  In the era of data-driven economies, incentive systems and loyalty programs,
have become ubiquitous in various sectors, including advertising, retail,
travel, and financial services. While these systems offer advantages for both
users and companies, they necessitate the transfer and analysis of substantial
amounts of sensitive data. Privacy concerns have become increasingly pertinent,
necessitating the development of privacy-preserving incentive protocols.
Despite the rising demand for secure and decentralized systems, the existing
landscape lacks a comprehensive solution. We propose the Boomerang protocol, a
novel decentralized privacy-preserving incentive protocol that leverages
cryptographic black box accumulators to securely store user interactions within
the incentive system. Moreover, the protocol employs zero-knowledge proofs
based on BulletProofs to transparently compute rewards for users, ensuring
verifiability while preserving their privacy. To further enhance public
verifiability and transparency, we utilize a smart contract on a Layer 1
blockchain to verify these zero-knowledge proofs. The careful combination of
black box accumulators with selected elliptic curves in the zero-knowledge
proofs makes the Boomerang protocol highly efficient. Our proof of concept
implementation shows that we can handle up to 23.6 million users per day, on a
single-threaded backend server with financial costs of approximately 2 USD.
Using the Solana blockchain we can handle 15.5 million users per day with
approximate costs of 0.00011 USD per user. The Boomerang protocol represents a
significant advancement in privacy-preserving incentive protocols, laying the
groundwork for a more secure and privacy-centric future.
