Surely, you can include the md5 signature of the document as a parameter
tacked onto the URN, using ";" as a separator as Dan suggested. This way
is safe if the document including the URN is trusted. The actual ownership
of the document can be checked provided it includes a public key signature
and related issuer certificates.
Best wishes,
Dave Raggett,
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Bristol, England dsr@hplb.hpl.hp.com