Re: Key distribution

[Moving to public-html-media]

Sent from my iPhone

On May 10, 2013, at 6:21 AM, Casey Callaghan <caseyc37@gmail.com> wrote:

Having a look over the documentation on EME (encrypted media extensions), I
find the following:

> The user should not be restricted from accessing content for which legal
rights have been obtained.

(source: https://dvcs.w3.org/hg/webtv/raw-file/tip/mpreq/cpreq.html)

I also find the following statement in the First Working Public Draft (
https://dvcs.w3.org/hg/html-media/raw-file/tip/encrypted-media/encrypted-media-fpwd.html
):

> Support simple decryption without the need for DRM servers, etc.

This is a necessary corollary of the previously quoted statement; if
servers are needed to view legally purchased content (even if only to
obtain decryption keys), then the legally purchased content will be
unavailable if and while said servers are down.

However, as soon as secure decryption is discussed, I find that a DRM
server begins to form a vital part of the process. I have no doubt that
many content providers will accept only the most secure decryption methods
for their content; this leads to well-known problems should the content
provider's servers ever go offline.

This can be mitigated, to some degree, with multiply redundant servers or
cloud computing. However, these solutions may be expensive and are unlikely
to be kept running when it would be unprofitable to do so (for example,
when the sales of a given piece of media have ended; possibly after an
interval after that ending). This could also be impractical for smaller
content providers, without large budgets.

Therefore, in order to resolve this, I would like to propose for
consideration the following idea (based on the serverless encryption scheme
for Bitcoin):

- that, when a user purchases legal access to a given piece of media, a
message (signed with the content provider's private key) must be sent to
all clients informing them of this purchase;
- that all clients may (and are indeed encouraged to) keep a record of all
such messages from all providers;
- that any client, in possession of both the signed message from the
content provider (verified by means of the content provider's public key)
giving a given user legal permission to view certain media, and the data
required to decrypt that media (either the CDM or the key obtained from the
same content provider), may provide either the CDM, or the key, or both to
the user on authorised request.
- that any client which does so must inform the content provider's server
and all other clients of such access, if the key is limited in any way.

In this way, a DRM server going offline does not prevent a user from
viewing content to which they purchased a valid license before the server
went offline. This appears to be a necessary consequence of the stated aims
of this standard.


You are making an assumption that the legal rights purchased are perpetual
and independent of the continued operation of the service from which they
are purchased.

This may not be true in all cases and indeed may never be true for exactly
the reasons you give above.

For example, in the case of Netflix, the legal right to watch the content
extends for only 8 hours or until the Netflix client application is closed,
whichever is sooner. After that the right must be requested again and this
is only possible if Netflix servers are still operating and you are still a
subscriber.

I would agree that the mechanism above is an interesting concept for
perpetual, service-independent rights, but support for this case is not one
of our requirements (as discussed in the bug titled 'EME depends on servers
with a finite life' - I don't have the number to hand).

...Mark

Casey