Java and security bits

Hashing a file in 3 lines

As I was working on a Peabody contribution recently, I remembered a short program I wrote a couple of years ago. It shows that you can calculate the message digest of a file in 3 lines of code. Of course, I had since lost the program, so I started over:

import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import static java.nio.channels.FileChannel.MapMode.READ_ONLY;
import java.math.BigInteger;
import java.util.*;
import java.security.MessageDigest;

public class Digest {

    public static void main(String[] args) throws Exception {
        if (args.length != 1) {
            System.out.println("Usage: java Digest <file>");
            return;
        }
        FileChannel channel = new RandomAccessFile(args[0], "r").getChannel();
        ByteBuffer buffer = channel.map(READ_ONLY, 0, (int)channel.size());
        digest("md5", buffer);
        digest("sha1", buffer);
        channel.close();
    }

    private static void digest(String algorithm, ByteBuffer buffer) throws Exception {
        MessageDigest md = MessageDigest.getInstance(algorithm);
        md.update(buffer.duplicate());
        byte[] digest = md.digest();
        new Formatter(System.out).format
                ("%-5s: %0" + (digest.length * 2) + "x%n",
                algorithm, new BigInteger(1, digest));
    }
}

Obviously, this is more than three lines, but the actual digesting is only three lines: getting the MessageDigest object [MessageDigest.getInstance()] hashing the file [md.update(ByteBuffer)] and getting the result [md.digest()].

The methods that take ByteBuffers were added to MessageDigest, Signature, and Cipher in JDK 5.0. They were designed for scalable network I/O using channels and securing them via an SSLEngine, but they work just as well for memory mapped files or just plain byte arrays. FWIW, I think ByteBuffers are underrated. They are to byte arrays as Collections are to Object arrays: more powerful and easier to use. But I disgress.

PS: I think it is also pretty clear that this program is not the smartest way to digest a file using multiple algorithms. Apart from the fact that it does not work for files larger than 2G at all (the limit for ByteBuffers), it also processes the entire file first using MD5 and then again using SHA1. That should really be done in chunks to take advantage of the OS page cache, but if I did all that I could not claim that I hashed the file using three lines of code ;-)

Posted at 00:03 Apr 12, 2006 by Andreas Sterbenz in Java  |  Comments[5]