Re: Signature in Wrong Position

On 17 January 2016 at 13:55, Henry Story <henry.story@bblfish.net> wrote:

> I was looking at the Linked Data Signatures document
>
>   https://web-payments.org/specs/source/ld-signatures/
>
> I am not sure if this is the right list to discuss this.
>
> I am really keen to have something like this to work, and I like
> most of it off the bat. But looking a bit closer I noticed what
> I think is a serious error that has an easy fix though.
>
> The mistake is to put the signature *inside* the graph.
> Take example 1:
>
> {
>   "@context": "https://w3id.org/identity/v1",
>   "title": "Hello World!",
>   "signature": {
>     "type": "LinkedDataSignature2015",
>     "creator": "http://example.com/i/pat/keys/5",
>     "created": "2011-09-23T20:21:34Z",
>     "domain": "example.org",
>     "nonce": "2bbgh3dgjg2302d-d2b3gi423d42",
>     "signatureValue":
> "OGQzNGVkMzVm4NTIyZTkZDYMmMzQzNmExMgoYzI43Q3ODIyOWM32NjI="
>   }
> }
>
> In short: Signatures should be external to the graph, since that is the
> object
> of the signature. By placing it inside the graph as above the problem is
>
> 1. the algorithm needs to remove elements from the signed graph
> 2. The algorithm won't be able to cope with alignement of ontologies such
> as
>   a different relation being discovered to be owl:sameAs the :signature
>   relation above. If another such widely used vocabulary is found, or
> perhaps even
>   if the current vocabulary is extended then the signature verification
> systems won't
>   necessarily be able to understand to remove those relations too.
>
> It is much better to put the signature outside of the graph. Then
>
> 1. The algorithm won't need to contain any condition about removing any
> relation
> which will be a lot better for large graphs, and speed up verification.
> 2. New relations could be found or the vocabulary could be extended
> without changing
> the signature verification algorithm.
>
> Another way to think of this is that a named graph can always be mapped to
> a literal.
> ( This is why one can think of named graphs as already implicitly
> contained in RDF/XML )
> Signing documents is always external to the document in those
> circumstances.
>

I think it depends on the use case.

For LinkedDataSignature2015 the use case is so sign a canonical form of a
graph pointed to by a subject.

You could also imagine another algorithm that signed named graphs, I dont
think the two are mutually exclusive.

Each system has pros and cons, but if you canonicalize things well, the
problem goes away, I think.  I think the way things are done currently
reduces complexity, which in turn, increases adoption.

For example, looking at your webid, you have a key in there, but it is
unnamed ie a blank node.  This makes it impossible to do signing.  That is
one of the reasons that work on WebID and work on web signatures diverged.
The convenience of using (which actually you argued for, and manu and
others argued against) bnodes comes at a cost.  The cost in this case was
to have two different communities working on liked data PKI solutions,
neither with really a lot of resources.  So bear in mind there's always a
trade off between convenience, utility and generalization.

Not sure it would be correct to argue this is an error, simply a design
decision.

Work on signing other structures I think is also welcome, but maybe there's
not (yet) enough use cases?


>
> Henry Story
>
>
>
>