W3C home > Mailing lists > Public > public-html-media@w3.org > August 2015

Re: Resolving issues around avoiding key requests

From: Mark Watson <watsonm@netflix.com>
Date: Wed, 12 Aug 2015 16:19:39 -0700
Message-ID: <CAEnTvdCiViBgQhWFCWD0aKh=RdeuY2mi4p90pTdA5H4tBOMDtw@mail.gmail.com>
To: "Jerry Smith (IEP)" <jdsmith@microsoft.com>
Cc: David Dorwin <ddorwin@google.com>, Joe Steele <steele@adobe.com>, "<public-html-media@w3.org>" <public-html-media@w3.org>

I agree with Joe and Jerry.

Regarding *Issue 52*: it has always been assumed that the initData is
opaque, that the CDM uses information in the initData to construct a key
request and that the server uses that information to construct a response
containing the content key. Any mechanism that adheres to that model should
be allowed. Clearly, we all expect the opaque portion of the initData to
contain information that identifies the content key. Whether that
information is in the form of a key id or the key itself seems irrelevant
to the specification and to interoperability. Is it allowed for the
initData to contain a partial key and then an id for the remainder ? If it
contains one bit of the key and an id for the remainder is it ok ? If it
contains 127 bits of the key and an identifier for the last bit of the key
is that ok ? In neither case does the initData contain a "key". What huge
problem does that one additional bit of the key cause ?

I also agree with Joes description of the way this change was introduced
without discussion: I think the onus should be on David to describe a
problematic scenario that this prohibition on keys in initData precludes
before we introduce that prohibition. In the meantime the prohibition
should be removed.

Regarding *Issue 41*: our model for key exchange messages is that the
number of exchanges needed, and their timing, is up to the CDM. CDMs may
need more than one request / response exchange in order to establish the
keys. They may initiate additional exchanges at any time (e.g. for license
renewal). It seems consistent and unproblematic from an interop pov if the
number of exchanges is zero. It has always been possible that the CDM has
other information (from the initData, from other message exchanges) which
mean there is no exchange necessary. Perhaps the beginning of the content -
unbeknownst to the application - is not encrypted and the CDM learns this
from the initData. It seems arbitrary to require the number of message
exchanges at session start to be non-zero.

Regarding *Issue 53*: it's not clear to me that changes to the
specification are required to enable this, though I agree it warrants
discussion. Certainly I don't see any reason why this issue should block
either 52 or 41, which stand on their own. There is nothing in the
specification preventing key exchanges from delivering CDM-specific
information to the CDM that has a scope greater than the session in which
it is delivered. Such information would not be deleted when the session is
closed and could be used by other sessions. Again, the model is that the
CDM executes as many message exchanges as it needs (zero or more) whenever
it needs to and signals the status with respect to content keys to the
application via KeyStatus.


On Mon, Aug 10, 2015 at 10:37 AM, Jerry Smith (IEP) <jdsmith@microsoft.com>

> Keys in initData is specifically allowed by the ISO specification on
> Common Encryption.  I’ve included a reference on this in a comment on Issue
> #52 below.  I do not believe we should make the EME spec more restrictive
> than the controlling ISO CENC documentation.
> Keys distributed in initData support license renewal use cases with
> seamless and efficient key distribution.  This method is commonly used by
> streaming media services that use CENC today.  We should align the EME spec
> to support it.
> My recommendation is that we approve and implement issue #52 now, and
> review and confirm the algorithm changes in issue #41 to support them.
> Jerry
> *From:* David Dorwin [mailto:ddorwin@google.com]
> *Sent:* Sunday, August 2, 2015 9:09 PM
> *To:* Joe Steele <steele@adobe.com>
> *Cc:* <public-html-media@w3.org> <public-html-media@w3.org>
> *Subject:* Re: Resolving issues around avoiding key requests
> 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.
> Questions:
>    - 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 Wednesday, 12 August 2015 23:20:09 UTC

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