Re: Signature in Wrong Position

> On 17 Jan 2016, at 22:27, Melvin Carvalho <melvincarvalho@gmail.com> wrote:
> 
> 
> 
> On 17 January 2016 at 22:44, Henry Story <henry.story@bblfish.net <mailto:henry.story@bblfish.net>> wrote:
> 
>> On 17 Jan 2016, at 20:38, Melvin Carvalho <melvincarvalho@gmail.com <mailto:melvincarvalho@gmail.com>> wrote:
>> 
>> 
>> 
>> On 17 January 2016 at 13:55, Henry Story <henry.story@bblfish.net <mailto:henry.story@bblfish.net>> wrote:
>> I was looking at the Linked Data Signatures document
>> 
>>   https://web-payments.org/specs/source/ld-signatures/ <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 <https://w3id.org/identity/v1>",
>>   "title": "Hello World!",
>>   "signature": {
>>     "type": "LinkedDataSignature2015",
>>     "creator": "http://example.com/i/pat/keys/5 <http://example.com/i/pat/keys/5>",
>>     "created": "2011-09-23T20:21:34Z",
>>     "domain": "example.org <http://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.
> 
> There should be no difference in the use cases.
> 
>> 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. 
> 
> You mean you can't refer to the key from outside the graph with a URI because it is a blank node? True, but that's not a problem for 
> signatures. You can for example add the public key in the signature.
> 
> {
>   "@context": "https://w3id.org/identity/v1 <https://w3id.org/identity/v1>",
>   "title": "Hello World!",
>   "signature": {
>     "type": "LinkedDataSignature2015",
>     "creator": { "cert:modulus": "DAB9D1E941F6F8...",
>                       "cert:exponent": 65537 }                
>     "created": "2011-09-23T20:21:34Z",
>     "domain": "example.org <http://example.org/>",
>     "nonce": "2bbgh3dgjg2302d-d2b3gi423d42",
>     "signatureValue": "OGQzNGVkMzVm4NTIyZTkZDYMmMzQzNmExMgoYzI43Q3ODIyOWM32NjI="
>   }
> }
> 
> Well that wouldnt work, as you dont know what kind of key it is.  But I guess going off topic.

you actually do as it is implied by cert:exponent that the blank node is a cert:RSAPublicKey. 
See the online documentation https://www.w3.org/ns/auth/cert#exponent <https://www.w3.org/ns/auth/cert#exponent>

But even if that had not been specified, I am sure you can see that would just be a minimal step to 
add the type.

>  
> 
> Anyway, that's neither here nor there. The question was one of where signatures should go: inside the graph or outside. One can
> put the signature outside the graph with keys too, just as one can if one uses URIs to identify the key. 
> 
> This is not something that affects any use cases. It's a question of simplicity of algorithms.
> It was pointed out in private conversation that the reason that this decision may have been made is because the other
> way around is less easy for beginner coders to get right for some reason. 
> 
> Makes sense.  I do think the current formulation works, and is not in error.  There's just different ways of doing things.
>  
> 
> My Json-LD is not so good. In N3 ( https://www.w3.org/2000/10/swap/Primer <https://www.w3.org/2000/10/swap/Primer> ) 
> What would it look like with the signature outside of the graph? 
> 
> 
>> 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.
> 
> WebID does not disallow URIs for keys, as should be clear from reading the spec, and just from basic RDF principles. That point
> was made a few times on the webid mailing list.
> 
> Not really, using bnodes here is a serious anti pattern.  This is axiom 0 of the web.  Anything of significance should be given a URI.  This mistake was one reason signatures have been developed independently.

That's what some folks say. Many who have been working in the LinkedData space for a very long time don't agree.
BNodes are part of RDF, available in JSON-LD, Turtle, Trig, RDF/XML, etc... all of which are used in the LinkedData space,
and there are many use cases where they are very useful.
But yes, this is completely off topic.

>  
> 
>> 
>> 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
>> 
>> 
>> 
>> 
> 
>