feat: Add Private Basis (Chaumian E-Cash) Proof of Concept (issue-12)

[Ayush090207]

We're Team Neuronest participating in the Unstoappable Hackathon LNMIIT 2025

Team Members:

Himanshu Jasoriya
Aditya Gautam
Ayush Sharma

This commit introduces a privacy-enhanced variant of the Basis off-chain cash system using Chaumian blind signature techniques for unlinkable bearer notes.

Issue: #12 - Private Offchain Cash

New Components

Documentation (docs/)

• basis_current_design.md: Technical analysis of transparent Basis design
• basis_private_chaumian_poc.md: Complete protocol specification for private variant
• basis_private_summary_for_pr.md: PR summary with privacy analysis and limitations

ErgoScript Contract (contracts/offchain/)

• basis_private_reserve.es: Modified reserve contract with nullifier-based redemption
• basis_private_reserve.md: Technical documentation for the contract

Rust Tracker Implementation (basis-private-tracker/)

• Complete Rust implementation with blind signature issuance
• Nullifier-based double-spend prevention
• 11 comprehensive tests (unit + integration)
• CLI demo (tracker_poc.rs)

Updated Files

• README.md: Added section describing private Basis PoC

Privacy Features

✅ Withdrawal-Redemption Unlinkability: Blind signatures prevent linking
✅ Off-Chain Transfer Privacy: Notes transferable without tracker visibility
✅ User Anonymity: Pseudonymous with rotatable keys
✅ Double-Spend Prevention: Nullifier AVL tree enforcement
✅ Proof-of-Reserves: Verifiable on-chain backing

Known Limitations (Documented)

⚠️ On-chain timing analysis possible (mitigated by batching) ⚠️ Denomination linkability (use standard denominations) ⚠️ Placeholder cryptography (production needs secp256k1 library) ⚠️ No change mechanism in PoC (future: split protocol) ⚠️ Windows build requires MSVC tools (compiles on Linux/macOS)

Testing

All tests pass on Linux/macOS:

• Unit tests: 4 (types.rs)
• Integration tests: 7 (tracker.rs, lib.rs)
• Demo: tracker_poc binary

Next Steps

• Cryptographic security review
• ErgoScript testnet deployment
• Production cryptography implementation
• Change protocol design

See docs/basis_private_summary_for_pr.md for complete analysis.

Note: This is a PROOF OF CONCEPT for research and demonstration. Requires cryptographic audit before production use.