the file has been decrypted 
                byte[] data = this.encryptionManagement.DecryptFile(fileStream, document.EncryptionInfo.Password, document.EncryptionInfo.AlgorithmName);
                await output.WriteAsync(data, 0, data.Length);
           }
           else
           {
                fileStream.Seek(startIndex, SeekOrigin.Begin);
                int bytesRead;
                var toRead = (int)Math.Min(count, bufSize);

                while (toRead > 0 && (bytesRead = await fileStream.ReadAsync(buffer, 0, toRead)) > 0)
                {
                     await output.WriteAsync(buffer, 0, bytesRead);
                     count -= bytesRead;
                }
            }
            catch (HttpException ex)
            {
                var msg = ex.Message;
                // The remote host closed the connection (for example Cancel or Pause pressed).
            }
       }
  }
}

Here is the logic to decrypt the file if anyone need it.

 public byte[] DecryptFile(Stream inputStream, string passPhrase, string algorithmName)
    {
        using (var outputStream = new MemoryStream())
        {
            this.decryptInputStreamToOutputStream(inputStream, outputStream, passPhrase, algorithmName);
            var bytes = outputStream.ToArray();
            return bytes;
        }
    }

private void decryptInputStreamToOutputStream(Stream inputStream, Stream outputStream, string passPhrase, string algorithmName)
    {
        inputStream.Position = 0;

        algorithmName = this.getAlgorithmName(algorithmName);
        using (var algo = SymmetricAlgorithm.Create(algorithmName))
        {
            // Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes.
            var saltBytesCount = algo.BlockSize / 8;
            var saltBytes = new byte[saltBytesCount];
            inputStream.Read(saltBytes, 0, saltBytesCount);

            // Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes.
            var ivBytesCount = algo.BlockSize / 8;
            var ivBytes = new byte[ivBytesCount];
            inputStream.Read(ivBytes, 0, ivBytesCount);

            using (var password = new Rfc2898DeriveBytes(passPhrase, saltBytes))
            {
                var keyBytes = password.GetBytes(algo.KeySize / 8);

                using (var decryptor = algo.CreateDecryptor(keyBytes, ivBytes))
                {
                    using (var cryptoStream = new CryptoStream(outputStream, decryptor, CryptoStreamMode.Write))
                    {
                        int count = 0;
                        byte[] data = new byte[this.chryptoChunkBlockSizeBytes];
                        do
                        {
                            count = inputStream.Read(data, 0, this.chryptoChunkBlockSizeBytes);
                            cryptoStream.Write(data, 0, count);
                        }
                        while (count > 0);

                        cryptoStream.FlushFinalBlock();
                    }
                }
            }
        }
    }

Many Thanks

Regards

Answer 1

Your Problem may be the Length of the file. As asked in another question, the lengt of the file must be correctly supplied. During the decryption, the size may vary from the original file. So be sure to suply the decrypted file size as ContentLength

public long ContentLength
{
   //Supply Decrypted file length here....
   get { return fileInfo.Length; }
}

You may also try to set Content lengt before Output.WriteAsync... like this:

context.Response.ContentLength = data.LongLength();