Re: [GNAP] Feedback on polymorphism

[Justin Richer <[e-mail]>]

That’s not exactly what I’m saying — I’m more making the argument that things that complexity in the AS isn’t an automatic (or even high priority) negative, if it offers a measurable benefit like simplicity to the rest of the system. Obviously things that make an AS very difficult to implement are going to be a problem.

Also, I’m not sure I was clear about characterizing my implementation in Java. I explicitly set out to build it the hard way: I wanted something that would run through a completely automated parser, without any pre-processing or special code paths, into static types and concrete object classes. There are no generic “Object”s or “List of Maps of Maps” or things like that in my implementation, and my code doesn’t even call the JSON parser directly, since it’s all wired directly through Spring. The difficult part wasn’t writing the code that says “for this specific field, if it’s a string do A, if it’s an object do B”, the actual difficult part was wiring it up such that it’s transparent to the rest of my code base. If I had been doing my own parsing by hand and then handing it to “real” classes, or had resorted to generic types upon parsing and pulling out raw values when I need them, it would have been a lot simpler. But both of these are fragile, as you point out, and the way I did it I was actually guaranteeing strict parsing and strict types, even with all the polymorphism in place. This is the approach I would expect a fully compliant AS to do, and something that conformance testing, schemas, and careful protocol design can ferret out from bad implementations.

 — Justin


> On Oct 25, 2020, at 2:42 PM, Yaron Sheffer <[e-mail]> wrote:
> 
> Hi Justin,
>  
> I agree that there are going to be more clients than AS’s, and that clients should be simpler to write.
>  
> But I’m not following your next argument. It sounds like “The AS is complex anyway, and it’s security-critical anyway, so why not have it absorb some additional complexity?”. With polymorphism in the protocol, AS developers are likely to move to weak typing (generic types, e.g. interface{} in Go). The entire implementations will pay the price (in more brittle code) for the benefit of the few polymorphic fields that indeed may be easier to validate. I don’t think this is the right call.
>  
> Thanks,
>                 Yaron
>  
> From: Justin Richer <[e-mail]>
> Date: Sunday, October 25, 2020 at 18:33
> To: Fabien Imbault <[e-mail]>
> Cc: Yaron Sheffer <[e-mail]>, Dick Hardt <[e-mail]>, Mika Boström <mika.bostrom=[e-mail]>, GNAP Mailing List <[e-mail]>
> Subject: Re: [GNAP] Feedback on polymorphism
>  
> I agree fully with Fabien’s statements on who we’re making things easy for. If this security protocol isn’t easy and straightforward for client developers — with or without a library — then it simply will not be adopted. It might be hard to see when looking at the whole protocol, but this goal is actually one of the drivers for the way polymorphism is used in the current draft. The driving philosophy of OpenID Connect’s development was “keep the simple things simple, make the complex things possible”, and that’s a philosophy I strongly believe we need to continue here in GNAP.
>  
> Importantly, a client developer shouldn’t need to have to support all possible permutations in order to get something right. OAuth has shown us that even when a protocol has a lot of options, client software is going to be coded to use only one set of options. If the protocol can support that reality, I think we are doing well. In the current draft, generally speaking it’s the AS that needs to handle different options, not the client. Take for example requesting resources: a very simple client might just send an array of strings, like OAuth scopes. The AS needs to be able to handle the string inputs as well as the rich object descriptors, but the client software doesn’t even need to know that the object format exists in order to create compliant JSON.  The client developer can do this without any special libraries, it can just bang out the JSON directly. Such a client won’t be able to do the more fancy stuff that the request objects allow, but for this client, that’s fine since all it needs to do is produce an array with strings that the AS will accept. The same is true across the various other options, like requesting multiple access tokens vs a single one, or providing an instance identifier vs. a set of information for the client instance itself. Client developers should be fully able to ignore parts of the protocol that they aren’t using, especially the more advanced stuff. An AS shouldn’t get to have such a choice, but as the AS is the lynchpin of the security model, I am OK with things being slightly more complex to support, especially if it shifts that complexity away from clients.
