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Re: crypto-ISSUE-25 (Global Unique ID): How do we provision Global Unique ID for pre-provisionned symetric keys [Web Cryptography API]

From: Mark Watson <watsonm@netflix.com>
Date: Tue, 21 Aug 2012 21:13:57 +0000
To: Ryan Sleevi <sleevi@google.com>
CC: Web Cryptography Working Group <public-webcrypto@w3.org>
Message-ID: <1EB7E5C3-C3AB-4986-B250-31584469E969@netflix.com>

On Aug 21, 2012, at 12:48 PM, Ryan Sleevi wrote:

On Tue, Aug 21, 2012 at 12:05 PM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:

On Aug 21, 2012, at 11:27 AM, Ryan Sleevi wrote:

On Tue, Aug 21, 2012 at 10:51 AM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:


While not trying to be dismissive, it remains unclear to me why you
feel that the application-specific, context-specific unique identifier
needs to be more special than any other pre-provisioned attribute.

Because the need for such an attribute is neither application- not context- specific, as I have explained. All applications with pre-provisioned symmetric keys (PSKs) will need such a thing because PSKs are useless without it (where useless = no different from keys generated randomly by the client).

Please note, I am arguing just for this to be supported in the specification. It's supported if you have KeyAttributes and the notion that some of those may be pre-provisioned along with the key itself. It's not supported if you remove KeyAttributes because there's nothing in the Local Storage specification about pre-provisioning (and if there was I think the link between the key and its attributes would still be too tenuous).

I'm fine with Local Storage for attributes that need to be stored by the JS application itself.

As I understand your response, it's primarily (solely?) focused on
pre-provisioned keys, which are already an implementation-specific
detail that is not going to be common across user agents, and is thus
inherently non-portable to begin with.

Support for PSKs is essential to our use-case and included in the charter.

You are right that not all WebCrypto implementations will support them, which just means they are an optional capability. It will need to be possible for an application to discover whether a pre-provisioned key exists. Applications such as ours will behave differently, depending on whether a PSK exists or not.

I don't see any mention about PSKs or pre-provisioned keys as being
part of the charter. That said, no details about the (underlying) key
storage mechanism are included in the charter, and that was sort of my
point - that it's an implementation specific detail.

The charter states, as a goal, "The provisioning and use of keys within Web applications can be used for scenarios like increasing the security of user authentication and determining whether a particular device is authenticated for particular services."

Ok, so "provisioning" is not qualified as explicitly including "pre-provisioning", but without pre-provisioning the goal of device authentication is not possible.

If you look at discussions related to the charting process, for example [2], it's clear that pre-provisioned keys were envisaged and that this identity requirement was understood. The first and for a long time the only use-case submitted for WebCrypto was focussed on the use of pre-provisioned keys. They were even discussed (without dissent) at the "Identity in the Browser" workshop early last year that kicked off this whole activity.

[2] http://lists.w3.org/Archives/Public/public-identity/2011Dec/0031.html<http://lists.w3.org/Archives/Public/public-identity/2011Dec/0031.htm>

Given that this is included as a clear goal in our charter I don't think it's sufficient to simply say it can all by done by implementations without any mention in the specification of how.

It's in the implementation gray area - there's nothing in LocalStorage
or in IndexedDB preventing pre-provisioning either. Politics of
specifying another WG's interface aside, if we accept that
pre-provisioned keys are a detail that user agents may or may not
implement, and that the knowledge about whether the keys were or were
not pre-provisioned relies on the peer (the web application) having
some out-of-band knowledge, then it seems reasonable to think that the
user agent and the web application could cooperate on any Web Platform
API, to the betterment or worsening of the platform.

I don't think that's sufficient. The specification needs to say how pre-provisioning will look to the script, not least so that the script can tell if pre-provisioned keys are present (for its origin).

An application using PSKs will have out-of-band knowledge of those identity and key values and properties of the devices they were pre-provisioned into, but a script running on a device has no idea what kind of device that is and whether it has pre-shared keys. If I have to work with N different ways keys and their identities are pre-provisioned for N different devices then I'd need to trial-and-error through all those options when starting my script. Even if I did know the type of device, I'd have to code N different approaches.

So, I don't think it should be left implementation-dependent.

