Use a combination of SHA1+MD5

Question

I'm trying use a secure way to create checksum for files (Larger than 10GB !).

SHA256 is secure enough for me but this algorithm is so process expensive and it is not suitable. Well I know that both SHA1 and MD5 checksums are insecure through the collisions.

So I just think the fastest and the safest way is combining MD5 with SHA1 like : SHA1+MD5 and I don't think there is way to get file (Collision) with the same MD5 and SHA1 both at a same time .

So is combining SHA1+MD5 secure enough for file checksum? or is there any attack like collision for it ?

I use c# mono in two way (Bufferstream and without Bufferedstream)

    public static string GetChecksum(string file)
    {
        using (FileStream stream = File.OpenRead(file))
        {
            var sha = new SHA256Managed();
            byte[] checksum = sha.ComputeHash(stream);
            return BitConverter.ToString(checksum).Replace("-", String.Empty);
        }
    }

    public static string GetChecksumBuffered(Stream stream)
    {
        using (var bufferedStream = new BufferedStream(stream, 1024 * 32))
        {
            var sha = new SHA256Managed();
            byte[] checksum = sha.ComputeHash(bufferedStream);
            return BitConverter.ToString(checksum).Replace("-", String.Empty);
        }
    }

Update 1: I mean SHA1 hash + MD5 hash. First calculate SHA1 of file then calculate MD5 of file then add this two sting together.

Update 2 :

As @zaph mentioned I implement my code(C# MONO) again according what I read here but it doesn't make my code as fast as he said ! It makes my speed for a 4.6 GB file from (approximate) 12mins to about 8.~ mins but sha1+md5 takes me less than 100 secs for this file. So I still think it isn't right to use SHA256 instead.

Answer 1

There should be only a small difference between SHA-256 and a combination of MD5+SHA1.

The only way to know is to benchmark:

On my desk top:
SHA-256: 200 MB/s
MD5: 470 MB/s
SHA1: 500 MB/s (updated, previously incorrect)
MD5+SHA1 240 MB/s

These times are only for the hashing, disk read time is not included. The tests were done with a 1MB buffer and averaged over 10 runs. The language was "C" and the library used was Apple's Common Crypto. The cpu was a 2.8 GHz Quad-Core Intel Xeon (2010 MacPro, my laptop is faster).

In the end it is 23% faster to use the combined MD5+SHA1.

Note: Most Intel processors have instruction that can be used to make crypto operations faster. Not all implementations utilize these instructions.

YOumight try a native implementations such as sha256sum.