Re: [EME] specification question: key Sytem selection

On Jul 2, 2012, at 2:42 AM, Zhouhaojun wrote:

Hi Mark,

I think Eric’s question is that it’s not clear how UA get to know the key systems supported by the file.

The UA, together with the CDMs it supports, need to get that information from the file. File format specifications need to define how they signal the encryption system used and content protection mechanisms supported by the file.

For example the mp4 file format has various fields which describe the Protection Scheme (in mp4 language) that the file supports. If this Protection Scheme is "Common Encryption" (cenc) then the Protection System Specific Information (push) boxes tell you which actual Protection Systems ( = keysystems) are supported.


Here is my understanding: UA can get the information of key systems from the HTMLSourceElement.keySystem (which is mentioned explicitly in the draft)  or from the metadata in the file (which is not mentioned in the draft explicitly, maybe we need some words to explain it).

How UA get initData is not clear too.

It has to get it from the file. We assume that if the UA supports use of encrypted media with file format X then it will know enough about that file format to get the initData out.

See example 7.2.2 “other content decryption module”, the app get initData from key needed event before calling generateKeyRequest. So it seems that UA can get the initData from the file and set the MediaKeyNeededEvent.initData. Some clarifying words may be needed also.

Btw, can MediaKeyNeededEvent.keySystem contain multiple supported key systems, so app can select preferred one from them?

No, we just leave the keysystem as null here and the app needs to try the ones it supports as I described.

Since the functional split between UA and CDM is not fully defined in the specification, it could be that determining if a file supports a keysystem requires some work by the CDM. So we don't want to require the UA to do that for every keysystem. With the current design it is done only for the keysystems that the app supports and in app preference order.

…Mark



Best Regards,

Haojun Zhou
Email: zhouhaojun@huawei.com<mailto:zhouhaojun@huawei.com>
Huawei Nanjing R&D Center, 101 Software Avenue, Yuhuatai District, Nanjing, Jiangsu, PRC.



发件人: Mark Watson [mailto:watsonm@netflix.com]
发送时间: 2012年7月1日 11:43
收件人: Sunyang (Eric)
抄送: public-html-media@w3.org<mailto:public-html-media@w3.org>
主题: Re: [EME] specification question: key Sytem selection

The key system is selected with the call to generateKeyRequest(). If that keysystem is not supported (by the browser, or by the file) then the call will fail and the app can try a different keysystem.

Note that all three of the app, the file and the browser may support multiple keysystems. Only a keysystem in the intersection of these three sets may be used. There may still be more than one. The app needs to choose its favorite from that set, as there may be commercial implications of the choice for the app. The way we chose to implement that was by allowing the app to attempt generateKeyRequest with different key systems, in preference order, until one succeeded. That seems actable given that the set of keysystems in the intersection I describe above is likely to be very small (coming up with an example where there are 3, say, is already a challenge).

…Mark

On Jun 29, 2012, at 8:30 PM, Sunyang (Eric) wrote:


After reading the specification, I have a question on Key System selection.

If media element’s source attribute has a keySystem attribute then we use this as selected keySystem, this is OK.
If no selected KeySystem, the specification says “jump to Key Presence”

The question is :

At “jump to Key Presence” is a else if of “selected key system”, so here we have no idea about key system,
We may need let app or CDM to analyze the initData to find the KeySystem, but I have not seen any text for
Analyzing initData here, or I miss something?

So where we select the Key System if no selected key system in media element?


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