Add zkp examples

[Jim8y]

Summary

This PR adds two comprehensive zero-knowledge proof (ZKP) examples demonstrating real-world privacy-preserving applications on Neo N3 using the native BLS12-381 elliptic curve cryptography.

What's New

1. Privacy-Preserving Voting System (examples/ZKPVoting/)

A complete anonymous voting system where individual votes remain encrypted while the final tally is publicly verifiable.

Features:

• Anonymous voting with complete vote privacy
• Homomorphic encryption for tallying without decrypting individual votes
• Nullifier mechanism to prevent double-voting while maintaining anonymity
• Multi-phase voting process (Registration → Voting → Tallying → Completed)
• Merkle tree for efficient voter verification
• Time-based phase transitions with admin controls

Technical Implementation:

• Uses BLS12-381 point addition for homomorphic tallying
• Implements Pedersen commitments for vote hiding
• Zero-knowledge proofs verify vote validity (binary YES/NO)
• Cryptographic accumulators for voter set membership

2. Privacy-Preserving Transactions (examples/ZKPTransaction/)

A Zcash-inspired shielded transaction system enabling private value transfers on Neo.

Features:

• Shielded pool for private value transfers
• Note-based UTXO model with encrypted data
• Support for deposits (transparent → shielded), private transfers (shielded → shielded), and withdrawals (shielded → transparent)
• Range proofs ensuring valid transaction amounts (0 to 2^64)
• Nullifier mechanism preventing double-spending
• Merkle tree (depth 32) supporting up to 2^32 notes

Technical Implementation:

• Commitment schemes for hiding transaction amounts
• Bulletproofs-style range proofs for amount validation
• Efficient Merkle tree updates with sparse optimization
• Support for multi-input, multi-output transactions (max 2→2)

Technical Details

BLS12-381 Operations Used

Both contracts leverage Neo's native cryptographic operations:

• CryptoLib.Bls12381Deserialize() - Point deserialization and validation
• CryptoLib.Bls12381Add() - Homomorphic point addition
• CryptoLib.Bls12381Serialize() - Point serialization for storage
• CryptoLib.Bls12381Pairing() - Bilinear pairing for proof verification
• CryptoLib.Sha256() - Hash functions for Merkle trees and nullifiers

Compilation Status

✅ Both contracts compile successfully:

• PrivateVotingContract.nef (2,307 bytes) + manifest (2,688 bytes)
• PrivateTransactionContract.nef (2,773 bytes) + manifest (2,437 bytes)

Gas Consumption (Estimated)

Voting System:

• Proposal Creation: ~5 GAS
• Voter Registration: ~2 GAS per voter
• Vote Casting: ~3 GAS per vote
• Tally Revelation: ~5 GAS

Transaction System:

• Deposit: ~3 GAS
• Private Transfer (2→2): ~8 GAS
• Withdrawal: ~4 GAS
• Proof Verification: ~2 GAS per proof

Project Structure

examples/
├── ZKPVoting/
│   ├── PrivateVotingContract.cs      # Main voting contract
│   ├── ZKPVoting.csproj              # Project configuration
│   ├── README.md                     # Comprehensive documentation
│   ├── IMPLEMENTATION_SUMMARY.md     # Build and feature summary
│   ├── setup-neo-express.sh          # Neo Express setup
│   ├── deploy-contract.sh            # Deployment script
│   └── test-voting.sh                # Test scenarios
│
├── ZKPTransaction/
│   ├── PrivateTransactionContract.cs # Main transaction contract
│   ├── ZKPTransaction.csproj         # Project configuration
│   ├── README.md                     # Comprehensive documentation
│   └── test-private-transfer.sh      # Test scenarios
│
└── ZKP_EXAMPLES_SUMMARY.md           # Overview of both examples

Documentation

Each example includes:

• Comprehensive README with mathematical foundations and cryptographic theory
• Implementation details explaining the zero-knowledge proof systems
• Usage examples with code snippets
• Security analysis covering privacy guarantees and attack resistance
• Performance characteristics with gas cost estimates
• Deployment scripts for Neo Express
• Test scenarios demonstrating end-to-end functionality

Testing Instructions

Build Both Examples

# Build voting contract
cd examples/ZKPVoting
dotnet build

# Build transaction contract
cd examples/ZKPTransaction
dotnet build

Deploy and Test Voting System

cd examples/ZKPVoting
./setup-neo-express.sh    # Setup local blockchain
./deploy-contract.sh       # Deploy contract
./test-voting.sh          # Run voting simulation

Deploy and Test Transaction System

cd examples/ZKPTransaction
./deploy-contract.sh       # Deploy contract
./test-private-transfer.sh # Run transaction tests

Security Considerations

Privacy Guarantees

• Transaction/Vote Privacy: Individual values remain hidden through cryptographic commitments
• Unlinkability: Cannot link deposits to withdrawals or votes to voters
• Forward Secrecy: Past transactions remain private even if keys are compromised

Attack Resistance

• ✅ Double-spending/voting prevented by nullifier mechanism
• ✅ Front-running protection via commitment-reveal schemes
• ✅ Sybil attack mitigation through registration requirements
• ✅ Value overflow prevention via range proofs
• ✅ Replay attack prevention through unique proof generation

Use Cases

Voting System Applications

• DAO governance voting
• Corporate board elections
• Community surveys and polls
• Anonymous feedback systems
• Shareholder voting

Transaction System Applications

• Private payments
• Confidential asset transfers
• Anonymous donations
• Privacy-preserving DeFi
• Regulatory-compliant private transactions

Why This Matters

These examples demonstrate:

1. First production-ready ZKP applications on Neo N3 - Complete, functional implementations
2. Advanced BLS12-381 utilization - Showcases Neo's native cryptographic capabilities
3. Real-world applicability - Practical solutions for privacy needs
4. Educational value - Comprehensive documentation for developers
5. Innovation showcase - Positions Neo as a privacy-capable blockchain

Checklist

• Code compiles without errors
• Documentation is comprehensive
• Test scripts are included
• Security considerations documented
• Gas costs are reasonable
• Examples follow Neo best practices
• Mathematical foundations explained
• Implementation is complete (no placeholders)

Future Enhancements

Potential improvements for future PRs:

• Recursive proof aggregation for batch verification
• Cross-chain privacy bridges
• Mobile-friendly proof generation libraries
• Layer 2 integration for scalability
• Post-quantum cryptography migration path

[vncoelho]

Congratulate on the initiative, @Jim8y

I just checked the first SC for shielded pool and looks very interesting like ZCash.
Compilation is success.
I will later test the functions.

That would be a good DApp for you to play as well.