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Re: Resolving issues around avoiding key requests

From: David Dorwin <ddorwin@google.com>
Date: Sun, 2 Aug 2015 21:09:15 -0700
Message-ID: <CAHD2rshkeePQR-dYZSS3gYBWMfwJx4rUdsAbk8Tw5nj3=bFYww@mail.gmail.com>
To: Joe Steele <steele@adobe.com>
Cc: "<public-html-media@w3.org>" <public-html-media@w3.org>
I have provided specific comments on the three issues inline. To summarize,
I believe 52 (removal of a single word) depends on 41, resolving 41 should
also consider 53, and that 53 requires a significant amount of work to
develop a consistent, cohesive, and interoperable solution.

No new information has been provided that demonstrates that inclusion of
these features - in an interoperable and consistent manner - is certain or
that such resolution of the issues is trivial. Given the finite cycles of
the editors, my recommendation is to continue to focus on facilitating
interoperability for the existing feature set and resolving issues that
affect many/most authors and implementers and block LC/CR.

On Thu, Jul 30, 2015 at 11:07 AM, Joe Steele <steele@adobe.com> wrote:

> There are three open issues relating to keys being provided in a manner
> that may allow key requests to be avoided. I am listing them below.
> *Please review all three issues carefully and respond to this thread with
> your opinions and questions.*
> Issue 52 - Remove reference to keys in Initialization Data definition
> https://github.com/w3c/encrypted-media/issues/52
> The addition of keys to the list of restricted data in the initData was
> made without any group input as far as I can tell. This restriction flies
> in the face of the CENC specification which members of this group agreed to
> support. I believe that the burden should be on the author of this list to
> provide justification. In the F2F meeting the justification offered for
> this restriction (as I recall) was that this would lead to an interop
> problem. I disagree with this and say that this can only lead to an interop
> problem if in fact those keys are used by the CDM directly, which *could*
> require the algorithm changes proposed in issue 41. However it does not
> *necessarily* lead to an interop problem. This edit is also required to
> allow for the DRM specific key metadata specified by CENC. I believe this
> issue can be resolved independent of issues 41 and 53.

Some corrections:

   - As I have repeatedly explained, adding the word "key(s)" to the
   definition was not a change in behavior; it made the definition consistent
   with the algorithms, which provide no mechanism for extracting keys from
   Initialization Data. Allowing keys (other than as a detail of the opaque
   blob to be sent to the license server) would require defining the
   extraction and processing of such keys (Issue 41).
   - Just as MSE supports a subset of MP4 streams to ensure compatibility
   with the algorithms and increase the likelihood of interoperability, EME
   supports a subset of possible CENC streams with a preference for openness
   and interoperability. In other words, EME is not a superset of CENC.


   - Could you clarify why you believe that keys in initData is currently
   supported by the spec? Specifically, how/where in the application flow/spec
   are they used?
   - Regarding "this can only lead to an interop problem if in fact those
   keys are used by the CDM directly": Since “DRM specific key metadata” is
   opaque - and in Adobe’s case encrypted - I believe only the CDM / license
   server can access such keys. Am I missing something? How would such keys be
   used indirectly by the CDM or by some other entity?

> Issue 41 - Update algorithms to reflect keys being provided in the
> Initialization Data
> https://github.com/w3c/encrypted-media/issues/41
> The justification for this proposal is that requesting a key from a
> license server is expensive. If it is unnecessary to request a key because
> the keys are already available to the CDM in a manner that complies with
> all other security/privacy restrictions in the spec, I believe the spec
> should support not issuing a key request. It has been argued that this will
> lead to interop issues. I would point out that protocol and policy
> differences can already cause key requests to happen on a different
> schedule between Key Systems. This is not fundamentally different and I do
> not believe that this change will cause much if any additional work for
> developers. *This is blocked by issue 52*.

As you note under Issue 53 below, the spec does not currently allow use of
keys not present in an active session (other than keys that are part of the
DRM protocol, such as platform or individualization/provisioned keys).
Thus, there is currently no mechanism for "the keys [to] already [be]
available to the CDM." I don't oppose such a concept, but I think it
requires careful consideration and design to maintain consistency with the
rest of the spec as well as related features, such as issue 53.

This is *blocking* issue 52 for the reason explained in that section above.
(In reality, they would likely be resolved together.)

> Issue 53 - Allow for long-lived key encryption keys (aka "master" keys) to
> increase performance
> https://github.com/w3c/encrypted-media/issues/53
> The justification for this proposal is supporting increased performance
> for DRMs that can avoid additional license requests. Most if not all DRMs
> already support some type of key chaining, where a key is either generated
> on the client or delivered to the client and then that client key is
> subsequently used for decrypting the content keys provided via key server
> responses (aka licenses). This chaining today is invisible to the
> application and is considered “out of scope” for this specification. I am
> proposing that we bring a variety of key chaining in scope, specifically
> when more than two keys are involved in the chain for performance reasons.
> The main problem with the current text is that it assumes that keys
> acquired during a session will go away once the session is closed, even
> when there are other active sessions on the same MediaKeys object. This
> would need to change to make retaining master keys across sessions
> compliant with the spec. *This is blocked by issue 41*, since using a
> master key would imply that a key request is not required in some cases.

I have ideas for how this might work, but they involve significant
changes/additions to the spec, including new session type(s).

> Joe
Received on Monday, 3 August 2015 04:10:03 UTC

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