>  
> I am in favor of including JSON Schema in the draft in a non-normative way, and you can see an earlier version of that experiment that Fabien and I undertook as part of the Design Team. The schema will be available for developers who want to / can use it, but even for people who aren’t using the schema directly it can provide additional context and information. I am willing to put some cycles into creating schemas for the main request and response messages and put those into a pull request to create an appendix in the draft. I’d also be fine with a non-normative JSON-LD reference for LD processors — but (1) I’m not going to write that :) and (2) it can’t be required for the protocol to be understood.
>  
>  — Justin
> 
> 
>> On Oct 25, 2020, at 8:47 AM, Fabien Imbault <[e-mail] <mailto:[e-mail]>> wrote:
>>  
>> Hi Yaron,
>>  
>> As Justin explained that's the kind of idea we tested. For instance https://github.com/fimbault/test_gnap_schema <https://github.com/fimbault/test_gnap_schema> (in rust).
>>  
>> Cheers
>> Fabien 
>>  
>> Le dim. 25 oct. 2020 à 13:36, Yaron Sheffer <[e-mail] <mailto:[e-mail]>> a écrit :
>>> Personally, I would support polymorphism in the protocol if it (the RFC) came with a well-defined JSON Schema [1] document, so a recipient can automatically validate incoming messages at run-time. I would expect senders to also validate messages as part of their testing. It’s not an ideal solution, because:
>>>  
>>> Some people at the IETF don’t like JSON Schema, for various reasons.
>>> JSON Schema is not a standard, so it’s painful to require it normatively.
>>> Even if it’s normative, some recipients will not validate messages anyway.
>>>  
>>> This would be shifting some of the complexity from the library developer to the spec developer, which is the right thing to do IMO.
>>>  
>>> Thanks,
>>>                 Yaron
>>>  
>>> [1] https://json-schema.org/ <https://json-schema.org/>
>>>  
>>>  
>>> From: Fabien Imbault <[e-mail] <mailto:[e-mail]>>
>>> Date: Sunday, October 25, 2020 at 12:39
>>> To: Yaron Sheffer <[e-mail] <mailto:[e-mail]>>
>>> Cc: Dick Hardt <[e-mail] <mailto:[e-mail]>>, Mika Boström <mika.bostrom=[e-mail] <mailto:[e-mail]>>, GNAP Mailing List <[e-mail] <mailto:[e-mail]>>, Justin Richer <[e-mail] <mailto:[e-mail]>>
>>> Subject: Re: [GNAP] Feedback on polymorphism
>>>  
>>> Hi Yaron,
>>>  
>>> Thanks for the feedback. 
>>>  
>>> Regarding client libraries, I think we can indeed learn a great deal from cryptographic libraries. Cryptographic design provides a great amount of flexibility for the specialists (including many parameters that you really need to get right). We might think this is great to provide options but it actually increases the cognitive load of library users.
>>>  
>>> Look instead at what Google has provided with tink as an alternative and you'll see it is a much easier way for cryptographic engineers (who aren't cryptographers) to avoid mistakes or misuses. 
>>>  
>>> That's the *security* issue I'm referring to (not the fact that being open they're tasty targets, although that may be related in some cases). And tink is the kind of design we should be trying to achieve. 
>>>  
>>> I agree that it should be applicable to a wide range of well known programming tools, including the likes of java and go. 
>>> But I don't really see a limitation here. Might not be the most idiomatic feel, but it can be made to work. 
>>>  
>>> Just so I understand, what alternatives would you prefer to polymorphism? I can suggest json-ld but even Justin will Teel you it's even more complex :-) 
>>>  
>>> Cheers
>>> Fabien 
>>>  
>>> 
>>> Le dim. 25 oct. 2020 à 11:17, Yaron Sheffer <[e-mail] <mailto:[e-mail]>> a écrit :
>>>> Hi Fabien,
>>>>  
>>>> I think your product management model has a lot of merit, but it does not capture the security issue well.
>>>>  
>>>> I agree that as we look at different customer personas, the “end user” (the developer that’s using the libraries) should be our highest priority. But I disagree about end-user mistakes being the top *security* issue. Libraries are often open source in our space and used very widely. So they make for tasty targets, and people are attacking them and are still finding security issues in libraries that we’ve been talking about forever [1].
