Re: [EME] Updated proposal for secure proof of key release

I filed a couple bugs to track some minor issues with the existing
spec. Comments inline.

On Wed, Jan 16, 2013 at 1:45 PM, Mark Watson <watsonm@netflix.com> wrote:

>
>  On Jan 16, 2013, at 12:20 PM, David Dorwin wrote:
>
> Hi Mark,
>
>  Comments and suggestions inline.
>
>  David
>
> On Tue, Jan 15, 2013 at 9:36 AM, Mark Watson <watsonm@netflix.com> wrote:
>
>> Here are some additional changes which I think are required to implement
>> David's alternative proposal for [1]
>>
>>  Address [2].
>>
>>  To Section 1.2.3 (SessionID):
>> - To "If session IDs are supported, a new one will be generated each time
>> createSession() successfully creates a MediaKeySession" add "for a new key
>> session."
>> - Change "although the CDM may retain them for longer periods" to
>> "although the CDM may retain key sessions and their associated IDs for
>> longer periods.".
>> - Change "If secure proof of key release is supported each Session ID
>> shall be unique within the origin." to "If the CDM retains key sessions
>> across browsing sessions then each Session ID shall be unique within the
>> origin."
>>
>
>  I suggest changing  "If" to "When". If a CDM supported both retained and
> unretained sessions, it does not *always* have to use unique IDs.
>
>
>  Ok.
>
>
>
>>  To David's createSession() proposal (see [1]):
>> - To "Let key request be a key request generated by the CDM using
>> initData, if provided" add "or null if no key request is generated."
>>
>
>  So keymessage's message attribute can be null? That affects the event
> definition (3.1) and seems undesirable. My proposed patch just says a
> response is generated and assigned to the message attribute. That can be an
> empty array in the key release case. Do we really need to support null or
> specify *what* the value is?
>
>
>  Empty array might work.
>
>  If we say that createSession() can retrieve an old session, and we say
> this is generic, in support of "whatever capabilities CDMs want to offer",
> not just specifically for secure proof of key release, then there is the
> case of a session being retrieved in a state where no messages need to be
> sent. For example a persisted session that already has an unexpired license
> matching the supplied initData. But we need to generate some kind of event
> to indicate when the sessionId has been assigned. A null message seemed
> reasonable for this.
>
> Sending keymessage to indicate "the CDM did what the app asked but has
nothing to send to the server" seems useful and null/empty seems like a
good normative definition for this. The only question is whether to use
null or an empty array. If an app gets a null value, it may not handle an
exception and stop working. If it gets an empty message and sends it to the
server, it would seem things have a better chance of succeeding and the
server can detect the issue. I'd like to hear from others that have more
experience with these types of issues and API designs. In the end, it's a
small issue. I opened https://www.w3.org/Bugs/Public/show_bug.cgi?id=20689 to
track this discussion.

>
>   - To "Note: cdm must not use any data, including media data, not
>> provided via initDate" add "or already present in the cdm".
>>
>
>  This is a note about creating the key request. Since one is not created
> in your use case, I don't think this applies. More importantly, such a
> change has much larger implications than key release. You can instead add
> "to generate a key request" to the current text if you want.
>
>
>  Hmm - I didn't read carefully enough where is refers to "key request".
> If we're making this generic, shouldn't it just be "key message" ? In the
> secure proof of key release case, where I retrieve a session related to a
> deleted key, the message will be the secure proof of key release message.
>

The purpose of this text was to enforce that the information about the
media that is used to generate a key request be provided via the initData
parameter. This doesn't necessarily apply to other uses of createSession().
Thus, by adding "to generate a key request", we still enforce this for key
requests while lifting the limitation for other uses.

