I find myself disagreeing with Sunlight Foundation. Worrying about authenticity is not a "red herring". It is profoundly important and critical if we are to move to paperless legislative processes and "born digital" law corpora.
It really is not all that complicated once you take a pragmatic approach to it. Sadly, there is much confusion and mis-information around digital signatures and crypto and the very meaning of the term "authenticity"...
There is no technological silver bullet here. What Congress needs is a Legislative Enterprise Architecture that is designed from the get-go to enable authentication not just of XML but of all promulgated artifacts. Doing that has a lot more to do with provenance than anything else and provenance is something you embed at the heart of your Legislative Enterprise Architecture.