>>>>  
>>>> (Yes, my example is actually an app flaw, but I think it could have been prevented by a better designed library.)
>>>>  
>>>> In other words, we do need to care about how easy it is to implement the protocol correctly by *library* developers. From Justin describing his own experience and other people on the thread that Dick referred to, I would say that JSON polymorphism is painful for Java and Go developers. With all due respect, I care about them much more than I care about Haskell, as many more implementations are likely to use these languages.
>>>>  
>>>> Thanks,
>>>>                 Yaron
>>>>  
>>>> [1] https://www.zofrex.com/blog/2020/10/20/alg-none-jwt-nhs-contact-tracing-app/ <https://www.zofrex.com/blog/2020/10/20/alg-none-jwt-nhs-contact-tracing-app/>
>>>>  
>>>> From: TXAuth <[e-mail] <mailto:[e-mail]>> on behalf of Fabien Imbault <[e-mail] <mailto:[e-mail]>>
>>>> Date: Sunday, October 25, 2020 at 01:25
>>>> To: Dick Hardt <[e-mail] <mailto:[e-mail]>>
>>>> Cc: Mika Boström <mika.bostrom=[e-mail] <mailto:[e-mail]>>, GNAP Mailing List <[e-mail] <mailto:[e-mail]>>, Justin Richer <[e-mail] <mailto:[e-mail]>>
>>>> Subject: Re: [GNAP] Feedback on polymorphism
>>>>  
>>>> Hi Dick,
>>>>  
>>>> Independantly from the debate on polyphormism, I beg to differ on your order preference. 
>>>>  
>>>> Your assumption is that AS devs matter the most, because they're doing the important security implementation. But eating our own dogfood might help a lot to change that view. Most security issues occur because users of the spec are unable to deal with the complexity that is passed onto them. 
>>>>  
>>>> 99% of the people that will actually use the output of the work are application developers (client or RS) and their own users. 
>>>>  
>>>> Our intent as well as the protocol drive the usage. Client libraries may help, but they're not a silver bullet, especially because GNAP ultimately has no control about what people do there (for better or worse). And everything we do here will help get to the better part. 
>>>>  
>>>> I'm not saying we don't intend to also care about AS developers (beginning with ourselves) but it's a second order optimisation. And since it's a tendancy we're leaning towards by default, I'm pretty sure we won't forget that anyway. 
>>>>  
>>>> Fabien 
>>>>  
>>>>  
>>>> 
>>>> Le sam. 24 oct. 2020 à 23:50, Dick Hardt <[e-mail] <mailto:[e-mail]>> a écrit :
>>>>> I'm confused by your logic Fabien.
>>>>>  
>>>>> If a client library developer wants to expose polymorphism, they can do that independent of what is in the protocol. 
>>>>>  
>>>>> I differ on who our stakeholders are. 
>>>>>  
>>>>> I think our stakeholders are in least to most important:
>>>>>  
>>>>> AS developer
>>>>> RS developer
>>>>> client developer
>>>>> user
>>>>>  
>>>>> A client library developer can expose whatever interface they want to simplify implementation.
>>>>>  
>>>>> I list the user so that we don't lose site of a critical role.
>>>>>  
>>>>> /Dick
>>>>>  
>>>>>  
>>>>> Error! Filename not specified.ᐧ
>>>>>  
>>>>> On Fri, Oct 23, 2020 at 6:27 PM Fabien Imbault <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>> Hi there, 
>>>>>>  
>>>>>> Let me try to approach the issue under a different light. More like a product manager would deal with feature selection to make it intuitive for its users. 
>>>>>>  
>>>>>> For most people, riding a bike is far easier than using a unicycle. Feels more stable. And yet it's way easier to design for a single wheel than to build with 2. Because then you'll need a lot more accessories (chain, chain ring, etc.). Even so producing a bike doesn't have to be a brittle process, it can be industrialized. 
>>>>>>  
>>>>>> Back to the GNAP topic. 
>>>>>> Ultimately we should strive to make the spec as simple as can be. But we need to ask: simple for whom? For the bike owner or for the bike vendor? 
>>>>>> (short answer: the priority should be simplicity for spec users, not spec implementers and even less spec designers). 
