Accountable Decryption and Non-Delegatable DRM

This document distills a conceptual framework for a cryptographic protocol that enables the creation of new decryption keys for a single encrypted payload while strictly forbidding sub-delegation (the ability for a key-holder to mint further keys).

1. Core Cryptographic Requirements

The primary objective is a system where a central authority or master secret can issue “leaf-only” decryption capabilities.

• One Ciphertext, Many Keys: A single large payload is encrypted once, but can be accessed by multiple unique keys issued over time.
• Non-Delegation: Possession of a decryption key $sk_i$ must not provide the computational ability to derive a valid $sk_j$ or a master secret $msk$.
• Accountability: Since plaintext leakage is physically inevitable once decrypted, the protocol must ensure that any “pirate” decryption path is forensically attributable to a specific liable party.

2. Theoretical Models for Non-Delegation

Several cryptographic families approximate this “no-sub-delegation” operator:

• Identity-Based Encryption (IBE): A Private Key Generator (PKG) uses a master secret to derive keys for specific identities. Key-holders cannot compute keys for other identities.
• Traitor Tracing (TT): Specifically designed so that any “pirate decoder” built from a coalition of user keys can be traced back to at least one original key-holder.
• Functional Encryption (FE) / Obfuscated Decryption: Instead of a raw key, the user receives an obfuscated program that performs decryption but hides the underlying secret, preventing the extraction of a reusable “master” key.
• Proxy Re-Encryption (PRE): Uses non-transitive, unidirectional rules where a proxy transforms ciphertext for a specific reader without the reader gaining the power to re-delegate.

3. The “Ownership” Paradigm vs. Modern DRM

The discussion identifies a fundamental shift in the “topology of power” regarding digital rights:

• Modern DRM (Revocability-Centric): Built on the operator “access until revoked.” It prioritizes platform leverage, subscription gating, and vendor lock-in over actual security.
• Accountable DRM (Ownership-Centric): Built on the operator “durable access + liability.” Users “own” a non-revocable capability, but are legally and cryptographically liable if that capability is used to leak content.
• Spy-Grade Tradecraft: This model mirrors Cold War “canary traps” where micro-variations in documents (fingerprints) allowed for human attribution of leaks, now formalized through cryptographic primitives.

4. Emergent Fingerprinting and Signal Processing

A sophisticated implementation involves intertwining the decryption key with the signal processing stack:

• Keyed Decoders: The key $k$ defines a transform $T_k$ such that the output is a perceptually transparent but bit-wise unique version of the content.
• Emergent Properties: Fingerprints are not “added” to the file but are emergent properties of the decryption process (e.g., specific phase shifts, quantization biases, or dither patterns induced by the key).
• Collusion Resistance: Using fingerprinting codes to ensure that even if multiple users combine their data to “average out” the noise, the identity of the contributors remains recoverable.

5. Post-Quantum (PQ) Considerations

To ensure long-term durability of the “ownership” model, the protocol must be resistant to quantum adversaries:

• Lattice-Based Primitives: The most viable path for PQ-safe Traitor Tracing and Functional Encryption.
• Current State: While pairing-based Traitor Tracing is mature, lattice-based equivalents are currently in the research-grade phase and require further standardization for “off-the-shelf” use.

6. Gaps and Areas for Expansion

• Threshold Minting: Exploring how to distribute the “master secret” across a committee (MPC) to remove the “trusted server” single point of failure.
• Verification Latency: Determining the computational overhead of “emergent fingerprinting” on low-power playback devices.
• Legal Integration: Defining the standard of proof required to transition from a cryptographic “trace” to a legal “liability” claim.
• Incentive Alignment: How to incentivize platforms to adopt a model that reduces their leverage (revocability) in favor of user ownership.