That said, is there any reason why such an ID needs to be any
different from any other pre-provisioned attribute?

What other pre-provisioned attributes ?

Whatever a user agent that wishes to implement pre-provisioning may support.

Ok, so that should be standardized, for the reason above.

Is this really a
question about unique IDs (as has been framed by yourself and Mitch
several times), or can your need simply be addressed by having
whomever is doing the pre-provisioning of keys (an industry and
application-specific act) also provision those attributes?

Where ? As I've said, pre-provisioning of Local Storage is not something mentioned in the Local Storage specification and may be viewed as rather odd.

I'd like to focus on this specific point, since I strongly feel the
needs you have identified are already readily addressed by the spec,
and what you're wanting to do is already possible, and without having
to introduce any special handling for pre-provisioned keys, nor
require any language changes to the spec.

Could you explain in more detail why you believe that ?

I am beginning to feel that we need a specific section on pre-provisioned keys, since there's clearly a wide variance in the assumptions people have about them.

Text welcome at an informative level, but again, I think that
pre-provisioning is a concept that is inherently non-portable. It may
simply be sufficient to describe it as "It can happen". Much like the
only discussion about platform-originated keys (eg: native keys) are
only mentioned in the Scope section, as informative, and as a
security/privacy concern. I see pre-provisioned keys as being a
function of 'native cryptographic module' (eg: they weren't expressly
requested by the web application), but perhaps that's a semantic
distinction that isn't shared.

To be completely unambiguous: pre-provisioned keys are inherently
implementation specific, and thus I believe any attempt to define a
common nomenclature for them is fatally flawed.

I disagree. The notion of a pre-provisioned symmetric key with an associated identity is straightforward and application-independent and we can easily specify is, just as is done for TLS PSK. Just because it is an optional feature doesn't make it's definition 'implementation-specific'.

In my original message, I provided examples of exactly how it is
application dependent.

And I described why your examples were not quite correct: how a key is pre-provisioned is device-manufacturer specific (not application specific). How a key identity is shared with an application is an agreement between device manufacturer and application provider (you could call that application-specific). But what a pre-provisioned key *is* and the fact that it needs to be accompanied by an identity, are generic and universal.

Even the use of TLS PSK is
application/implementation dependent - the same key identifier used
for TLS PSK is not necessarily usable for any other purpose, nor is
the same identifier used by one TLS PSK peer equivalent to the
identifier used when negotiating (with the same key, if you were that
dangerous) to another peer.

Again, the concept of a PSK and associated identity are sufficiently generic that explicit support for the identity is embedded in the TLS PSK specification. Yes, a lot of other things are applications specific. I am not asking for more than TLS PSK does.

Assume we resolve ISSUE-17 and agree that, indeed, it's necessary to
have a specific attribute store for keys (for which the only use case
I seems to be pre-provisioned keys...). As discussed previously, the
KeyAttributes interface (
http://www.w3.org/2012/webcrypto/WebCryptoAPI/#dfn-KeyAttributes )
fully supports read-only properties. That's why explicit
getters/setters were defined, since WebIDL makes it clear that any
accesses will be redirected through these getters/setters (
http://www.w3.org/TR/WebIDL/ ) and that they need not support the

Yes, if you have the KeyAttributes interface, everything is fine (modulo the descriptive text which I think it would be wise to add on this issue). There is no need to say what the "name" of the key id is, since whomever pre-provisions the id itself can tell me what the name is as well.

We could consider keeping KeyAttributes only for functional or read-only attributes and require applications to use Local Storage for application-specific stuff.

Possible ways that Netflix can do what they want without a spec change:
* Presume a pre-provisioned Master Key (MK) which is secret from all
* Goal is a pre-provisioned Origin Key (OK), which is origin-specific

