java.security.MessageDigest question - seems to generate same MD5 Value

Ruaidhri

Hi, i've written a small method to compute the MD5 of a string which i'm passing to the method. the method then encodes the MD5 into Base64 with an another method ( not shown here ). i seem to be getting the same values each time for the MD5 tho. any suggestions?

public String computeMD5(String xml)
{
  String messagedigest = "";
  String digest64 = "";
try
 {
  MessageDigest md = MessageDigest.getInstance("MD5");
  byte[] data = xml.getBytes();
  byte[] digest = md.digest(data);
  digest = md.digest();
  messagedigest = new String ( digest );
  digest64 = net.iharder.Base64.encodeBytes(digest);
 }
catch(NoSuchAlgorithmException ex)
 { ex.toString(); }
   
 return digest64;

Enygma

I think it should be something like this:

   md = MessageDigest.getInstance("MD5");
   md.update(xml.getBytes());
   byte[] digestValue = md.digest();
   // then Base64 encode digestValue

seamus

It's quite possible that it's throwing an exception and returning the MD5 value of the hashed empty String.

Try using System.err.print() for outputting any caught exceptions. ex.toString() doesn't do any useful debugging

lynchie

What enygma said, you need to call update on it first before calling digest.

tempest

Ruaidhri wrote:

Hi, i've written a small method to compute the MD5 of a string which i'm passing to the method. the method then encodes the MD5 into Base64 with an another method ( not shown here ). i seem to be getting the same values each time for the MD5 tho. any suggestions?

public String computeMD5(String xml)
{
  String messagedigest = "";
  String digest64 = "";
try
 {
  MessageDigest md = MessageDigest.getInstance("MD5");
  byte[] data = xml.getBytes();
  byte[] digest = md.digest(data);

  // This line is overwriting the previously digested data....
  // So just take it out and all will be ok..
  digest = md.digest();

  messagedigest = new String ( digest );
  digest64 = net.iharder.Base64.encodeBytes(digest);
 }
catch(NoSuchAlgorithmException ex)
 { ex.toString(); }
   
 return digest64;

See the comment in the code...
The second call to digest undoes all of the good work performed by the digest(byte[] data) call...

You can do it with update either but that's not the problem you are seeing as digest(byte[]) calls update(byte[]) followed by digest().

Ruaidhri

yep that did the trick thanks!