>>>>>>  
>>>>>> The initial question that is asked is very interesting: isn't the design flawed if GNAP is using json polyphormism? Or if the AS needs to handle the state of the request? Or if we must handle token revocation? Or if we are looking for a global unique identifier? The argument stems of the fact that is still arguably harder and more error prone to implement. Fair enough. 
>>>>>>  
>>>>>> From a spec implementer's perspective, it may well be more complex. It mostly impacts the json library you'll end up using, plus a bit of input/output decoration around it. Even golang provides utilities for this, despite not exactly being made for this kind of purpose.
>>>>>> My practical experience implementing it is that it's not that big a deal. I mean, I wished it could be simpler, but it's manageable and there are other ways to reach levels of insurance that it does work as intended (json schema, test cases to validate the implementation, etc.). Arguably it is still easier from an implementation perspective than say, json-ld, which is massively used in the SSI community. 
>>>>>>  
>>>>>> But ultimately who are we designing for? Are we striving to go easy on the spec implementer? Or are we trying to make sure end-developers using the client libraries won't shoot themselves in the foot?
>>>>>>  
>>>>>> The job to be done (JTBD), from the end-developer's perspective, is to efficiently ship an application. And provide authN/authZ capabilities for end-users by relying on some well known implementation. 
>>>>>> In turn, this spec implementer will rely on cryptographic utility libraries that deals internally with the complexity of their own domain, so that we don't have to. And here we could launch another HN flame war that starts with the title "JWT sucks because". Which does have its set of very real issues but that's beyond the point. 
>>>>>> Note that any decent flamewar will be efficiently fueled by people hating medium. Is it outrageous for blog posts to be behind a paywall? Maybe but it's even more outrageous to lack consistency, either by not knowing how to get around a paywall if you're into a hacker punk movement, or on the contrary by to not paying a subscription if you believe that surveillance capitalism, to reuse Zuboff's terms, should be eradicated. 
>>>>>> What likely seems an unnecessary sidenote tries to illustrate the point: for Justin it was easier to publish on medium, because as a blog publisher, you might not want to deal with hosting your own blog. But maybe as a reader you'll find that annoying. Different audiences, different JTBD, different tradeoffs. 
>>>>>>  
>>>>>> Polyphormism is a tool that enables the end-developer to have minimal knowledge of what it means to deal with a GNAP client library. You prepare the request, send to the endpoint and you're good to go. Massively simpler than OAuth2 or any similar protocol by the way (as anyone with teaching experience on the subject might acknowledge). And  there's a lot more to be done to make sure we indeed reduce the complexity for the end-developer and the end-user. 
>>>>>>  
>>>>>> If we find a better way to deal with that simplicity balance, I'm all in. But the arguments need to be way more convincing than just saying that it may be difficult to implement or validate. 
>>>>>>  
>>>>>> Cheers.
>>>>>> Fabien
>>>>>>  
>>>>>>  
>>>>>>  
>>>>>>  
>>>>>>  
>>>>>> Le ven. 23 oct. 2020 à 22:35, Justin Richer <[e-mail] <mailto:[e-mail]>> a écrit :
>>>>>>>  
>>>>>>>  
>>>>>>> 
>>>>>>>> On Oct 23, 2020, at 3:52 PM, Dick Hardt <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>>>  
>>>>>>>> Justin
>>>>>>>>  
>>>>>>>> I did note that I was the one that argued for instance_id being in the object. Since it is in the object in the current draft, not including a pass by reference option would be preferable. 
>>>>>>>>  
>>>>>>>> As for concrete examples:
>>>>>>>> - version of client
>>>>>>>> - version of OS
>>>>>>>> - security attestation of OS / device
>>>>>>>> - location of client device
>>>>>>>> - network client is operating on
>>>>>>>>  
>>>>>>>> These are all attributes of the client that an AS may require on the initial grant request, and in future grant requests (which is when an instance_id) would be used.
