🤖 AI Summary
This work addresses the challenge of achieving transaction privacy on Algorand—a high-throughput public blockchain—where smart contracts face severe resource constraints and lack global state-based concurrency control. The authors propose a decentralized, non-custodial privacy protocol that leverages LSAG ring signatures with on-chain verification to ensure anonymity. Innovatively, they replace the conventional global Merkle accumulator with an O(1) commitment membership check based on Box Storage. To overcome opcode budget limitations, an embedded application call pool is introduced for dynamic budget expansion, enabling, for the first time on Algorand, efficient signature-obfuscating privacy without trusted setup. By integrating BN254-based cryptography with lightweight on-chain verification, the protocol delivers strong privacy guarantees and selective auditability while preserving Algorand’s high throughput, all without relying on zk-SNARKs or trusted setup assumptions.
📝 Abstract
While public blockchains provide transparent and auditable transaction histories, they inherently compromise user privacy. Existing privacy-enhancing protocols, such as those deployed on Ethereum, typically rely on succinct zero-knowledge proofs (zk-SNARKs) to obscure the transaction graph. However, implementing comparable cryptographic guarantees on high-throughput blockchains like Algorand is challenging due to strict per-call execution budgets and the state contention introduced by global Merkle accumulators. This paper presents Obscura, a decentralized, non-custodial privacy protocol tailored for constrained smart contract environments. Obscura achieves transaction anonymity using Linkable Spontaneous Anonymous Group (LSAG) signatures over the BN254 elliptic curve, verified entirely on-chain. To overcome limitations of the Algorand Virtual Machine (AVM), we introduce a novel state model that leverages Algorand's Box Storage for $O(1)$ commitment membership checks, eliminating the need for global Merkle accumulators, and a dynamic opcode-budget expansion mechanism via pooled inner application calls. Our implementation demonstrates that signer-ambiguous privacy is practical and efficient on Algorand without relying on trusted setups or succinct proofs. Obscura provides a robust privacy layer for transparent ledgers, bridging the gap between high-throughput blockchain architectures and the dual requirements of cryptographic privacy and selective auditability.