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Re: Thoughts on the LDS WG chartering discussion

From: David Booth <david@dbooth.org>
Date: Sat, 12 Jun 2021 22:10:35 -0400
To: semantic-web@w3.org
Message-ID: <ebdfeaa5-61d2-0ceb-1a1a-980e3bd6f315@dbooth.org>
On 6/12/21 11:34 AM, Eric Prud'hommeaux wrote:
> . . .
> One could implement URDNA without ever serializing N-Quads with a
> hashCode() function on Quad, BNode, IRI and Literal: . . . >
> [[
> Hash Quad.urdna (h: Hasher) {
>    return h.add(
>      g.urdna(h), h(" "),
>      s.urdna(h), h(" "),
>      p.urdna(h), h(" "),
>      o.urdna(h), " .\n"
>    );
> }
> 
> Hash IRI.urdna (h: Hasher) {
>    return h.add('<', value, '>');
> }
> 
> Hash BNode.urdna (h: Hasher) {
>    static int ordinal = 1;
>    return h.add("_:c14n_" + ordinal++);
> }
> ]]
> 
> Literal elided, and you'd want conditional code around Quad.g, but you
> get the idea. Note that this approach means you never need to
> duplicate your graph in memory as you do when constructing a
> potentially giant N-Quads string.

If I'm understanding your intent, you are essentially showing that the 
hash can be computed on the fly as a serialization is produced 
piecemeal, without ever materializing the whole serialization at once.

That could be done, but that is exactly what I would advise AGAINST 
doing, because in essence you are defining a home grown hashing 
algorithm, even if your algorithm makes use of an off-the-shelf hashing 
algorithm as a component.  From a security perspective, I think it is 
unwise to do that.

> 
>>From a geek perspective, this abstract hashing function looks a little
> arbitrary 'cause you could get just as strong a hash if you eliminated
> all of the string constants (" ", "<", ">", ...). But keeping them is
> cheap and allows you to serialize the result in a widely-supported RDF
> syntax. RDF C14N, XML C14N and JWS's C14N all canonicalize to
> serializable data, which makese it easier to implement and makes
> user's lives way easier 'cause they can printf the input to the
> hashing function.
> 
> I believe that describing hashing as a function over N-Quads suggests
> that folks need to serialize as N-Quads to use it. I think the wording
> describing it as function over an (abstract) RDF Graph (Dataset,
> whatever) to say exactly what I as an RDF user want to know.

I appreciate that desire, but I think defining the hash over the 
abstract RDF would raise security concerns that would be better avoided, 
because it inherently entangles the hashing algorithm with RDF-specific 
code.  I think a safer approach is to use an off-the-shelf cryptographic 
hashing algorithm on the fully materialized bytes of canonical N-Quads, 
which is essentially what the charter currently describes.