>>>>>>>>  
>>>>>>>  
>>>>>>> This is where our interpretations differ: I don’t see these as “attributes of the client” in the same way that the key, display information, class identifiers, and other items currently represented by an instance_id are attributes of the client instance. The attestation components don’t modify the instance so much as present additional information on top of the client instance itself. This is why I argue that they ought to be handled in a separate object, so you’d have something like this strawman:
>>>>>>>  
>>>>>>> {
>>>>>>>  
>>>>>>>   posture: {
>>>>>>>     software_version: 1.2.3,
>>>>>>>     os_version: 14.3.2
>>>>>>>     device_attestation: { … some structure or signed blob? … }
>>>>>>>     location: { lat: …, lon: …, alt: … }
>>>>>>>   },
>>>>>>>  
>>>>>>>   client: “client-541-ab"
>>>>>>>  
>>>>>>> }
>>>>>>>  
>>>>>>> This is a more fundamental question about GNAP than whether the syntax uses polymorphism: this is about GNAP being very explicit about the data model of its elements. OAuth 2’s incredibly loose and broad model of what the term “client” is referring to, exactly, is deeply problematic in practice. We’re even seeing that in the OAuth 2.1 work with having to define a “credentialed client”, and even then that doesn’t fully capture the different aspects that are out there. I think we’re getting closer here in GNAP with explicit definition of “client instance”, but we still need to be more precise about what exactly a client instance includes, and what it does not.
>>>>>>>  
>>>>>>>  — Justin
>>>>>>>  
>>>>>>>  
>>>>>>> 
>>>>>>>> /Dick
>>>>>>>>  
>>>>>>>> Error! Filename not specified.ᐧ
>>>>>>>>  
>>>>>>>> On Fri, Oct 23, 2020 at 12:42 PM Justin Richer <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>>>> Dick,
>>>>>>>>>  
>>>>>>>>> As you’ll recall, I argued against including the client instance identifier inside of the object as a mutually-exclusive field precisely because of the principle violation that you are pointing out here, and so it’s important to point out that the current text is a compromise that needs to be examined in the wider experience of the working group. I am on the side of removing the mutually-exclusive “instance_id” option within an object, but this needs to be explored.
>>>>>>>>>  
>>>>>>>>> The crux of my argument is that is exactly a case of pass-by-reference vs pass-by-value, and that runtime attestations are not part of the “client instance” value itself but rather belong outside of that object in a another part of the request. As stated in the editorial notes in this section, we need to look carefully at how these concepts fit within the model and where we would want to put them. Without concrete examples of what these extensions look like and how they’re generated, that is nearly impossible to do at this stage. I look forward to seeing examples of this kind of data and how it can fit into the protocol.
>>>>>>>>>  
>>>>>>>>>  — Justin
>>>>>>>>>  
>>>>>>>>> 
>>>>>>>>>> On Oct 23, 2020, at 3:07 PM, Dick Hardt <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>>>>>  
>>>>>>>>>> Hey Justin,
>>>>>>>>>>  
>>>>>>>>>> As the draft has evolved, I question the continued use of polymorphism. Note that I appreciate the elegance of using a string for pass-by-reference, and an object for pass-by-value.
>>>>>>>>>>  
>>>>>>>>>> In the current draft, the 
>>>>>>>>>>  
>>>>>>>>>>> Every time you create or process a field it will mean only one thing, and there’s only one field to look at to answer a question. 
>>>>>>>>>>  
>>>>>>>>>> is violated in 2.3.1.  Identifying the RC Instance <https://tools.ietf.org/html/draft-ietf-gnap-core-protocol-00#section-2.3.1>
>>>>>>>>>>  
>>>>>>>>>>  
>>>>>>>>>>>    instance_id  An identifier string that the AS can use to identify the
>>>>>>>>>>>       particular instance of this RC.  The content and structure of this
>>>>>>>>>>>       identifier is opaque to the RC.
>>>>>>>>>>>  
>>>>>>>>>>>    "client": {
>>>>>>>>>>>        "instance_id": "client-541-ab"
>>>>>>>>>>>    }
>>>>>>>>>>>  
>>>>>>>>>>>    If there are no additional fields to send, the RC MAY send the
>>>>>>>>>>>    instance identifier as a direct reference value in lieu of the
>>>>>>>>>>>    object.
