Question

I know this sounds like a broad question but I can narrow it down with an example. I am VERY new at Java. For one of my “learning” projects, I wanted to create an in-house MD5 file hasher for us to use. I started off very simple by attempting to hash a string and then moving on to a file later. I created a file called MD5Hasher.java and wrote the following:

import java.security.*;
import java.io.*;
public class MD5Hasher{
    public static void main(String[] args){
        String myString = "Hello, World!";
        byte[] myBA = myString.getBytes();
        MessageDigest myMD;
        try{
            myMD = MessageDigest.getInstance("MD5");
            myMD.update(myBA);
            byte[] newBA = myMD.digest();
            String output = newBA.toString();
            System.out.println("The Answer Is: " + output);
        } catch(NoSuchAlgorithmException nsae){
            // print error here
        }
    }
}

I visited java.sun.com to view the javadocs for java.security to find out how to use MessageDigest class. After reading I knew that I had to use a “getInstance” method to get a usable MessageDigest object I could use. The Javadoc went on to say “The data is processed through it using the update methods.” So I looked at the update methods and determined that I needed to use the one where I fed it a byte array of my string, so I added that part. The Javadoc went on to say “Once all the data to be updated has been updated, one of the digest methods should be called to complete the hash computation.” I, again, looked at the methods and saw that digest returned a byte array, so I added that part. Then I used the “toString” method on the new byte array to get a string I could print. However, when I compiled and ran the code all that printed out was this:

The Answer Is: [B@4cb162d5

I have done some looking around here on StackOverflow and found some information here:

How can I generate an MD5 hash?

that gave the following example:

String plaintext = 'your text here';
MessageDigest m = MessageDigest.getInstance("MD5");
m.reset();
m.update(plaintext.getBytes());
byte[] digest = m.digest();
BigInteger bigInt = new BigInteger(1,digest);
String hashtext = bigInt.toString(16);
// Now we need to zero pad it if you actually want the full 32 chars.
while(hashtext.length() < 32 ){
    hashtext = "0"+hashtext;
}

It seems the only part I MAY be missing is the “BigInteger” part, but I’m not sure.

So, after all of this, I guess what I am asking is, how do you know to use the “BigInteger” part? I wrongly assumed that the “toString” method on my newBA object would convert it to a readable output, but I was, apparently, wrong. How is a person supposed to know which way to go in Java? I have a background in C so this Java thing seems pretty weird. Any advice on how I can get better without having to “cheat” by Googling how to do something all the time?

Thank you all for taking the time to read. 🙂

Answer 1

You have actually successfully digested the message. You just don’t know how to present the found digest value properly. What you have is a byte array. That’s a bit difficult to read, and a toString of a byte array yields [B@somewhere which is not useful at all.

The BigInteger comes into it as a tool to format the byte array to a single number.

What you do is:

• construct a BigInteger with the proper value (in this case that value happens to be encoded in the form of a byte array – your digest
• Instruct the BigInteger object to return a String representation (e.g. plain, readable text) of that number, base 16 (e.g. hex)

And the while loop prefixes that value with 0-characters to get a width of 32. I’d probably use String.format for that, but whatever floats your boat 🙂