Could not verify xml signature

I have an simple process activity which verify xml signature.

i m getting following error

Error in validating xml signature ALC-DSS-304-004

caused by ALC-DSS-310-051 could not verify xml signature(in the operation verify)

Caused by null(XMLsignatureimpl.java151)

Please reply back if you have solution ovet it.

Regards

Abhishek