W3C home > Mailing lists > Public > public-webcrypto@w3.org > June 2012

Re: Use case classification, and associated security models

From: Ryan Sleevi <sleevi@google.com>
Date: Tue, 12 Jun 2012 14:29:32 -0700
Message-ID: <CACvaWvYJCL_nh8fiRkeCty8bTzM2ZTjq4y3+YEZ7G_JfffQW4g@mail.gmail.com>
To: Mitch Zollinger <mzollinger@netflix.com>
Cc: public-webcrypto@w3.org
On Tue, Jun 12, 2012 at 1:31 PM, Mitch Zollinger <mzollinger@netflix.com>wrote:

>  On 6/12/2012 12:53 PM, Ryan Sleevi wrote:
> On Tue, Jun 12, 2012 at 12:38 PM, Mitch Zollinger <mzollinger@netflix.com>wrote:
>>  On 6/12/2012 1:07 AM, Vijay Bharadwaj wrote:
>> <snip>
>> In all the above cases, once a key is obtained, all the actual crypto
>> operations are pretty much the same. So is we define all operations in the
>> API which require a key such that they take a key object as a parameter,
>> then the only difference between the above scenarios (from an API
>> perspective) is the operations used to instantiate that key object. The
>> above 3 scenarios would then correspond to 3 different instantiation
>> methods for key objects:
>> 1.       GenerateKey  create a new key for use with a specific
>> algorithm. Choice of crypto provider left up to the platform.
>> 2.       ImportKey  take a key blob obtained from key agreement and
>> create a key object from it. Choice of crypto provider left up to the
>> platform.
>> 3.       OpenKey  Locate a key on the host system that matches a set of
>> criteria. Choice of crypto provider to be made by platform depending on the
>> location of the key.
>> There is also a fourth primitive which is often used with scenario 3 
>> credential enrollment. This would be the operation where the user employs
>> the trusted key to obtain a credential (e.g. enrolling for a smart card
>> certificate by signing a request using ones existing smart card key).
>> Does that seem reasonable? Any other families of use cases that Im
>> overlooking?
>> Good write up.
>> I would like to better understand what you (and the group) think about
>> "crypto providers". When I think about crypto providers, I think of the JCE
>> (Java Cryptography Extensions) Crypto Providers & OpenSSL "engines". In
>> that context, it would seem to me that the app has varying behavior with
>> regard to crypto provider selection that may be orthogonal to the type of
>> the key. One example:
>> * If my app detects that a smart card is plugged in & I can use keys on
>> the smart card for service authentication, I will not ask the user a
>> security question when they connect to my webapp. However, if I'm using
>> another crypto provider (likely the default provided by the browser) --
>> with the same keys that would have been stored in the smart card if the
>> user had it plugged in -- then I will ask my security question.
>> Mitch
> I think that any app that would do what you propose, without any other
> additional proof, would be making a naive assumption that doesn't really
> improve the security. I certainly do not want to expose any such concept to
> the web API for exactly that reason.
>  In your example, you're relying on the user agent to honestly answer
> "smart card" or "not smart card" for where the key is stored, and then
> making a server side security decision based on that.
> I should have been more explicit. I'll echo much of what you say below (I
> agree with you):
> When I say "same key" I'm referring to a key with the same *key name* in
> both contexts, not actually the same keying material or same UUID (I'm
> taking the liberty of assuming the UUID exists for now.)
> The point is that the running web app / web service is making a decision
> on further authentication steps based on the provenance of the key: was it
> distributed out-of-bound and considered pre-shared or was it something I
> made up at webapp runtime?
>  Such a decision seems a lot like sticking form validation code in the
> client to prevent against SQL injection. Browsers can be mislead or
> manipulated, so I don't think it can be reasonably assumed to give an
> answer that is reliably accurate.
> No reliance on trustworthiness of client side JS code assumed here.
>  Using Vijay's ontology, if you want to make the distinction between
> secure element or not, you, as a service provider, would do so based upon
> whether or not you (as a service provider) were responsible for the
> provisioning ceremony of those keys. If you controlled the ceremony, you
> know the provenance of those keys, and you know how and where they're
> stored.
> Absolutely agree.
>  Whether this means mailing out a smart card, using some other
> application/protocol/platform. Examples might be GlobalPlatform, with it's
> proof of possessions, some work product of the IETF's KeyProv WG, something
> like Anders Rundgren's Keygen2, etc. However, the size and scope of work,
> and the many competing interests involved, seem very much out of scope for
> this WG (although admittedly very interesting and perhaps parallel work to
> solve)
>  To modify your scenario into something that I think would provide some
> reasonable assurances:
> * If your app detects that the user has a (pre-provisioned Key ID X), then
> you can allow that user to authenticate using Key X.
> * If your app does not detect Key X, but detects a (previously generated
> via <keygen> or the ephemeral API Vijay proposed), then it uses Key Y, plus
> some form of challenge/security question.
> * If your app detects neither Key X nor Key Y, it offers the user the
> chance to (online enroll a Key Y, contact you to obtain a smart card with
> Key X)
> Yep. No disagreement from me with this approach. I would rewrite this as:
> * My app wants to use "AuthKey" to authenticate the client, wherever
> "AuthKey" may be.
> * The app attempts to use "HSMProvider" as the crypto provider; if that
> provider does not work or is not available, it falls back to
> "DefaultProvider". (For the moment, I'm ignoring the bootstrap of "AuthKey"
> into each of these providers.)
> * The app authenticates using "AuthKey", which has an associated UUID
> which allows me to map the key to one of (a) a key that I provisioned into
> an HSM (and accessible via "HSMProvider") or (b) a key entrusted to the
> "Default" provider.
> * The server side decides, based on Provider/UUID whether to initiate
> further authentication steps.
>  Mitch
I'm not sure the providers are necessary here, which is part of why I'm
somewhat opposed to them.

Using the KeyQueryList sort of example from the other thread, I would think
you'd simply set a KQL for "some criteria" (admittedly, hand-wavey), and
you would get back (assuming the user approved/pre-approved) two keys.

At that point, you have two keys you're interested in. The question is
which (or perhaps both) you want to use for the signing operation. Your web
app may make a query to the server and say "I have these two keys, which
would you like to use" - at which point, the server might say "Use this
one, I know because I presided over the ceremony".

Your web app may have already sent that list to the client, and the client
JS then knows that key X is preferable over key Y, and upon seeing that it
has both Key X and Key Y, attempts the authentication protocol first with
Key X.

You may even show a UI (using your web apps branding) to allow the user to
select which key they wish to use, similar to sites that offer multi-login
or oauth/saml style workflows.

At no point does the application need to be exposed to the crypto provider
or how the key is backed - it's simply a key handle that it can use.

My guess (perhaps completely unfounded, based on our current participants)
is that most applications using this API will not care about
"DefaultProvider" or "HSMProvider", and certainly, I think it introduces a
whole host of new, cross-browser standardization issues. For example,
consider if we have two implementations, one based on PKCS#11 (eg: Mozilla
Firefox, Chrome for Linux) and one based on CryptoAPI/CNG (eg: Internet
Exporer, Chrome for Windows, Safari for Windows). Both OS-level APIs may be
talking to the same logical smart card/key store/secure element, but
because of the very different nature of the APIs, have very different
naming schemes to identify them.

Using the above example, is there a scenario/workflow where "Try to find
Key X, then try to find Key Y", in application logic, is more desirable
than "Query for Key X or Key Y", and then in application logic making a
decision based upon the response you got?
Received on Tuesday, 12 June 2012 21:30:01 GMT

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