>>>>>>>>>>>  
>>>>>>>>>>>    "client": "client-541-ab"
>>>>>>>>>>  
>>>>>>>>>> The instance identifier can be sent two ways. Polymorphism is a convenience for the client, but requires the server to have two code paths for "instance_id".  We discussed this in the design team, and I argued for having "instance_id" in the "client" object so that any updates, such as new devices assertions, could be in the "client" object. As noted above, while I appreciate the elegance of using a string (handle) to reference a previously provided object, it complicates how to update an existing object while providing the reference.
>>>>>>>>>>  
>>>>>>>>>> In your example of the "key" object below, setting "proof" to bearer would avoid the issue you describe:
>>>>>>>>>>  
>>>>>>>>>> { 
>>>>>>>>>>  "key": { 
>>>>>>>>>>      "proof": "bearer" 
>>>>>>>>>>     } 
>>>>>>>>>> }
>>>>>>>>>>  
>>>>>>>>>> In your example, when processing the "key" object, code is having to check both the JSON type of the property, as well as check the value of the "proof" property. In the example I provided, only the value of "proof" needs to be checked. The "proof" property is acting as a type for the "key" object.
>>>>>>>>>>  
>>>>>>>>>> Not being a Java programmer, I don't know how this would work in a Java implementation, but node.js, the processing would need to be done as above.
>>>>>>>>>>  
>>>>>>>>>> On a related note, there was significant negative feedback on handles and polymorphism in the Hacker News article https://news.ycombinator.com/item?id=24855750 <https://news.ycombinator.com/item?id=24855750> 
>>>>>>>>>>  
>>>>>>>>>> /Dick
>>>>>>>>>>  
>>>>>>>>>>  
>>>>>>>>>> On Fri, Oct 23, 2020 at 10:20 AM Justin Richer <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>>>>>> Hi Mika,
>>>>>>>>>>>  
>>>>>>>>>>> Thanks for bringing this topic here — I was able to see the forum discussion that brought you here, and hopefully I can help clear up what I mean with how polymorphism is used in the proposal. The short version is that the goal is to avoid the kinds of ambiguity that make insecure protocols, and so in that goal we’re fully aligned. I think that using polymorphism in very specific ways can help that goal — just as I agree that misusing it or applying it sloppily can lead to ambiguous and insecure systems.
>>>>>>>>>>>  
>>>>>>>>>>> Some background: I built out the XYZ protocol (one of the predecessors to the initial GNAP Draft) in Java using strongly typed parsers and Java objects specifically to prove to myself that it could be done in a way that made any sense in the code. (My own open source implementation is at https://github.com/bspk/oauth.xyz-java <https://github.com/bspk/oauth.xyz-java>, but note that it’s not yet up to date with the GNAP spec). It was important to me that I be able to use the system-wide configured parsers to implement this and not have to resort to stepping through elements completely by hand. Java doesn’t make it simple to get the hooks into the right places (especially with the Jackson parser that I used), but it is definitely possible to create a deterministic and strongly-typed parser and serializer for this kind of data structure. Some of the rationale for using polymorphism is covered in the trailing appendix of the draft document (https://www.ietf.org/archive/id/draft-ietf-gnap-core-protocol-00.html#name-json-structures-and-polymor <https://www.ietf.org/archive/id/draft-ietf-gnap-core-protocol-00.html#name-json-structures-and-polymor>), but it’s still good to discuss this here as the working group decides which approaches to take. 
>>>>>>>>>>>  
>>>>>>>>>>> The driving reason for using polymorphism at the protocol level was to simplify the protocol and make it :more: deterministic to create and process, not less. Every time you create or process a field it will mean only one thing, and there’s only one field to look at to answer a question. Without polymorphic field values, you usually need to rely on mutual exclusivity of fields, which is prone to failure and requires additional error checking. Take for example the key binding of access tokens. An access token could be bound to the RC’s key used during the request, to a different key chosen by the AS, or it could be a bearer token with no key at all. By making the “key” field polymorphic, we can define it in terms of boolean values and objects and express this set of mutually-exclusive options in a non-ambiguous way. Without that, you’d need to have different fields for the options and include additional checks in your parser to make sure they weren’t sent simultaneously, otherwise you could get hit with this potential security vulnerability in an object:
>>>>>>>>>>>  
>>>>>>>>>>> { 
>>>>>>>>>>>     key: {
>>>>>>>>>>>       proof: httpsig,
>>>>>>>>>>>       jwk: { … key value … }
>>>>>>>>>>>     },
>>>>>>>>>>>     bearer_token: true,
>>>>>>>>>>>     bind_to_rc_key: true
>>>>>>>>>>> }
>>>>>>>>>>>  
>>>>>>>>>>> This would be an illegal object as per this alternate proposal, but then you’d have to check each field and make sure it wasn’t put next to others in the same object. I’ve done this exercise with many other protocols and it’s both error prone and easy to ignore since all the “good” examples would pass code that doesn’t check this. With the polymorphic approach to this same field, each of these three mutually-exclusive states is written in a way that they cannot be sent together. It’s not just illegal, it’s impossible and enforced by the syntax of JSON itself.
