MPEG-CENC: Defective by Specification (2024)

A security analysis presents DeCENC, a generic exploit against the MPEG-CENC encrypted media container that demonstrates a fundamental design flaw: use of encryption without authentication. DeCENC enables decryption of encrypted video streams without direct knowledge of the content key by manipulating codec-level features in widely used codecs (h264/AVC and h265/HEVC). The work explains how common DRM architectures and Content Decryption Modules (CDMs) fit together, catalogs practical attack vectors (analog capture, HDMI interception, exfiltrating decrypted-but-undecompressed frames, stealing content keys or CDM secrets), and supplies a proof-of-concept implementation with testing guidance for ClearKey on GitHub. Explanations are self-contained because relevant MPEG specifications are paywalled.

Technically, the exploit constructs specially crafted video bitstreams that abuse features such as I_PCM macroblocks, NAL emulation-prevention bytes, chroma subsampling and limited-range color to induce predictable plaintext from AES-CTR-encrypted data, then substitutes or forges video stream units and metadata to coax decoders into revealing decrypted content. The write-up details AES-CTR background, bitstream crafting, metadata preparation, stream substitution, capability limits and practical mitigations, and concludes with reflections on why a specification-level omission of authentication makes encrypted containers inherently brittle in real-world DRM deployments.