W3C home > Mailing lists > Public > public-webcrypto@w3.org > March 2014

Re: WebIDL usage for Algorithms

From: Mark Watson <watsonm@netflix.com>
Date: Tue, 11 Mar 2014 08:07:59 -0700
Message-ID: <CAEnTvdDg89=VqUT52zT++NWxudF96Ny+WX832U9BA8X5zWoSZg@mail.gmail.com>
To: Ryan Sleevi <sleevi@google.com>
Cc: "public-webcrypto@w3.org" <public-webcrypto@w3.org>, Richard Barnes <rlb@ipv.sx>
On Mon, Mar 10, 2014 at 7:28 PM, Ryan Sleevi <sleevi@google.com> wrote:

> I haven't reviewed the proposed language, but as expressed to you in
> email, I agree it needs to change from Algorithm into a more opaque type to
> support such an action.
>
> I don't think we can use prose to wipe away a normative reference to
> WebIDL.
>
We can do whatever we like, it's a question of whether we *should*. We can
say "in this specification the symbol '5' is used to represent the the
result of the sum '2+2' if we want to, but we probably shouldn't.

We have said we want all errors to be asynchronous*. This means that WebIDL
type mapping cannot happen for* any of the method parameters* until we say
so. It's not even an option to replace all method parameters with "any",
because even type mapping to IDL "any" can throw an exception. Replacing
just some of the types with "any" doesn't help.

So, we have to say that implementations of WebCrypto must be compliant to
WebIDL *except* that the procedures specified in Section 4.4.7 of (WebIDL
http://www.w3.org/TR/WebIDL/#es-operations) are delayed until the
asynchronous part. There is actually a possibility of a TypeError here
before we get to argument type mapping - in the identification of the
"this" value - so we might need to be more explicit than presently about
which procedures are deferred to the asynchronous part.

It does seem that, longer term, it would make sense for WebIDL to
incorporate Promises explicitly, so that the delay of these WebIDL steps to
the asynchronous part is placed on a common footing. Then one could expect
generated bindings to be developed for methods with Promise return types
that do this work in the asynchronous part.

Furthermore, it does seem that Dictionary inheritance is not as useful as
it could be without proper support for polymorphism. WebIDL could consider
having Dictionary IDL types carry with them a reference to the original
object so that downcasts are possible.

Note that all of the above points to things WebIDL could do that would -
eventually - lead to better development tool support for what we want to do
in WebCrypto. But none of this discussion changes what we want to achieve
in terms of how our methods are called and how they behave. Implementers
are going to have to support the desired behavior "by hand" until WebIDL
has better support for it.

...Mark

* I'm assuming we do not want to revisit this and say that *some* errors
are synchronous: if even one is synchronous then it might as well be many.




