Zlib-compatible compression streams?

Question

Are System.IO.Compression.GZipStream or System.IO.Compression.Deflate compatible with zlib compression?

Answer 1

I ran into this issue with Git objects. In that particular case, they store the objects as deflated blobs with a Zlib header, which is documented in RFC 1950. You can make a compatible blob by making a file that contains:

• Two header bytes (CMF and FLG from RFC 1950) with the values 0x78 0x01
  • CM = 8 = deflate
  • CINFO = 7 = 32Kb window
  • FCHECK = 1 = checksum bits for this header
• The output of the C# DeflateStream
• An Adler32 checksum of the input data to the DeflateStream, big-endian format (MSB first)

I made my own Adler implementation

public class Adler32Computer
{
    private int a = 1;
    private int b = 0;

    public int Checksum
    {
        get
        {
            return ((b * 65536) + a);
        }
    }

    private static readonly int Modulus = 65521;

    public void Update(byte[] data, int offset, int length)
    {
        for (int counter = 0; counter < length; ++counter)
        {
            a = (a + (data[offset + counter])) % Modulus;
            b = (b + a) % Modulus;
        }
    }
}

And that was pretty much it.

Answer 2

DotNetZip includes a DeflateStream, a ZlibStream, and a GZipStream, to handle RFC 1950, 1951, and 1952. The all use the DEFLATE Algorithm but the framing and header bytes are different for each one.

As an advantage, the streams in DotNetZip do not exhibit the anomaly of expanding data size under compression, reported against the built-in streams. Also, there is no built-in ZlibStream, whereas DotNetZip gives you that, for good interop with zlib.