Thanks,
David Booth

> 
> 
>>> On 2021-06-11 17:14, David Booth wrote
>>>> 2. It is misleading, because *any* RDF canonicalization algorithm is
>>>> fundamentally about serialization -- *not* the abstract RDF Dataset. The
>>>> proposed algorithm is really only abstract in the sense that it can be
>>>> used with a family of serializations.
> 
> I think the above shows that it's not fundamentally about a
> serialization, more "compatible with" serialization. It doesn't
> undermine your diff/patch utility below, but it does convey that it's
> a function over a Dataset, not a string of unicode characters.
> 
> 
>>> 2) It is true that the RDF standard states that "Blank node identifiers
>>> are not part of the RDF abstract syntax, but are entirely dependent on
>>> the concrete syntax or implementation." The standard also states that
>>> "the set of possible blank nodes is arbitrary", and to have a set of
>>> elements, we must be able to establish equality and inequality over
>>> those elements; otherwise the set is not defined. So how can we deal
>>> computationally with an arbitrary set of elements? We label blank nodes
>>> in a one-to-one manner as a proxy and work with the labels. (We could,
>>> equivalently, consider the set of labels to *be* the "arbitrary set of
>>> blank nodes" mentioned by the standard.)
>>
>> That's an interesting point!  You're right.  It hadn't occurred to me that
>> the labels themselves could be considered the "arbitrary" set of blank
>> nodes.
>>
>> But again, I see that as a purely theoretical point, because from a
>> practical perspective, in order to do anything useful with a canonicalized
>> RDF dataset -- such as storing it, transmitting it, comparing it to another
>> canonicalized RDF dataset, or computing its hash -- then you either need to
>> serialize it, or you need to create RDF-specific tools for those operations,
>> which would pretty much defeat the purpose of doing the canonicalization in
>> the first place.  After all, canonicalization is not an end goal, it is
>> merely a means to enable other operations to be performed more easily or
>> without requiring RDF-specific tools.
>>
>> For example, to my mind a key purpose in standardizing a canonical form of
>> N-Quads (aside from digital signatures) is *specifically* to enable
>> *standard* diff and patch tools to work on RDF.
>>
>> Thanks,
>> David Booth
>>
>>> If the labels of two blank nodes are equal, the blank nodes are equal.
>>> If the labels are not equal, the blank nodes are not equal. We can use
>>> string libraries for this. Almost every implementation that works with
>>> (abstract) RDF datasets does this; it's not something particular to
>>> canonicalisation. So this is part of an implementation of the algorithm
>>> and is completely compatible with what the standard suggests of abstract
>>> RDF datasets, and those implementations that work with them.†
>>>
>>>
>>>
>>>
>>> That said, I agree with you that the explainer could be made more
>>> precise in this manner (without affecting readability) in that it does
>>> refer to blank node labels as part of the dataset rather than the
>>> implementation. Here are the problematic sentences I can see:
>>>
>>> * "Two isomorphic RDF Datasets are identical except for differences in
>>> how their individual blank nodes are labeled." This could be removed to
>>> just leave the sentence that follows it: "In particular, R is isomorphic
>>> with S if and only if it is possible to map the blank nodes of R to the
>>> blank nodes of S in a one-to-one manner, generating an RDF dataset R'
>>> such that R' = S." (This itself could be more formal, but I think that
>>> would be out of scope for the explainer; also it is defined elsewhere.)
>>>
>>> * "Such a canonicalization function can be implemented, in practice, as
>>> a procedure that deterministically re-labels all blank nodes of an RDF
>>> Dataset in a one-to-one manner" We could change "re-labels" to simply
>>> "labels" to avoid the impression that the (abstract) RDF Dataset already
>>> has labels.
>>>
>>> * "without depending on the particular set of blank node labels used in
>>> the input RDF Dataset" We could change to "without depending on the
>>> particular set of blank node identifiers used in the serialization of
>>> the input RDF Dataset"
>>>
>>> * "It could also be referred to as a “canonical relabelling scheme”"
>>> This could rather be "It could also be referred to as a “canonical
>>> labeling scheme”", to avoid implying that labels already exist.
>>>
>>> I have created a PR for this:
>>>       https://github.com/w3c/lds-wg-charter/pull/91
>>>
>>> If there's some other sentence that you see as imprecise in that way,
>>> let me/us know!
>>>
>>> Best,
>>> Aidan
>>>
>>> † Well, there is a perhaps one weird issue that if "the set of possible
>>> blank nodes is arbitrary", then it could also be a uncountably infinite
>>> set that cannot be "labelled" one-to-one by finite strings. The set of
>>> blank nodes could be the set of reals, for example. So when we talk
>>> about labelling blank nodes, we assume that they are countable (finite
>>> or countably infinite). I am not aware that anything in the RDF standard
>>> restricts the set of blank nodes to be finite, countably infinite, or
>>> something else. Just to clarify that if this does turn out to be a
>>> problem, it will likely be a theoretical flaw for most standards built
>>> on top of RDF, and probably for the semantics of RDF graphs. I don't
>>> know if this actually matters in any meaningful sense, but I think it
>>> would be slightly comforting to know that somewhere, something normative
>>> says or implies that the set of blank nodes is countable. :)
>>>
>>
> 
Received on Sunday, 13 June 2021 02:11:12 UTC

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