Question

Using .net, I would like to be able to hash the first N bytes of potentially large files, but I can’t seem to find a way of doing it.

The ComputeHash function (I’m using SHA1) takes a byte array or a stream, but a stream seems like the best way of doing it, since I would prefer not to load a potentially large file into memory.

To be clear: I don’t want to load a potentially large piece of data into memory if I can help it. If the file is 2GB and I want to hash the first 1GB, that’s a lot of RAM!

Answer 1

You can hash large volumes of data using a CryptoStream – something like this should work:

var sha1 = SHA1Managed.Create();

FileStream fs = \\whatever
using (var cs = new CryptoStream(fs, sha1, CryptoStreamMode.Read))
{
    byte[] buf = new byte[16];
    int bytesRead = cs.Read(buf, 0, buf.Length);
    long totalBytesRead = bytesRead;

    while (bytesRead > 0 && totalBytesRead <= maxBytesToHash)
    {
        bytesRead = cs.Read(buf, 0, buf.Length);
        totalBytesRead += bytesRead;
    }
}

byte[] hash = sha1.Hash;