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Re: extensions, continued.. (was: 05/24/2016 WebAuthn Summary

From: Dirk Balfanz <balfanz@google.com>
Date: Tue, 07 Jun 2016 04:45:34 +0000
Message-ID: <CADHfa2A3+NPMbpDjnc9rb9P+eQRRCcX_UCfOG=93Du83iRtSMg@mail.gmail.com>
To: "Mandyam, Giridhar" <mandyam@qti.qualcomm.com>, "public-webauthn@w3.org" <public-webauthn@w3.org>
On Sat, Jun 4, 2016 at 6:09 PM Mandyam, Giridhar <mandyam@qti.qualcomm.com>
wrote:

> >Does anyone else remember us discussing (perhaps in the FIDO 2.0 WG)
> these extensions?
>
>
>
> Yes, I do.  But I cannot reproduce the communications that occurred within
> one SDO (in this case the FIDO Alliance) on the mailing list of another
> SDO.  I’m not sure this is relevant to your argument anyways.
>

I agree it's not relevant other than these three extensions haven't gotten
as much scrutiny (at least from this spec editor; perhaps from others, too)
as the other extensions have.


> >How can the client do its job of protecting user privacy if the
> authenticator is allowed to add data to the assertion that the client
> doesn't understand?
>
I’ll answer with an example.  Chrome browser supports EME.
> MediaKeySessions involve the exchange of messages between the DRM engine
> (CDM) and license server that the client may not understand, and will
> likely involve unique and privacy-impacting identifiers.
>

DRM protocols that expose unique identifiers are not exactly an example of
clients protecting user privacy. FIDO historically draws the line quite
differently from where DRM systems draw the line. In fact, FIDO
historically draws the line even more conservatively than the rest of the
web does (not even considering DRM). Let me give you an example of that:
when we first introduced app IDs and facets, it was technically possible
for two origins, let's say google.com and youtube.com, to collaborate and
access the same key on an authenticator (something that could come in quite
handy for those two origins, as you can imagine). Note that absent app IDs
and FIDO, any two origins on the web, if they choose to collaborate, can
already track a user and agree on a common identifier (by iframing each
other, run federation protocols, etc). FIDO at that point decided that we
didn't want to introduce an *additional* channel for different origins to
track users; so we changed the app id spec to no longer make it possible
for exampleA.com and exampleB.com to have access to the same keys from an
authenticator.

So, historically speaking, *that's* the bar that FIDO has set (and that,
presumably, we want to inherit here in the webauthn WG), not the bar set by
DRM systems. DRM systems, to be sure, set the bar differently for good
reasons - but we're not here to build DRM, we're here to build
authentication systems that protect the user's security and privacy.

Answers to your questions below.

  You can see https://www.w3.org/Bugs/Public/show_bug.cgi?id=26332 for more
> on this topic.  The solution to this was requiring secure context for
> access to EME (
> https://www.chromium.org/Home/chromium-security/deprecating-powerful-features-on-insecure-origins),
> which is what the WebAuth spec already recommends.
>
>
>
> To me, a greater threat to user privacy than unprompted extensions is that
> secure contexts is only a SHOULD requirement in the spec instead of a MUST
> – see https://w3c.github.io/webauthn/#secure-contexts.  But I imagine
> this has already been discussed to death.
>
>
>
> > I therefore propose that these three extensions be changed to included
> a client argument that signals to the authenticator that the information
> described in the extension should be provided by the authenticator.
>
> Qualcomm cannot support the proposal as written.  I would like to pose
> some clarifying questions however:
>
>
>
> a)      Can the client argument be null?
>
yes.


> b)      What occurs if the client drops an extension that is associated
> with valid client data?  Will the RP know?  e.g. is an exception thrown?
>
No exceptions is thrown. What happens, and whether the RP will know,
depends on the extension. Sometimes, the RP won't know whether or not the
client supported the extension. For example, in the authenticator selection
extension, the client MUST choose a (random) authenticator if none of the
specified authenticators are present. So when the RP asks for authenticator
A, and gets authenticator B, this might be because no authenticator A was
present or because the client didn't understand the extension - the RP will
never know. For most extensions, however, I would assume it will be pretty
obvious from the authenticator response whether the client supported it or
not.


> c)      What does this mean for unprompted extensions in the packed
> attestation (https://w3c.github.io/webauthn/#sec-raw-data-packed) without
> a corresponding client argument?  It seems that the client cannot just drop
> an unprompted extension and still expect the signature to pass.
>
Right. "Dropping an unprompted extension" presumably means that the client
somehow chops off parts of the authenticator response, which will obviously
break the signature. That's one of the reasons unprompted extensions are a
bad idea: we can't ask clients to "drop an unprompted extension". We can
only ask clients to drop extensions that are requested by the RP. Because
clients must be able to drop extensions they don't understand, all
extensions must be requested by the RP.


