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Making Sense of CMAF and Common Encryption

Posted by Oleksandr Paraska on Dec 15, 2016 4:34:18 PM



A recent article on Streaming Media magazine brought up a point that Common Media Application Format (CMAF) is not good enough when it comes to encrypted content. At BuyDRM we are pretty excited about CMAF, and we have adopted a slightly more optimistic point of view.

[Please see update made on this topic on June 14th, 2017 here]

The idea of Common Media Application Format has been around for quite some time already. BuyDRM has been moving towards it since 2012. Those days it wasn’t possible to download Smooth Streaming assets in Silverlight and then play them back offline. So BuyDRM’s KeyOS Player downloaded the highest possible bitrate chunks from a stream and stored them all in one file together with the manifest. In its essence CMAF is a standardization of that idea of combining asset fragments and a manifest in one file.

Retrospectively, the CMAF takeoff event was Apple announcing support of fMP4 chunks in HLS on their platforms. That gave hope to the idea of “one silo to stream them all”, which was the industry’s Holy Grail for quite some time already. It meant it would be possible to stream both HLS and DASH from the same source, which accordingly, meant cutting storage and overhead prices in half.

CMAF and Common Encryption

CMAF resolves a lot of challenges indeed. However, there is one nuance in section 8.2 of CMAF specification, which defines content encryption. On one hand, it merely defines that content has to be encrypted using Common Encryption, which seems fairly straightforward. But now, with Apple and its preference of AES-CBC encryption in the mix, the Common Encryption specification itself has evolved to include both AES-CBC and AES-CTR encryption schemas. This means that a CMAF file can be encrypted in 2 different ways, which are mutually incompatible.

It would seem that just like that, CMAF will fail to deliver on its promise of a unifying file format. If it is still needed to store 2 versions of one asset the improvement isn’t exactly noticeable. Concluding that would be a pessimistic point of view, however. The saving grace of CMAF is that it will allow for flexibility and it will be up to DRM systems operators to adapt to the constantly changing marketplace. As of now, Google Widevine has already added support for CBC encryption in Android N and on the Chromecast platform which means no matter what encryption schema is used, your content will play back on Google's Android platform and platforms that support Widevine.

With the CMAF standard rumored to be ratified in February, 2017 we are still waiting for clarification from major OEMs like Microsoft, Google, and Apple regarding support for both encryption schemas. The fact that neither Microsoft nor Apple have publicly presented their vision yet and that both of them are initiators of the CMAF specification suggests CMAF may still be saved.

Taking all of this into account, the future compatibility between CMAF and CENC seems a likely reality in 2017. 

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