>>>>>>>>>>>  
>>>>>>>>>>> { 
>>>>>>>>>>>     key: {
>>>>>>>>>>>       proof: httpsig,
>>>>>>>>>>>       jwk: { … key value … }
>>>>>>>>>>>     }
>>>>>>>>>>> }
>>>>>>>>>>>  
>>>>>>>>>>> // bearer token
>>>>>>>>>>>  
>>>>>>>>>>> {
>>>>>>>>>>>     key: false
>>>>>>>>>>> }
>>>>>>>>>>>  
>>>>>>>>>>> // bound to the RC’s presented key
>>>>>>>>>>>  
>>>>>>>>>>> {
>>>>>>>>>>>     key: true
>>>>>>>>>>> }
>>>>>>>>>>>  
>>>>>>>>>>> If someone sends a different type for this field, like an array or number or a null, this doesn’t have a defined interpretation in the protocol and would be a protocol level error.
>>>>>>>>>>>  
>>>>>>>>>>> While it might sound like polymorphism means that any field could have any type or value, the opposite is true: each possible value is explicitly typed, it’s just that there are potentially different types that express meaning for the field. This applies to all members of all objects (dictionaries) as well as all members of an array (list). Every time you process a field value or other element, you look at the type and then the value to determine what to do with that typed value.
>>>>>>>>>>>  
>>>>>>>>>>> In your example below, each field within the dictionary would also need to be typed, and each type would need to have a clear indication of its meaning. To take your strawman key format below, the “modulus” field could be defined polymorphically as either a “bigint” (a JSON number) or an “encoded string” (a JSON string). The definition would further say what exactly the encoding of the string would be. That means that when you read the “modulus” field there wouldn’t be any confusion on what the value was or how it was represented, regardless of the input format. Seeing a number there means exactly one interpretation and seeing a string means exactly one (different) interpretation — but importantly, both of them are a “modulus”, since that’s the field that determines the type. An implementation would likely use an internal BigInteger type of object to represent the field value after parsing, so the question is how to go from the JSON value (which is typed) into the BigInteger value.You don’t just apply the type rules on the “public_key” field, you apply it to all sub-fields of that object. 
>>>>>>>>>>>  
>>>>>>>>>>> So let’s dig into the specific bug you bring up in the strawman, because it’s interesting: A JSON encoder that encodes numbers as strings, and not numbers, is not compliant with the JSON definitions of the field in question. For another example, the quoted string value of “true” is not equivalent to the boolean value true in JSON, and they shouldn’t be treated the same by a parser implementation when mapping to a concrete object. It’s in this kind of automated guessing that this class of bugs occur, and that’s going to be the case whether or not you take  advantage of JSON’s polymorphic nature. I’ve run into cases where a parser library was trying to be overly “helpful” in doing this kind of mapping, but ended up introducing errors in more strict components downstream. This is something that protocol designers need to be aware of and guard against in the design of the protocol to reduce possible ambiguities. Within GNAP today, we generally have things that branch whether they’re an object (for a rich description of something) or some non-structured special value (for a reference or other item). 
>>>>>>>>>>>  
>>>>>>>>>>> The design team created some simple JSON Schemas for parts of the protocol during our discussion, but we didn’t include them in the design document due to both lack of time to keep it updated with the rapid changes to the protocol during the design team discussion, and not knowing if there would be interest in such material. I personally think it would be helpful to include as an informative reference in the final document, but that’s something for the working group to take up eventually.
