Boomerang: A new privacy-preserving protocol from Brave Research to decentralize Brave Rewards

Brave Rewards

Brave Rewards allows users to earn Basic Attention Tokens (BAT) for privacy-preserving ads—called Brave Ads—they choose to see in the Brave browser. Users can control how many ads they see, and easily support their favorite content creators with on-demand and automatic contributions using their BAT. Users’ browsing data remains private and never leaves their device. Users can also use earned BAT to buy gift cards, exchange for other currencies, and more.

Privacy Pass

Brave Rewards currently uses a privacy-preserving protocol called “Privacy Pass” as a key  component of its system, enabling users to earn rewards in a privacy-respecting way. For example, when users engage with Brave Ads or perform certain actions (such as viewing an ad or clicking on one), Privacy Pass generates cryptographic tokens (not to be confused with BATs) to validate these interactions without revealing specific details about the user’s identity or browsing habits.

These cryptographic tokens act as a form of proof that the user has completed the required actions, allowing them to earn Basic Attention Tokens (BAT) through Brave Rewards. By incorporating Privacy Pass, Brave Rewards ensures that users can participate in earning rewards without sacrificing their privacy, maintaining a balance between user anonymity and the verification of legitimate interactions.

Boomerang

Privacy Pass, while effective in preserving user privacy in systems like Brave Rewards, operates in a centralized manner, lacking the decentralized properties commonly sought in blockchain-based solutions.

Developed by the Brave Research team, the Boomerang protocol is a novel, decentralized, and privacy-preserving incentive protocol that leverages Black Box Accumulators and Zero-Knowledge Proofs to securely store user interactions. By utilizing this protocol, Brave can replace its current rewards system, based on Privacy Pass, with a more secure and decentralized alternative.

The Boomerang protocol simultaneously ensures privacy and verifiability by utilizing Zero-Knowledge Proofs based on Bulletproofs, and verifying the proofs in a smart contract on a Layer 1 blockchain. This approach allows users to prove their eligibility for rewards without revealing any sensitive information about the reward calculation (for example, about which specific advertisements the user interacted with), while still maintaining transparency and public verifiability. The protocol also employs Black Box Accumulators to securely store user interactions with Brave Ads, further enhancing privacy and security. 

Boomerang leverages Bulletproof zero-knowledge proofs as its fundamental mechanism, ensuring privacy while enabling verifiable reward generation. These zero-knowledge proofs are verified on a Layer 1 blockchain, ensuring public verifiability and transparency in the reward generation process. Figure 1 below shows the interaction between an user, the issuer (Brave) and a L1 blockchain that verifies the rewards proofs that are generated by the user. 

Figure 1: Local reward generation and public verification on a L1 blockchain smart contract.

The Boomerang protocol presents a significant advancement in privacy-preserving incentive protocols, paving the way for a more secure and decentralized future for Brave Rewards. Stay tuned for further developments.

Find out more about Boomerang in the technical report.