CBCS is the AES-CBC encryption scheme of Common Encryption, using cipher-block-chaining with pattern encryption (encrypting a recurring pattern of blocks rather than every byte). It is the scheme FairPlay requires, and it has become the modern convergence target for cross-DRM, single-copy delivery.
The significance is interoperability. The older cenc scheme (AES-CTR) and cbcs are not interchangeable on the same bytes, so when FairPlay demanded cbcs and Widevine/PlayReady used cenc, platforms had to encrypt and store content twice. Once Widevine and PlayReady added cbcs support, a single cbcs-encrypted CMAF set could finally be unlocked by all three DRMs — halving packaging, storage, and CDN cache footprint.
Pattern encryption (e.g. encrypt 1 of every 10 blocks) was designed to reduce decryption overhead on constrained devices while keeping the content effectively unusable without the key. For new deployments, standardizing on cbcs is the default recommendation.
