C# - GZipStream magic number incorrect?

Question

So, I'm trying to make a program which turns a computer into a proxy using this. It all works well, except for gzip/deflate pages.

Whenever I try to uncompress, I get an InvalidDataException stating the magic number in the GzipHeader is incorrect.

I use this function:

private byte[] GZipUncompress(byte[] data)
{
    using (var input = new MemoryStream(data))
    {
        input.Seek(0, SeekOrigin.Begin);

        using (var gzip = new GZipStream(input, CompressionMode.Decompress))
        using (var output = new MemoryStream())
        {
            output.Seek(0, SeekOrigin.Begin);
            gzip.CopyTo(output);

            return output.ToArray();
        }
    }
}

to decompress data. The error:

_{(source: gyazo.com)}

Any help would be appreciated.

EDIT: I seem to have gotten somewhere!

As usr suggested, I should write a HTTP parser to get the body and decompress that.

Before parsing: http://pastebin.com/Cb0E8WtT

After parsing: http://pastebin.com/k9e8wMvr

This is the method I use to get to the body:

    private byte[] HTTParse(byte[] data)
    {
        string http = ascii.GetString(data);
        char[] lineBreak = crlf.ToCharArray();
        string[] parts = http.Split(lineBreak);

        List<byte> res = new List<byte>();

        for (int i = 1; i < parts.Length; i++)
        {
            if (i % 2 == 0)
            {
                Regex r = new Regex(@"(.)*: (.)*");
                Regex htt = new Regex(@"HTT(.)*/(.)*.(.)* d{1,50} (.)*");
                if (!r.IsMatch(parts[i]) && !htt.IsMatch(parts[i]))
                {
                    //Console.WriteLine("[TEST] " + parts[i]);
                    res.AddRange(ascii.GetBytes(parts[i]));
                    res.AddRange(ascii.GetBytes("\r\n"));
                }
                
            }
        }
        return res.ToArray();
    }

However, I still get an error saying "The magic number in GZip header is not correct. Make sure you are passing in a GZip stream."

EDIT (2): After copying an answer from here, I have managed to successfully uncompress the body.

The new problem: Firefox.

_{(source: gyazo.com)}

I'm now unsure whether or not I even needed to uncompress gzip pages..

Where have I gone wrong now?

Answer 1

You said, that you use this code for gzip/deflate. But deflate is not the same as gzip, especially it has no magic header like gzip does. Deflate is defined in RFC1951, gzip in RC1952. Also, browsers like Firefox and Chrome (but not Internet Explorer) also accept "raw deflate" according to RFC1950. So before you apply decompression to the body you must first check based on the "Content-Encoding" header which compression is used.