> On Mar 10, 2014 7:13 PM, "Mark Watson" <watsonm@netflix.com> wrote:
>
>>
>>
>> Sent from my iPhone
>>
>> On Mar 10, 2014, at 6:39 PM, Richard Barnes <rlb@ipv.sx> wrote:
>>
>> So it seems like the editors' inclination (and what's in WebKit/Chrome)
>> is option (1), requiring custom bindings.  I can agree with that, and
>> understand the utility of still having the dictionaries around in that case.
>>
>> It still seems like we need to change the use of Algorithm in the
>> SubtleCrypto methods to prevent binding at that method invocation.
>>
>>
>> We have text that says the type mapping doesn't happen on method
>> invocation but when explicitly stated in the asynchronous part. Do you
>> think we need more ?
>>
>> ...Mark
>>
>>
>> --Richard
>>
>>
>>
>> On Mon, Mar 10, 2014 at 11:50 PM, Mark Watson <watsonm@netflix.com>wrote:
>>
>>> Richard,
>>>
>>> Yes, Ryan and I discussed this and I've proposed a solution in the ED.
>>>
>>> First, since we decided all errors must be asynchronous, we can't have
>>> WebIDL type mapping happening at the usual time. We explicitly note this
>>> and then explicitly specify when to do type mapping. The type mapping is
>>> done in the "normalize to X" algorithm which takes an object and does
>>> WebIDL type mapping to dictionary type X (as well as resolving any aliases,
>>> though we still don't have any of these defined).
>>>
>>> As you say, this mapping discards members not mentioned in X, so
>>> "normalize to Algorithm" leaves you only with name. The individual
>>> algorithm operations call "normalize to Y" where Y is the appropriate
>>> subclass.
>>>
>>> ...Mark
>>>
>>>
>>> On Mon, Mar 10, 2014 at 4:18 PM, Ryan Sleevi <sleevi@google.com> wrote:
>>>
>>>>
>>>> On Mar 10, 2014 4:12 PM, "Richard Barnes" <rlb@ipv.sx> wrote:
>>>> >
>>>> > TL;DR: Usage of Dictionary inheritance for Algorithms is broken.
>>>>  Should use WebIDL object type or callback interfaces instead.
>>>> >
>>>> > BACKGROUND:
>>>> > In the current spec, algorithm identifiers and parameters are passed
>>>> to encrypt(), decrypt(), etc. using dictionaries that inherit from
>>>> Algorithm, which is itself a dictionary type.  For example:
>>>> >   dictionary Algorithm { DOMString name; }
>>>> >   dictionary AesCbcParams : Algorithm { CryptoOperationData iv; }
>>>> >   typedef (Algorithm or DOMString) AlgorithmIdentifier;
>>>> >   Promise<any> encrypt(AlgorithmIdentifier algorithm, ... )
>>>> >
>>>> > PROBLEM:
>>>> > According to the WebIDL spec for handling of dictionary types, the
>>>> implementation is required to create the dictionary object by copying out
>>>> of the source object the fields specified in the type definition.
>>>> > http://www.w3.org/TR/WebIDL/#es-dictionary
>>>> >
>>>>
>>>> Yes, I raised this privately with Mark when discussing things I didn't
>>>> think were LC ready.
>>>>
>>>> > For WebCrypto, this means that polymorphism via inheritance of
>>>> dictionaries doesn't work.  If someone creates a dictionary that matches
>>>> AesCbcParams and passes it to encrypt(), then the WebIDL implementation
>>>> will only store the "name" field, and throw everything else away.  Anything
>>>> using the WebIDL object will not be able to see the "iv" parameter.
>>>> >
>>>>
>>>> Did the wording Mark proposed to handle this not address it?
>>>>
>>>> Note the asynchronous behavior of WebIDL conversions that are written.
>>>>
>>>> > As far as implementation: I discovered this bug while trying to
>>>> implement using the Firefox WebIDL implementation, so clearly Firefox has
>>>> this problem :)  It appears that Chromium is internally treating
>>>> "algorithm" arguments as generic objects (not following the WebIDL spec for
>>>> dictionaries), then using a "parseAlgorithm" method to pull out relevant
>>>> information.
>>>> >
>>>>
>>>> Both Blink and WebKit have implemented this with custom bindings.
>>>>
>>>> > PROPOSED RESOLUTION:
>>>> > It seems to me that we have a couple of options here.
>>>> >
>>>> > 1. "typedef object Algorithm" - Don't have the WebIDL handle any of
>>>> the internal structure of Algorithm objects, but instead lay this out in
>>>> the text of the spec.  In this case, there's no inheritance to speak of,
>>>> just lists of what parameters must be present in each use of Algorithm.  We
>>>> would need to specify at what point in the process values are extracted
>>>> from the object.
>>>> >
>>>>
>>>> Or just call it "any"
>>>>
>>>> > 2. "callback interface Algorithm" - This would use inheritance and
>>>> have the expected behavior, since callback interfaces keep around a copy of
>>>> the provided JS object.  However, there might be some weird behaviors if
>>>> people pass in exotic objects, e.g., side-effects or changing values.
>>>>  Also, the spec would need to carefully define exactly when the properties
>>>> are accessed, and all per-spec access to the properties would need to be
>>>> done on the main thread, since it needs to be able to run script.
>>>> >
>>>>
>>>> Strongly opposed to this. Its going to be quite ugly and requires
>>>> painting over all of the ES5/ES6 interactions.
>>>>
>>>> > I have a slight preference for (2).  It continues to delegate most of
>>>> the work to WebIDL, and it seems unlikely that people will do exotic enough
>>>> things to cause trouble, given that the way WebCrypto uses these objects is
>>>> fairly straightforward.
>>>> >
>>>> > --Richard
>>>> >
>>>> >
>>>>
>>>> Considering that the WebIDL spec changed in order to support other use
>>>> cases of ours - notably, the Promise<resolve type> notation - have you
>>>> considered approaching the WebIDL WG with this?
>>>>
>>>> >
>>>> >
>>>> >
>>>> >
>>>>
>>>
>>>
>>
Received on Tuesday, 11 March 2014 15:08:28 UTC

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