False tar file detection

TL;DR: TarArchive.IsTarFile() says a file is a tar file when it actually isn’t.

I tried extracting a .gz file (not .tar.gz) using the ReaderFactory.Open() method and the extraction resulted in very strange file names, which lead me to investigate, and I discovered that the decompressed gz file is detected as a valid tar file.

Steps to reproduce:

var bytes = new List<byte>(System.Text.Encoding.ASCII.GetBytes("hello world"));

while (bytes.Count < 512)
    bytes.Add(0);

var stream = new MemoryStream(bytes.ToArray());
bool result = SharpCompress.Archives.Tar.TarArchive.IsTarFile(stream); // returns true

This behaviour occured with multiple (similar) files. The code above generates a simplified version of one of the files I tried to decompress. It behaves the same even if I read the file from disk.

[adamhathcock]

Read the source in is_tar.c in libmagic which calculates the checksum of the block. So easy enough

[adamhathcock]

Should port the is_tar detection from https://github.com/threatstack/libmagic/blob/master/src/is_tar.c