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Getting MD5 Sums in Java

At 1:42 PM on Nov 15, 2006, R.J. Lorimer wrote:

A human-readable hexedecimal string representing the MD5 summation of the contents of a file or even just a text string can be extremely valuable in programming contexts as well when handling user input. The MD5 concept is covered in detail on Wikipedia.org .

MD5 is very valuable, but the process of calculating MD5 for a array of bytes in Java is not as straightforward as it may seem. In many languages and operating systems (like Linux) calculating MD5 is as simple as saying 'md5sum [filename] --[ascii|binary]'; the Java implementation is lower level, and is meant to be able to MD5 arbitrary lengths of binary data. In addition, the Java API handles other digest/checksum algorithms, such as SHA and MD2.

Here is a complete method for calculating MD5 from a file (as a common example):

First, we need to get an implementation of java.security.MessageDigest that is meant to handle MD5 specifically:

MessageDigest digest = MessageDigest.getInstance("MD5");

There are several algorithms typically available. References to what versions are typically available and the standard identifiers are detailed and referenced in the MessageDigest javadoc. Now that we have this digest, we need to start calculating the MD5, using a piece of the data at a time. To do this, we need to break the binary data up into byte[] chunks. For this process we can use a standard java.io.InputStream (in this case FileInputStream ) buffering into a byte[].

File f = new File("c:\\myfile.txt");
InputStream is = new FileInputStream(f);				
byte[] buffer = new byte[8192];
int read = 0;
try {
	while( (read = is.read(buffer)) > 0) {
		// --- MD5 work here.---
	}		
	
}
catch(IOException e) {
	throw new RuntimeException("Unable to process file for MD5", e);
}
finally {
	try {
		is.close();
	}
	catch(IOException e) {
		throw new RuntimeException("Unable to close input stream for MD5 calculation", e);
	}
}

This loop should look familiar to most people, and doesn't really have any unique MD5 component in it. This algorithm is, in general, the same that could be used to copy a file in Java or numerous other IO processing tasks. Note that if you are MD5-ing something completely different from a file, you don't need this loop. You could easily be processing user input, segments from an XML file, or even a serialized object. The point is, all MD5 needs is byte data.

During each iteration of the loop, we need to take the new read-in data and tell the message digest class to add new data as part of the calculation - we do this by calling MessageDigest.update(byte[]) (or one of its variants). In this case, we can use the variant that also takes and offset and length:

digest.update(buffer, 0, read);

The last step which is often important, however isn't required in all cases is to take the finished result of the MD5 calculation, and turn it into some managable format. By default, the MessageDigest class provides a byte[] representing the finished calculation. It is common practice in the modern tech world to throw MD5 data around as a hexadecimal text string. I've seen common solutions using Apache commons libraries (such as the org.apache.commons.codec.binary.Hex.encodeHex(byte[]) method in the Commons Codec project), although using java.lang.BigInteger it is possible to perform the calculation (albeit possibly at a higher processing expense) - the only trick with BigInteger is knowing which bit represents the sign bit (hence the '1' in the following code):

BigInteger bigInt = new BigInteger(1, md5sum);
String output = bigInt.toString(16);

Here is the a full example:

public static void main(String[] args) throws NoSuchAlgorithmException, FileNotFoundException {
	MessageDigest digest = MessageDigest.getInstance("MD5");
	File f = new File("c:\\myfile.txt");
	InputStream is = new FileInputStream(f);				
	byte[] buffer = new byte[8192];
	int read = 0;
	try {
		while( (read = is.read(buffer)) > 0) {
			digest.update(buffer, 0, read);
		}		
		byte[] md5sum = digest.digest();
		BigInteger bigInt = new BigInteger(1, md5sum);
		String output = bigInt.toString(16);
		System.out.println("MD5: " + output);
	}
	catch(IOException e) {
		throw new RuntimeException("Unable to process file for MD5", e);
	}
	finally {
		try {
			is.close();
		}
		catch(IOException e) {
			throw new RuntimeException("Unable to close input stream for MD5 calculation", e);
		}
	}		
}