>>>>>>>>>>>  
>>>>>>>>>>>  — Justin
>>>>>>>>>>>  
>>>>>>>>>>> 
>>>>>>>>>>>> On Oct 23, 2020, at 10:18 AM, Mika Boström <mika.bostrom=[e-mail] <mailto:mika.bostrom=[e-mail]>> wrote:
>>>>>>>>>>>>  
>>>>>>>>>>>> Hello, everyone.
>>>>>>>>>>>>  
>>>>>>>>>>>> For background: GNAP/TxAuth/XYZ/Oauth3 came up on a discussion forum and when I made note about certain concerns, I was requested to send my comments to this working group.
>>>>>>>>>>>>  
>>>>>>>>>>>> In short, I believe that the use of polymorphic JSON in the protocol invites subtle and confusing implementation problems. I also searched through the WG archives, and noticed that similar concerns were noted, briefly, in a thread in July. 
>>>>>>>>>>>>  
>>>>>>>>>>>> The problem with polymorphic values, as I see it, is that implementations will need to branch on the (inferred) type of a given field. This isn't quite as bad if the types are strictly different, but allows for subtle bugs when the value in question is a dictionary. What makes this unappealing is that "subtle bugs" in security protocols have a habit of turning into vulnerabilities.
>>>>>>>>>>>>  
>>>>>>>>>>>> Let's say we have these imaginary payloads, both possible and valid in the same protocol step:
>>>>>>>>>>>>  
>>>>>>>>>>>> # payload 1
>>>>>>>>>>>> {
>>>>>>>>>>>>   ...,
>>>>>>>>>>>>   "public_key": {
>>>>>>>>>>>>     "alg": "rsa",
>>>>>>>>>>>>     "modulus": <BIGINT>
>>>>>>>>>>>>   }
>>>>>>>>>>>> }
>>>>>>>>>>>>  
>>>>>>>>>>>> # payload 2
>>>>>>>>>>>> {
>>>>>>>>>>>>   ...,
>>>>>>>>>>>>   "public_key": {
>>>>>>>>>>>>     "alg": "rsa",
>>>>>>>>>>>>     "modulus": "<encoded string>"
>>>>>>>>>>>>   }
>>>>>>>>>>>> }
>>>>>>>>>>>>  
>>>>>>>>>>>> In both cases, the type of "public_key" field is a dictionary. In both cases, they even have the same keys. However, the values in the dictionaries are entirely different, and an implementation will have to branch to at least two possible decoding mechanisms. To make things worse, some JSON implementations may choose to encode non-dictionary values as strings, so it is possible for an originator to transmit what they expect and believe to be payload 1 format, but which the receiver will interpret to be in payload 2 format. And if the encoded string contains only digits, it will even parse correctly as a bignum.
>>>>>>>>>>>>  
>>>>>>>>>>>> While the above is clearly a manufactured scenario, it nonetheless demonstrates the potential for logic bugs with polymorphic JSON. With richer types and more complex dictionaries, there will surely be more room for errors.
>>>>>>>>>>>>  
>>>>>>>>>>>> Ambiguity in protocols is always a source of implementation complexity and interoperability snags, but in an AuthN/AuthZ protocol it is worse: it's terrifying. If GNAP/Oauth3 is intended to supersede Oauth1/2, wouldn't it be in everyone's interest to keep implementation complexity and mistake potential to a minimum?
>>>>>>>>>>>>  
>>>>>>>>>>>> Best regards,
>>>>>>>>>>>> Mika
>>>>>>>>>>>>  
>>>>>>>>>>>> -- 
>>>>>>>>>>>> Mika Boström
>>>>>>>>>>>> Smarkets
>>>>>>>>>>>> -- 
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>>>>>>>>>>>> https://www.ietf.org/mailman/listinfo/txauth <https://www.ietf.org/mailman/listinfo/txauth>
>>>>>>>>>>>  
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>>>>>>>>>> Error! Filename not specified.ᐧ
>>>>>>>>> 
>>>>>>>>>  
>>>>>>> 
>>>>>>>  
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