Option 1: Implicit keys on demand:
- When the UA navigates to one of the authorized origins
  - If an OK does not yet exist, generate one
    - Pre-provision an attribute/value pair with "MKID" and the ID
of the master key
    - Pre-provision an attribute/value pair with "OKID" and whatever
industry-specific origin key
  - Expose it via window.crypto.keys
Option 2: Explicit key derivation
- When the UA navigates to one of the authorized origins
  - Expose the MK as a handle via window.crypto.keys
  - Implement a custom key derivation mechanism ("Netflix-TV-keys")
  - Have your application request that a key be derived from the MK
  - The user agent performs the following steps:
    - Take the origin of the currently executing script (which all
UAs must track anyways), and feed that as input into your derivation
scheme. Thus, it's *never* necessary the application provide it
    - Derive the OK from the MK
    - On the generated OK, set the OKID and MKID as appropriate
    - Return the OK to the application
   - The key is now available to the application, and may be
available later via window.crypto.keys (if the caller requested the OK
be persistent)

Both assume that there is a sensible place to pre-provision attributes. Local Storage is a sensible place for the application to store stuff, but I don't think it works for pre-provisioning key attributes.

So then is this just ISSUE-17? It seems the argument is that existing
storage mechanisms are unsuitable for pre-provisioning, ergo a new
mechanism is needed. That's a fine argument to make, but does seem to
get at the heart of ISSUE-17.

No, ISSUE-17 says: "For attributes defined by the application, the question is…"

So, ISSUE-17 is constrained to attributes defined by the application, which I take to mean those where the JS script wants to attach its own attributes to a key for later retrieval. If that is the problem, then I agree with your arguments that Local Storage seems sufficient.

The global identity for a pre-shared symmetric key is not one of those attributes. It does not start life in the script or even with the application-provider. It's pre-provisioned, probably by the device maker.

If ISSUE-17 includes pre-provisioned information then I would argue that Local Storage is unsuitable for that, unless the Local Storage specification is amended to make that use-case clear.

That said, the proposal for KeyAttributes being *explicitly* read-only
is interesting as well. For example, WebIntents does this via the
'extras' member of the IntentParameters, which is a key/value map of
extra meta-data to be sent with the Intent. That said, I understand
there is some ongoing discussion/controversy regarding it, so it may
not be the best API to cite.

I think both options could work, with some tweaks. If by 'ID of the master key' you mean a global id for that, then I would not expect that to be exposed. The idea of explicitly exposing a key derivation function that factors in the current origin is interesting. That could be under the covers (the application only ever knows about the OK), but there may be some advantage in giving the application control of the derivation. You'd need to derive an orgin-specific key identifier too.

In fact if we could specify the above approach that would make things significantly easier for device manufacturers.

I think that the MK/OK scheme is something specific to a given
implementation, and highlights both the flexibility and dangers of the

It could be left implementation specific. But we could also decide to standardize it if we thought it was generically useful. We don't need to have that debate right now.

This similarly relates to the discussions about MUST vs SHOULD
for algorithms, since the concern was raised that if anyone can
implement any algorithm, then you'll have new implementations
effectively barred from entry or working with a site, due to their
lack of support for a given algorithm.

If we can reach consensus that the fundamental API is flexible enough,
and possible to standardize, then I think we could begin discussions
about whether it's worthwhile to specify such a key
agreement/negotiation scheme that is specific to the context of user
agents and the security boundaries they operate in (eg: having
unspoofable inputs from the U-A in addition the the web application's
inputs). This is similar to the PBKDF2 text in the current draft.

Like all crypto-politics, I imagine that conversation will be a fun
one, so I would rather wait until we've shipped something (eg: not
just FPWD) before going down that road.

Again, pre-provisioned keys are user-agent specific.

Support of pre-provisioned keys is user-agent-specific. That's different.

There is no way
to have pre-provisioned keys to universally work across user agents,
since that universally assumes the same key storage and access APIs,
which is simply impossible. So your user agent can do whatever it
wants - because it's the only one who understands this concept.

But there are some concepts which are generic, which we can standardize. We could standardize the origin-specific key derivation function and the concept of a common Master Key as you describe above. We would not make any statements about the properties this key has. Some implementations might just re-generate it randomly every time. Especially if you're in "incognito mode". Others would have it stored in hardware. Doesn't matter because the API is not guaranteeing any special properties for this key. Any special properties are learned out-of-band.

If we had consensus to do so, yes. I don't believe attempting to
define a new cryptographic algorithm for user agents to derive keys
from origin inputs is something as an implementor we'd be interested
in standardizing at this time.

OK, let's leave the MK -> OK derivation for later.

Received on Tuesday, 21 August 2012 21:14:27 UTC

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