[encrypted-media] Define key ID representation (byte order)

ddorwin has just created a new issue for 
https://github.com/w3c/encrypted-media:

== Define key ID representation (byte order) ==
`MediaKeyStatusMap` exposes key ID values to the application. We 
should clarify and eliminate all ambiguity about the representation of
 the IDs.

Since `keyStatuses` is populated from the license rather than media 
data, the representation could differ across implementations in a way 
that is visible to applications. For example, a license format using a
 GUID representation might need to take special care when exposing the
 IDs.

The simplest solution is probably to say that the bytes are to be 
presented as they would appear in media data (as defined by the 
container format spec). For example, the first byte in the stream 
would be byte 0 in the `BufferSource`.
Note: Any reference to the media data must refer to the common fields,
 such as the key ID to use for a given block, and not any 
system-specific fields, such as 'pssh' boxes, which could have the 
same issue.

Note: We may want to document the representation in/near the Key ID 
definition and refer to that from `MediaKeyStatusMap` to avoid any 
similar issues if key IDs are exposed in other ways. It may also be 
worth referencing in the Clear Key specification.

See https://github.com/w3c/encrypted-media/issues/68

Received on Tuesday, 7 July 2015 19:13:15 UTC