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Re: Signing and Verifying RDF Datasets for Dummies (like Me!)

From: Eric Prud'hommeaux <eric@w3.org>
Date: Wed, 16 Jun 2021 07:44:16 +0200
To: Peter Patel-Schneider <pfpschneider@gmail.com>
Cc: semantic-web@w3.org
Message-ID: <20210616054416.GP8464@w3.org>
On Mon, Jun 14, 2021 at 07:43:32PM -0400, Peter Patel-Schneider wrote:
> On Mon, 2021-06-14 at 23:57 +0200, Eric Prud'hommeaux wrote:
> > On Sun, Jun 13, 2021 at 04:26:57PM -0400, Peter F. Patel-Schneider
> > wrote:
> > > 
> > > On 6/13/21 10:45 AM, Eric Prud'hommeaux wrote:
> > > > On Sun, Jun 13, 2021 at 08:55:34AM -0400, Peter F.
> [...]
> > > > 
> > > > If you're OK with appending a signature to a document, how about
> > > > appending a signature to an N-Quads representation of a graph?
> > > Oh, yes, this is fine. N-Quads has a unique expansion to triples.
> > > (Module
> > > case normalization of language tags.)
> > 
> > Crap, I missed a step here. I wanted to explicitly ask about adding a
> > signature to the RDF graph ( la the examples in
> > <   
> > https://janeirodigital.github.io/rdf-sig-playground/index?manifestURL=examples/toy.yaml
> > >
> > ). I think your answer about JSON-LD below says you're OK with
> > signatures in the graph.
> 
> I am very uncomfortable with anything that is different in any way from
> current cryptographic practice.  My understanding is that this practice
> dictates a strict separation between the thing that is being protected
> and the protecting information.   So in a signed document the thing
> that is being signed is either separate or is encapsulated.  I was thus
> being a bit simplistic in saying that I was OK with appending a
> signature - I am only OK with this if there is a separation between the
> signature and the thing being signed.  This is decidedly not what
> appears to be happening in linked data signatures or in the interface
> that you mention.
> 
> One of these problems easily shows up when you take the output of the
> signining interface and feed it back as input to itself be signed.  The
> verifier refuses to verify the signed graph.   That's a major problem -
> why should it not be possible to sign and successfully verify a graph
> that has a signature in it?   This kind of problem shows up in the
> algorithms in https://w3c-ccg.github.io/ld-proofs/

The single-proof constraint that you consider a critical flaw, I
consider to be a design limitation that's easy to work with. You can
probably go further with rooted graphs or fresh graph labels but
that's reseearch and the current approach is already quite useful.

Likewise, the context-changing attacks that convince you that the work
can't be done, I consider pretty easy to addresss in a spec and test
suite. It's not a coincidence that the Digital Bazaar stack avoids
this problem by using static documents. Here, the specification needs
to capture more of the current practice.


> peter
> 
> 
Received on Wednesday, 16 June 2021 05:44:43 UTC

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