CENC (common encryption)
CENC (common encryption) is an MPEG standard that sets one shared way to encrypt video, so a single encrypted file can be unlocked by different DRM systems. Its cbcs mode is the version that works with Apple FairPlay, Google Widevine and Microsoft PlayReady together. Without CENC, a service would need a separate encrypted copy of every video for each DRM.
Why it matters
Without CENC, a service would encrypt and store a separate copy of each video for each DRM system. That multiplies storage, packaging time and CDN cache use. CENC lets one encrypted copy serve every device, which cuts cost and simplifies operations for any multi-DRM service.
How it works
- The video is split into samples (single frames or chunks of audio), and parts of each sample are encrypted with AES-128.
- Metadata in the file records which parts are encrypted and the ID of the key used.
- Each DRM system adds its own signaling data, so its player knows how to request a license.
- The player gets the key from its own DRM's license server and decrypts the shared file.
CENC cbcs vs cenc: what is the difference?
Both are encryption schemes inside the common encryption standard. The cenc scheme uses AES counter mode, while cbcs uses AES cipher block chaining with a pattern that encrypts only some blocks. Apple devices use cbcs, and modern Widevine and PlayReady devices support it too, so cbcs is the usual choice when one file must play everywhere. Some older devices support only cenc.
Is CENC a DRM?
No, CENC is not a DRM system. It is a standard way to encrypt media files so that more than one DRM system can unlock them. The DRM systems, such as Widevine, FairPlay and PlayReady, handle licenses, keys and playback rules. CENC simply makes sure they can all work with the same encrypted file.
Where it's used
OTT services and online learning platforms package each title once in CENC's cbcs mode, so the same encrypted copy plays on iPhones, Android, Chrome and smart TVs, cutting storage and packaging work.
By the numbers
- ISO/IEC 23001-7 specifies AES-128 encryption of media samples in two protection schemes, counter mode (CTR) and cipher block chaining (CBC). Its fourth edition was published in August 2023. (ISO: ISO/IEC 23001-7 Common encryption)
- Apple's HLS authoring specification requires video encrypted with common encryption to use a 1:9 encrypt:skip pattern, meaning 10 percent partial encryption. (Apple Developer: HLS Authoring Specification for Apple Devices)
