encryptionScheme

Encryption scheme this header declares, e.g. AESCTR or AESCBC, or null when the header does not say.

PlayReady spells it ALGID — document-level under <PROTECTINFO> in v4.0.0.0, and per-KID from v4.2.0.0 on. Widevine spells it protection_scheme, a 4CC packed into a uint32. It used to be dropped on conversion and replaced with a hardcoded AESCTR.