Re: Secure release and persistence

On Wed, Apr 29, 2015 at 9:22 AM, David Dorwin <ddorwin@google.com> wrote:

>
> On Tue, Apr 28, 2015 at 12:07 PM, Jerry Smith (WINDOWS) <
> jdsmith@microsoft.com> wrote:
>
>>  This makes sense to me, Mark.  You are looking for markers to bound the
>> usage of keys so that keys used for extended playback can be distinguished
>> from keys used to start, but not watch content.  Persisting first and
>> latest decrypt times accomplishes this.
>>
>
> I'm not sure what you're referring to with these two types of keys, but I
> don't think Mark has mentioned anything about different types of keys. As
> far as I know, the Netflix use case is for a single key used for the entire
> duration of the content. Clearly, there is still a lot of confusion (on one
> or both of our parts) about what exactly this feature is and what it is
> used for.
>

​I didn't sense any confusion in Jerry's mail. I think this comment is a
little self-serving. He is right that the secure release information can
give us reliable information about how long each key is used for. There are
no different "kinds" of keys, but there can be multiple licenses involved
in a session.​

...Mark



>
>>
>> Jerry
>>
>>
>>
>> *From:* Mark Watson [mailto:watsonm@netflix.com]
>> *Sent:* Tuesday, April 28, 2015 8:02 AM
>> *To:* public-html-media@w3.org
>> *Subject:* Secure release and persistence
>>
>>
>>
>> All,
>>
>>
>>
>> Hopefully this will catch you in your newly-freed "EME hour" :-)
>>
>>
>>
>> As promised at the F2F I will draft spec updates this week to fill in the
>> missing details of this feature. However, I would like to make one
>> significant change in response to comments at the F2F and previously
>> regarding exactly what is persisted.
>>
>>
>>
>> The spec describes persistence of a "record of license destruction".
>> Aside from the fact that we have no concept in our spec of "license", this
>> is not typically how this feature is implemented in DRMs and suggests a
>> need to persist data at close / shutdown / crash or other events that cause
>> license destruction.
>>
>>
>>
>> In practice, what is persisted  is a record of available keys at a point
>> in time. This is updated regularly *during streaming*. There is no update
>> on close / shutdown / crash. In a later browsing session, this record is
>> compared with the actually available keys and a discrepancy is taken as
>> evidence that keys were destroyed.
>>
>>
>>
>> I propose that we describe this persisted data explicitly as "for each
>> key in the session, the *first decrypt time* and the *latest decrypt
>> time*".*
>>
>>
>>
>> Are there any comments on that before I implement it ?
>>
>>
>>
>> Based on this change in description, I suggest we call this session type "
>> *tracked*" - or something similar - since really we are 'tracking' the
>> usage of keys. This name will also invite appropriate scrutiny of the
>> persisted data properties.
>>
>>
>>
>> ...Mark
>>
>>
>>
>> * there may of course be other CDM-specific information persisted and
>> included in the release message. For example if the CDM has a concept of
>> licenses, then license correlation identifiers may be present. Also, how
>> the CDM communicates time in its messages may be CDM-specific.
>>
>>
>>
>>
>>
>>
>>
>
>