> d)      Similar question for the assertion.
>

Same response as above.

Dirk.


>
>
> -Giri Mandyam
>
>
>
>
>
> *From:* Dirk Balfanz [mailto:balfanz@google.com]
> *Sent:* Saturday, June 04, 2016 11:14 AM
> *To:* Hodges, Jeff <jeff.hodges@paypal.com>; Vijay Bharadwaj <
> vijaybh@microsoft.com>
>
>
> *Cc:* public-webauthn@w3.org
> *Subject:* Re: extensions, continued.. (was: 05/24/2016 WebAuthn Summary
>
>
>
> Hi there,
>
> Until Adam pointed this out to me in Berlin, I had no idea that these
> other three extensions exited. Now, it's certainly my fault - and no one
> else's - for not reading the attestation document more carefully (which is
> where these three extensions were originally defined), but I honestly don't
> remember these three extensions being discussed the way we discussed the
> authenticator-selection and transaction-authorization extensions. Does
> anyone else remember us discussing (perhaps in the FIDO 2.0 WG) these
> extensions?
>
> In particular, I don't understand why they are defined as "unprompted"
> extensions. This is a privacy problem. How can the client do its job of
> protecting user privacy if the authenticator is allowed to add data to the
> assertion that the client doesn't understand? I get Jeff's point about
> innovation if the RP and authenticator can agree on something even if the
> client doesn't know what that something is, but I believe we should err on
> the side of privacy here.
>
> I would also point out that technically speaking, unprompted extensions
> are not allowed according to the current text, which states that "*an
> extension must specify, at minimum, an extension identifier and an
> extension client argument sent via the {{getAssertion()}} or
> {{makeCredential()}} call*".
>
> I therefore propose that these three extensions be changed to included a
> client argument that signals to the authenticator that the information
> described in the extension should be provided by the authenticator.
>
> Thoughts? Opposing or supporting views?
>
> Dirk.
>
>
>
>
> On Fri, May 27, 2016 at 1:07 PM Hodges, Jeff <jeff.hodges@paypal.com>
> wrote:
>
> On 5/27/16, 12:50 PM, "Vijay Bharadwaj" <vijaybh@microsoft.com> wrote:
>
>
>
> You mean you object to allowing the client a say in which extensions are
> emitted? We’re not talking about removing any existing extensions, just
> about clearly defining the circumstances under which an authenticator might
> emit them.
>
>
>
> Yes, we would object to altering the present design that allows for
> authenticators to implement and emit extensions of their own volition, as
> pesently specified (c.f., AAGUID extension, SupportedExtensions extension,
> User Verification Index (UVI) extension).  We feel it is a
> subtle-but-important aspect of fostering the overall ecosystem.
>
>
>
> This entire thread has become quite frayed... having a concrete extension
> proposal on the table may help it coalesce -- I suggest that Giri write up
> the postulated "opaque data" extension using the framework that's presently
> defined in the spec and then hopefully we can more objectively assess it.
>
>
>
> HTH,
>
>
>
> =JeffH
>
>
>
>
>
>
>
>
>
> *From:* Hodges, Jeff [mailto:jeff.hodges@paypal.com
> <jeff.hodges@paypal.com>]
> *Sent:* Friday, May 27, 2016 12:48 PM
> *To:* Vijay Bharadwaj <vijaybh@microsoft.com>
> *Cc:* public-webauthn@w3.org
> *Subject:* Re: extensions, continued.. (was: 05/24/2016 WebAuthn Summary
>
>
>
> On 5/27/16, 12:37 PM, "Vijay Bharadwaj" <vijaybh@microsoft.com> wrote:
>
> One issue with that is that some of the extensions that are currently
> defined (in fact, 3 out of 5) are emitted unprompted by the authenticator.
> Though if we wanted to make this rule, I would be fine with it and we could
> add it in the spec if others agree.
>
>
>
> Essentially the authenticator would still be allowed to ignore requested
> extensions, just not add new ones on its own.
>
>
>
> We paypal object to obviating existing extensions.
>
>
>
>
>
>  *From:* J.C. Jones [mailto:jjones@mozilla.com <jjones@mozilla.com>]
>
> *Sent:* Friday, May 27, 2016 12:33 PM
>
> That's how you'd enforce it: if the authenticator doesn't obey the
> contract, the signature won't be valid when the RP checks it.
>
> Roughly the contract would be: Authenticators will only emit extensions
> they were prompted to emit.
>
>
>
>
>
>
>
>
>
>
Received on Tuesday, 7 June 2016 04:46:16 UTC

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