Common Encryption Schemes for Secure AV1 Distribution

This article explores the Common Encryption (CENC) scheme implementations used to protect AV1 video streams across modern digital rights management (DRM) ecosystems. It covers the standardization of AV1 protection within ISO Base Media File Format (ISOBMFF) and Common Media Application Format (CMAF) containers, details the primary encryption schemes (cbcs and cenc), and explains how Open Bitstream Units (OBUs) are encrypted at the subsample level to ensure broad cross-platform compatibility.

Standardization and Container Framework

Securing the AV1 video codec relies on the ISO/IEC 23001-7 standard, commonly known as Common Encryption (CENC). The Alliance for Open Media (AOMedia) defines the mapping of AV1 bitstreams into ISOBMFF and CMAF containers. Under this framework, media protection metadata—such as encryption algorithms, key identifiers (KIDs), initialization vectors (IVs), and subsample mapping—is stored within standard ISOBMFF protection boxes (such as sinf, senc, and pssh). This allows multiple DRM systems (Google Widevine, Microsoft PlayReady, and Apple FairPlay) to decrypt the same media file using a shared decryption methodology.

The Primary CENC Schemes: cbcs vs. cenc

While ISO/IEC 23001-7 defines four distinct encryption schemes, secure AV1 distribution in commercial streaming relies almost exclusively on two:

1. cbcs (AES-CBC with Pattern Protection): The cbcs scheme uses the Advanced Encryption Standard (AES) in Cipher Block Chaining (CBC) mode with a 10% pattern protection scheme (typically a 1:9 crypt-to-skip pattern, where one 16-byte block is encrypted followed by nine 16-byte unencrypted blocks). This scheme has become the preferred standard for modern AV1 delivery because it enables unified CMAF delivery across all major platforms. Apple FairPlay exclusively requires cbcs, and recent versions of Google Widevine and Microsoft PlayReady provide native support for it on AV1-capable hardware.

2. cenc (AES-CTR Counter Mode): The original cenc protection scheme uses AES in Counter (CTR) mode without pattern protection, encrypting targeted payload bytes completely. Historically, cenc was the default implementation for MPEG-DASH streaming on Android, Windows, and smart TV platforms. While widely supported across legacy devices, maintaining cenc alongside cbcs requires dual-packaging and increased storage overhead. Consequently, modern AV1 pipelines often prioritize cbcs unless legacy hardware support mandates AES-CTR.

Subsample Encryption for AV1 Bitstreams

AV1 relies on Open Bitstream Units (OBUs) to structure its video data. Rather than encrypting the entire media sample, Common Encryption implementations for AV1 use subsample encryption. This technique selectively encrypts only the sensitive compressed video data while leaving structural metadata exposed for container-level processing.

• Clear Headers: OBU headers, Sequence Headers, Metadata OBUs, and Frame Header OBUs remain completely unencrypted. This allows demuxers, parsers, and decoders to read essential playback parameters (such as resolution, profile, and timing information) without requiring access to DRM keys.
• Encrypted Payloads: Tile Group OBUs and Frame OBUs containing the actual transform coefficients and motion vectors are encrypted. Within these units, the uncompressed header data remains in the clear, and the encrypted range covers the compressed tile data.
• Initialization Vector Alignment: Depending on whether cbcs or cenc is used, IVs are reset per sample or subsample, ensuring cryptographic integrity across segments.

Industry Convergence and Unified Delivery

Secure AV1 distribution increasingly converges on CMAF combined with the cbcs encryption scheme. Because modern consumer silicon with hardware AV1 decoders also features universal support for AES-CBC with pattern protection, content distributors can package AV1 content a single time. By referencing distinct DRM vendor identifiers within the pssh (Protection System Specific Header) boxes, a single set of encrypted AV1 chunks can be decrypted by Widevine, PlayReady, or FairPlay clients across web, mobile, and connected television ecosystems.