ZeeStar: Private Smart Contracts by Homomorphic Encryption and Zero-knowledge Proofs

Summary

C-Star is a system designed to enable data privacy in smart contracts on public blockchains, using homomorphic encryption and zero-knowledge proofs to keep sensitive data secure while allowing contract execution. Developers can implement private smart contracts without cryptographic expertise by using privacy annotations in a custom programming language. The system automates encryption and certifies correctness through zero-knowledge proofs, which can be compiled to Solidity contracts for Ethereum. C-Star optimizes gas costs with Graph16 zk-SNARKs, addressing efficiency challenges while ensuring privacy, though access patterns remain visible. It represents a practical solution for secure and private smart contract development.

Takeaways

• 😀 C-Star is a system designed to enable data privacy for smart contracts on public blockchains like Ethereum.
• 😀 Traditional public blockchains store data in clear text, which poses a problem for sensitive information like medical records.
• 😀 C-Star automates data privacy using homomorphic encryption and zero-knowledge proofs (zk-SNARKs), making it easier for developers to implement private contracts without cryptographic expertise.
• 😀 The system uses privacy annotations in a custom programming language to designate which data should be kept private, like balances in a blockchain application.
• 😀 C-Star distinguishes between 'self-owned' and 'foreign' data, ensuring that privacy restrictions are properly applied and respected during contract execution.
• 😀 C-Star automates the integration of cryptographic primitives, allowing developers to focus on logic without worrying about the complexities of cryptography.
• 😀 The system enables transactions like transferring encrypted balances between users using homomorphic encryption to ensure privacy during blockchain interactions.
• 😀 Developers can specify privacy requirements in C-Star's input contract, which are automatically compiled into Solidity code for Ethereum-compatible execution.
• 😀 The system guarantees privacy by ensuring that even if an attacker tries to tamper with the data, they cannot learn more than what is allowed by the privacy annotations.
• 😀 C-Star provides two main guarantees: correctness (ensuring contract logic is followed) and privacy (hiding sensitive data while maintaining transparency).
• 😀 The efficiency of C-Star was tested, showing reasonable transaction generation times and gas costs, comparable to existing blockchain applications like Uniswap, though gas costs can still be high due to cryptographic operations.

Q & A

What is the primary purpose of C-Star?

C-Star is a system designed to enhance data privacy in blockchain smart contracts by using homomorphic encryption and zero-knowledge proofs. It allows developers to implement private smart contracts without requiring cryptographic expertise.

How does homomorphic encryption contribute to C-Star's functionality?

Homomorphic encryption allows computations to be performed on encrypted data, enabling the privacy of sensitive information while still allowing interactions with the data in a smart contract, such as financial transactions, without revealing the actual values.

What are zero-knowledge proofs, and how are they used in C-Star?

Zero-knowledge proofs (ZKPs) are cryptographic techniques that allow one party to prove to another that they know a specific piece of information without revealing the actual information. In C-Star, ZKPs are used to verify the correctness of operations on encrypted data, ensuring privacy without exposing sensitive data.

What is the role of privacy annotations in C-Star smart contracts?

Privacy annotations in C-Star smart contracts mark which pieces of data are private and need to be encrypted. These annotations help specify ownership and privacy levels for data, such as encrypting balances under the owner's public key, ensuring that only the owner can access their encrypted data.

Can developers use C-Star without any cryptographic knowledge?

Yes, C-Star is designed so that developers do not need cryptographic expertise. They simply annotate their contracts with privacy types, and the system automatically handles the cryptographic operations and verification, streamlining the process of implementing private smart contracts.

How does C-Star handle the issue of data privacy while executing transactions between users?

C-Star encrypts sensitive data, like user balances, under their respective public keys. When a user performs a transaction, such as transferring tokens, the system uses homomorphic encryption to modify the encrypted balances and zero-knowledge proofs to ensure the transaction is legitimate without exposing the underlying data.

What are some of the limitations of previous works in data privacy for smart contracts that C-Star addresses?

Previous systems either relied on trusted managers or hardware (which introduces trust issues) or required manual integration of cryptographic primitives, which could be complex and error-prone. C-Star automates this process, allowing developers to implement privacy without needing to manually handle cryptographic operations.

What is the significance of using Groth16 zk-SNARKs in C-Star's implementation?

Groth16 zk-SNARKs are used in C-Star because they are efficient for generating zero-knowledge proofs. This helps reduce the computational cost of verifying encrypted transactions, making the system more practical for real-world use, especially on Ethereum, where gas costs are a concern.

What challenges does C-Star face with respect to gas costs on Ethereum?

Although C-Star aims to minimize computational costs, the use of zero-knowledge proofs and homomorphic encryption adds some overhead. This results in higher gas costs compared to regular Ethereum transactions, though C-Star's approach is still more efficient than some other privacy-focused systems.

How does C-Star manage transactions involving multiple parties and encrypted data?

C-Star allows transactions between multiple parties by using homomorphic encryption to modify encrypted data without decrypting it. It ensures that the privacy of each party is maintained through zero-knowledge proofs, which verify that the operations on the encrypted data were performed correctly.