>
>
>   - Add to the above note: "The CDM may use pre-existing session data
>> that matches the provided type and initData and that is not already
>> associated with a MediaKeySession to initialize the session."
>>
>
>  This seems pretty confusing. Is it necessary? Initializing a session is
> key system-/CDM-specific and nothing prevents the scenario in the proposed
> text.
> As mentioned above, the existing normative note is specifically related to
> generation of a key request, so I definitely don't think they should be
> folded together.
>
>
>  I think we need to say something too explain how you can retrieve old
> sessions in a CDM-independent way.
>
>
>
>>  New FAQ and answer:
>>
>>  "Is secure management of the entire key lifecycle supported ?
>>
>>  Yes.
>>
>>  If a CDM implements the capability to provide a secure proof of key
>> destruction (i.e. a signed attestation from the CDM that a given key has
>> been destroyed), this capability may be exposed by generating a keymessage
>> event when the key is destroyed, containing this attestation. The CDM
>> should persist a record of the session and the key destruction, in case the
>> keymessage cannot be delivered for any reason, for example network loss or
>> inability to deliver messages during page close. This persisted record
>> should be removed when an acknowledgement of the secure proof of key
>> release is received through an update() method. MediaKeySessions associated
>> with such persisted sessions could be retrieved at a later time (e.g. after
>> page unload and later page load) using the createSession() method with null
>> type and initData which implicitly match any persisted session for the page
>> origin."
>>
>
>  I think null type is a strong and potentially generally useful value
> that should not be dedicated to key release
>
>
>  I'm not suggesting it's dedicated to key release. It would match any
> persisted session, whatever state that is in.
>

Okay. My statement still applies to "persisted session". I think the use of
"null" would have to really stand out as "of course, that makes sense."
As it stands, we allow null but don't really have a use case and it's
basically there for key system-specific use. Maybe we should make it a
required parameter. I filed
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20691.

>
>   nor have its behavior defined in a FAQ or other non-normative text.
>
>
>  Actually it should be defined normatively that "null" matches
> everything, rather than nothing.
>

|type| is intended to tell the CDM how to interpret |initData|. I'm not
sure "everything" makes sense in this context.

