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Re: Need for a canonical byte stream for an RDF graph

From: Jeremy Carroll <jeremy@topquadrant.com>
Date: Thu, 23 Jun 2011 16:49:28 -0700
Message-ID: <4E03D108.50707@topquadrant.com>
To: public-rdf-wg@w3.org
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On 6/23/2011 4:39 PM, Gavin Carothers wrote:
> One of the commonly mentioned uses cases for named graphs and graph
> literals is their use with digital signatures. At the moment signing
> an RDF graph is impossible. Today applications and systems can sign
> serializations of an RDF graph. However, there are issues with all of
> those serializations, and the whole notion of signing the serialized
> form. For example as we know a given RDF graph can be serialized into
> a huge number of equivalent RDF/XML files. We also know that RDF/XML
> can't represent all RDF graphs. If we leave the creation of a
> canonical form up to each serialization we are likely to end up with a
> large number of canonicalization methods. One for RDF/XML, one for
> Turtle, one for JSON-LD, one for RDF-JSON, etc. If we look at RDFa,
> the situation gets really strange. What the heck do you sign? The
> whole HTML, what if you just want to sign the data?
> It seems that creating a standard canonical byte stream based on an
> abstract syntax graph would be very useful. It also seems reasonably
> easy. The simplest naive approach of sorting N-Triples works
> reasonably well. Until that is you encounter our good old friend Blank
> Nodes. Happily the majority of the work needed to address blank nodes
> can be found in Jeremy Carroll's paper "Signing RDF Graphs"
> http://www.hpl.hp.com/techreports/2003/HPL-2003-142.html from back in
> 2003.
> What follows is a simple summary of the method in "Signing RDF Graphs
> 2003". Please read the paper for all the gory details. Basically, we
> sort the triples in lexicographic order, ignoring blank nodes. There
> are some remaining issues with different triples that are identical
> when ignoring blank nodes. It is usually possible to number the blank
> nodes in document order (in the output document), to break these ties.
> There are cases for which it is not possible to make such a
> tie-breaking numbering. These cases can be resolved before saving the
> document by modifying the graph by explicitly and nondeterministically
> adding formally meaningless triples. Output it all as N-Triples. This
> process is always O(n log (n)), there are also a number of possible
> optimizations mentioned in the paper.
> The nondeterministic nature of the process isn't great. Happily lots
> and lots of real world RDF graphs will never get to the
> nondeterministic step. In fact it would be perfectly reasonable for a
> system or application present giant red flashing lights when
> publishing such data.
> There is also a bit of an issue in that at the moment the paper
> defines the canonical graph as after deleting all non-distinctive
> triples the first occurrence of each blank node identifier appear in
> numeric order starting at 1 without gaps. It is possible to make sure
> that the blank node labels stay consistent as well.
> There are other non-signing uses. By using a line oriented canonical
> form existing version control and differencing tools will work right
> out of the box. A normal unified diff is suddenly useful not just for
> code, but for data.
> Welcome further discussion and feedback, specifically from people
> looking at implementing canonicalization in a specific serialization,
> and if this can meet your use cases.
> Cheers,
> Gavin
> PS. House keeping, the addition of a semantics free predicate may mean
> reopening ISSUE-56.
Received on Thursday, 23 June 2011 23:49:49 UTC

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