URGENT: inconsistencies between Message Digests

Question

Hi
I'm developing a system wherein the contents of a JMS message are fed into a MessageDigest, and signed by a Signature with the sending node's Private Key. The signed bytes from the Signature are then set as a message property.
The node that receives the message then creates a MessageDigest from the message body text, and using another Signature, performs verification of the new MessageDigest with the signed bytes obtained from the JMS message property that was set.
The problem is that, despite explicitly using the UTF-8 encryption when converting from byte[] to String in my code, the MessageDigest of the message body is always different (and differs upon each program run, even when the message body contents are the same for repeated runs). I've checked and the text being used to create the message digest on each node is exactly the same (including when checking the bytes), yet as I have said, the digests are always different. However, I am running one node on Windows XP, and another node on Solaris, but I can't see how this might affect things if I've explicitly used UTF-8 encoding, and have the same Provider (SUN version 1.2) in use on both platforms.
Are any aspects of Message Digests time-dependent/platform-dependent/randomly-generated?
Should I not even be using Message Digests/Signatures and instead use Ciphers?
Can anyone help me out here? I would be extremely grateful for any assistance!
Thanks,
Simon Parkin

Answer 1

Thanks for the help.
I removed the MessageDigest code ... and I'm still having problems (in a different area now).
I hate having to do this, but here is the code listing for the functions I've created (minus the Exception catching code):
public String signedDigest(String messageText)
     PrivateKey key = (PrivateKey)KEYSTORE.getKey(LOCAL_ALIAS, LOCAL_PASSWORD.toCharArray());
     Signature kSig = Signature.getInstance(SIGNATURE_ALGORITHM);
     kSig.initSign(key);
     kSig.update(messageText.trim().getBytes("UTF-8"));     
     byte[] signedBytes = kSig.sign();
     return new String(signedBytes, "UTF-8").trim();
public boolean verifyContent(String messageText, String signedDigest, String receiverID)
     Certificate kCert = KEYSTORE.getCertificate(receiverID);     
     PublicKey kPub = kCert.getPublicKey();
     Signature kVer = Signature.getInstance(SIGNATURE_ALGORITHM);
     kVer.initVerify(kPub);
     kVer.update(messageText.trim().getBytes("UTF-8"));          
     boolean result = kVer.verify(signedDigest.trim().getBytes("UTF-8"));
     return result;
'signedDigest' creates the signed content in the sending node, and 'verifyContent' checks it at the receiver end. Now I've checked the content of the 'signedBytes' produced in 'signedDigest', and compared it to 'signedDigest.getBytes("UTF-8)' in 'verifyContent', and they are exactly the same, yet the verification never returns true. I'm using the right keys, and the 'messageText' used at both ends is the same, so I still can't understand why this isn't working.
Any further help would be very much appreciated.
Simon Parkin