CTR refers to the AES-CTR (counter mode) protection scheme of Common Encryption, identified by the four-character code 'cenc'. It was the original scheme used by Widevine and PlayReady, and it is not byte-compatible with the cbcs (AES-CBC) scheme that FairPlay requires.
That incompatibility is the historical reason platforms once had to encrypt and store premium content twice - cenc for Widevine/PlayReady and cbcs for FairPlay. The industry has since converged toward cbcs so a single CMAF copy can serve all three DRMs, but cenc/CTR remains widely deployed and is essential to understand when reasoning about why a stream may not be interoperable across DRM systems.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
The CTR (cenc) scheme is not byte-compatible with the cbcs scheme FairPlay needs, which is why many platforms store their catalog twice. The seven volumes cover the video stack; for CTR start with Volume 3 on DRM and players, then Volume 2 on CMAF, HLS, and DASH packaging, with encoding in Volume 1. Written by Nikolay Sapunov, CEO at Fora Soft.
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