embed digital signature in XML

[Lei Chen <Le...@sfcommerce.com>]

Hi folks,

I am trying to embed a digital signature into a XML document as processing
instructions like following:

                  <top>
                      <contract>
                            ...
                      </contract>
                      <?DigSignA234B5678E897?>
                   </top>

The signature was generated by using 'byte[] Signature.sign(bye[])' from
java.security package. However, when I ran following code:

                  ...
                  byte[] data = ...
                  byte[] sig = signature.sign(data);
                   ...
                   Element root = doc.createElement("top");
                   doc.appendChild(root);
                   ...
                   root.appendChild (
doc.createProcessingInstruction("DigSign" + new String(sig), null) );

I got 
java.lang.InstantiationError: org/w3c/dom/DOMException
 at
org.apache.xerces.dom.DocumentImpl.createProcessingInstruction(DocumentImpl.
java:566)

If I switch to sun's crimson parser, the error message is more informative:
org.apache.crimson.tree.DomEx: INVALID_CHARACTER_ERR: An invalid character
was specified, such as in a name.
 at
org.apache.crimson.tree.XmlDocument.createProcessingInstruction(XmlDocument.
java:984)

I guess some kind of encoding has to be done to the signature byte array
before I construct the
String and pass it to this method. But I don't know how to do it. Can
somebody point it out?

thanx in advance
-Lei