>
>   (Key systems may wish to use it for something else.) Is there any
> reason not to use type = "keyrelease" from your original proposal?
>
>
>  The original proposal was normative. I agree that we should try and find
> a way where secure proof of key release is not specified normatively, but
> can be built using the generic normative capabilities, in the same way as
> heartbeat etc. But that means we need to define those generic normative
> capabilities - specifically the retrieval of pre-existing sessions, which
> is what I was trying to do above.
>
>  Certainly a CDM "can" provide any capability it wants - but I think we
> need at least the basic concepts in the API in order for people to
> understand how the capability makes sense in the context of the API and to
> ensure some commonality across implementations. Otherwise it just looks
> like secure proof of key release is a hack, not something which was
> considered as a reasonable capability to be supported in the API.
>
>
>  Minor note: "release" and "destruction" seem to be used interchangeably.
> Is that intentional?
>
>
>  No.
>
>
>
>>
>>  …Mark
>>
>>  [1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=17199
>> [2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=20622
>>
>>  On Jan 15, 2013, at 8:03 AM, Mark Watson wrote:
>>
>>   I think we also need to address
>> https://www.w3.org/Bugs/Public/show_bug.cgi?id=20622
>>
>>  I will provide text for the FAQ answer.
>>
>>  As for requesting old sessions, I think we need to explicitly introduce
>> the idea that CDMs may persist sessions and so createSession() may retrieve
>> a preexisting session based on the type and initData. If the provided
>> type/initData match multiple pre-existing sessions, then any one is
>> returned (and so if they are both null, than any pre-existing session is
>> returned). We should also ask that a keymessage is always sent (even if it
>> might be null) when the session is created so that the user script has some
>> indication when the sessionId is assigned.
>>
>>  …Mark
>>
>>  On Jan 14, 2013, at 6:19 PM, David Dorwin wrote:
>>
>>  Mark and I discussed this and agree that we should try to reuse as much
>> of the base algorithms as possible. Mark would also like a consistent way
>> to retrieve saved key sessions and a FAQ answer. Therefore, I propose the
>> following tasks for resolution for this bug:
>>
>>    - Generalize createSession() algorithm so proposed 4.5.1 is covered
>>    by it. There are a few statements that assume generation of licenses.
>>    - Figure out where to document how to request old sessions, if
>>    supported.
>>    - Add a FAQ entry about how key release could be supported, similar
>>    to the entry for heartbeat.
>>
>>
>>  I’ve started working on the createSession() changes.
>>
>>  David
>>
>> On Fri, Jan 11, 2013 at 6:10 PM, Mark Watson <watsonm@netflix.com> wrote:
>>
>>> Hi David,
>>>
>>>  Please see inline …
>>>
>>>   On Jan 11, 2013, at 10:07 AM, David Dorwin wrote:
>>>
>>>   Hi Mark,
>>>
>>>  The more I think about key release, the more I think it should not be
>>> included in the spec.
>>>
>>>  Key management, policies, message protocol & structure, and key-system
>>> or CDM-specific details have been intentionally left out of the spec. As an
>>> example, even the key replacement algorithm is not specified. Key systems
>>> are free to define protocols that include a variety of messages as long as
>>> they fit within the defined APIs (primarily update() and keymessage).
>>> Heartbeat, license renewal, key rotation, etc. can all be implemented
>>> within the existing framework but are not included in the spec.
>>>
>>>  The key release proposal does not require altering the existing APIs
>>> or the defined algorithms. It simply defines additional key management,
>>> policy, parameter values, and messages, all of which are key
>>> system-specific. As with the basic message flow, heartbeat, etc., I expect
>>> that key systems will eventually converge on a set of best practices, but
>>> these are not usually defined in a standards-track spec.
>>>
>>>
>>>  The proposal does expose functionality at the API level and it's not
>>> all just key-system specific.
>>>
>>>  Specifically, the concept of "sessions" and "sessionIds" is an API
>>> concept, not a key-system-specific one. We say that anything that goes on
>>> *within* a session is keysystem-specific. For example, the number of
>>> message exchanges, when they occur in the session, what they do in terms of
>>> keys etc. Heartbeat and key rotation are both examples of where - within a
>>> session - some keysystem-specific magic can go on.
>>>
>>>  But the notion of a session itself is part of our API. We define how
>>> you create them in a key-system-specific way. They're created from a
>>> keysystem-independent object (the initData - although within it there may
>>> be multiple keysystem-specific parts).
>>>
>>>  The key part of the secure proof of key release proposal is that it is
>>> possible, from outside any session, to retrieve information related to
>>> pre-existing sessions. We're adding a new way to interact with the CDM
>>> outside the context of a session. I don't see how we could do that within
>>> the existing API.
>>>
>>>
>>>  Finally are there implementors and CDM providers that intend to
>>> support this? Are there any content providers besides Netflix that intend
>>> to use it?
>>>
>>>
>>>  Those are good questions. If the answer to the first one is no when we
>>> get to the appropriate milestones in the specification work, we'll have to
>>> pull this out. Just like anything else.
>>>
>>>  I'm not really empowered to speak on behalf of CDM implementors, but I
>>> know that it's not pointless putting this in the specification now.
>>>
>>>  …Mark
>>>
>>>
>>>  I do not think a Working Draft should include a feature where the
>>> answer to the above questions is "no", especially when it adds non-trivial
>>> complexity to implementations.
>>>
>>>  Regards,
>>> David
>>>
>>> On Fri, Jan 11, 2013 at 9:10 AM, Mark Watson <watsonm@netflix.com>wrote:
>>>
>>>>  https://www.w3.org/Bugs/Public/show_bug.cgi?id=17199
>>>>
>>>>  An updated proposal is attached.
>>>>
>>>>  I'd like to propose that we include this text in the First Public
>>>> Working Draft.
>>>>
>>>>  [It need not be Section 4, as written, since this interrupts the flow
>>>> of the rest of the document. I leave it to David to suggest the best place
>>>> for it.]
>>>>
>>>>  …Mark
>>>>
>>>
>>>
>>>
>>
>>
>>
>
>