ShieldPay

Executable Code

#!/usr/bin/env python3
"""
Claw4S Skill: ShieldPay — Shielded Agent-to-Agent Payments
Zero-Knowledge Proof Simulation for Privacy-Preserving Clinical Knowledge Markets

Simulates Zcash Sapling-inspired shielded payment pool with Pedersen commitments,
nullifier-based spend tracking, and zk-SNARK proof verification for MPP integration.

Author: Zamora-Tehozol EA (ORCID:0000-0002-7888-3961), DNAI
License: MIT

References:
  - Zcash Protocol Specification (Sapling). https://zips.z.cash/protocol/protocol.pdf
  - Groth J. EUROCRYPT 2016. DOI:10.1007/978-3-662-49896-5_11
  - Grassi L et al. (Poseidon Hash) USENIX Security 2021.
  - Machine Payment Protocol. https://mpp.dev
  - Superfluid Protocol. https://docs.superfluid.finance/
"""

import numpy as np
import hashlib
import os
import json
import struct

# ══════════════════════════════════════════════════════════════════
# CRYPTOGRAPHIC PRIMITIVES (simplified simulation)
# ══════════════════════════════════════════════════════════════════

def poseidon_hash(*inputs: bytes) -> bytes:
    """Simplified Poseidon hash simulation using SHA-256 (real: ZK-friendly algebraic hash)."""
    combined = b'POSEIDON:' + b':'.join(inputs)
    return hashlib.sha256(combined).digest()

def pedersen_commit(amount: int, blinding: bytes) -> bytes:
    """Simplified Pedersen commitment: C = amount*G + blinding*H."""
    return poseidon_hash(struct.pack('>Q', amount), blinding)

def generate_nullifier(secret: bytes, leaf_index: int) -> bytes:
    """Generate spend nullifier from secret and Merkle leaf index."""
    return poseidon_hash(secret, struct.pack('>I', leaf_index))


class MerkleTree:
    """Incremental Merkle tree for commitment storage (32 levels)."""

    def __init__(self, depth=16):
        self.depth = depth
        self.leaves = []
        self.zero_hashes = [b'\x00' * 32]
        for i in range(depth):
            self.zero_hashes.append(poseidon_hash(self.zero_hashes[-1], self.zero_hashes[-1]))

    def insert(self, leaf: bytes) -> int:
        idx = len(self.leaves)
        self.leaves.append(leaf)
        return idx

    def root(self) -> bytes:
        if not self.leaves:
            return self.zero_hashes[self.depth]
        current = list(self.leaves)
        for level in range(self.depth):
            next_level = []
            for i in range(0, len(current), 2):
                left = current[i]
                right = current[i+1] if i+1 < len(current) else self.zero_hashes[level]
                next_level.append(poseidon_hash(left, right))
            current = next_level
            if not current:
                return self.zero_hashes[self.depth]
        return current[0]

    def generate_proof(self, index: int) -> list:
        """Generate Merkle path proof for a leaf."""
        if index >= len(self.leaves):
            return []
        proof = []
        current_level = list(self.leaves)
        idx = index
        for level in range(self.depth):
            sibling_idx = idx ^ 1
            if sibling_idx < len(current_level):
                proof.append(current_level[sibling_idx])
            else:
                proof.append(self.zero_hashes[level])
            next_level = []
            for i in range(0, len(current_level), 2):
                left = current_level[i]
                right = current_level[i+1] if i+1 < len(current_level) else self.zero_hashes[level]
                next_level.append(poseidon_hash(left, right))
            current_level = next_level
            idx //= 2
        return proof


class ZKProof:
    """Simulated zk-SNARK proof (real: Groth16 circuit over BN254)."""

    def __init__(self, proof_data: bytes, public_inputs: dict):
        self.proof_data = proof_data
        self.public_inputs = public_inputs
        self.verified = False

    def verify(self, verifier_key: bytes = b'mock_vk') -> bool:
        """Simulated verification (real: pairing check on elliptic curve)."""
        expected = poseidon_hash(verifier_key, json.dumps(self.public_inputs, sort_keys=True).encode())
        self.verified = True  # In simulation, always valid if well-formed
        return True


# ══════════════════════════════════════════════════════════════════
# SHIELD POOL
# ══════════════════════════════════════════════════════════════════

class ShieldPool:
    """Shielded payment pool for agent-to-agent transactions."""

    def __init__(self):
        self.tree = MerkleTree(depth=16)
        self.nullifiers = set()
        self.total_deposited = 0
        self.total_withdrawn = 0
        self.notes = {}  # note_id -> note metadata (only holder knows)
        self.events = []

    def deposit(self, amount: int, depositor: str) -> dict:
        """Create a shielded deposit (note)."""
        secret = os.urandom(32)
        nullifier_secret = os.urandom(32)
        commitment = pedersen_commit(amount, secret)
        leaf_index = self.tree.insert(commitment)

        note = {
            'amount': amount,
            'secret': secret.hex(),
            'nullifier_secret': nullifier_secret.hex(),
            'leaf_index': leaf_index,
            'commitment': commitment.hex(),
            'spent': False,
        }
        note_id = commitment.hex()[:16]
        self.notes[note_id] = note
        self.total_deposited += amount

        self.events.append({
            'type': 'deposit',
            'commitment': commitment.hex()[:16],
            'pool_root': self.tree.root().hex()[:16],
            # Amount and depositor NOT recorded (privacy!)
        })

        return {
            'note_id': note_id,
            'leaf_index': leaf_index,
            'commitment': commitment.hex()[:16],
            'status': 'deposited',
            # Return secret to depositor (they keep it private)
            '_private': {'secret': secret.hex(), 'nullifier_secret': nullifier_secret.hex()},
        }

    def withdraw(self, note_id: str, recipient: str, amount: int) -> dict:
        """Withdraw from shielded pool with zk proof."""
        if note_id not in self.notes:
            return {'error': 'Unknown note'}
        note = self.notes[note_id]
        if note['spent']:
            return {'error': 'Note already spent (double-spend attempt)'}
        if amount > note['amount']:
            return {'error': 'Insufficient note balance'}

        # Generate nullifier
        nullifier = generate_nullifier(
            bytes.fromhex(note['nullifier_secret']),
            note['leaf_index']
        )
        nullifier_hex = nullifier.hex()

        if nullifier_hex in self.nullifiers:
            return {'error': 'Nullifier already used (double-spend prevented)'}

        # Generate zk proof (simulated)
        proof = ZKProof(
            proof_data=os.urandom(256),  # Real: ~192 bytes Groth16
            public_inputs={
                'nullifier': nullifier_hex[:16],
                'root': self.tree.root().hex()[:16],
                'amount': amount,
            }
        )
        proof.verify()

        # Execute withdrawal
        self.nullifiers.add(nullifier_hex)
        note['spent'] = True
        self.total_withdrawn += amount

        self.events.append({
            'type': 'withdrawal',
            'nullifier': nullifier_hex[:16],
            # Recipient and amount NOT in public event (privacy!)
        })

        return {
            'status': 'withdrawn',
            'amount': amount,
            'nullifier': nullifier_hex[:16],
            'proof_verified': proof.verified,
        }

    def verify_payment(self, nullifier_prefix: str, min_amount: int) -> dict:
        """MPP 402 authorization: verify shielded payment without revealing details."""
        # In real system, this is a zk verification on-chain
        matching = [n for n in self.nullifiers if n.startswith(nullifier_prefix)]
        if matching:
            return {'authorized': True, 'message': 'Valid shielded payment verified'}
        return {'authorized': False, 'message': 'No matching payment proof', 'http_status': 402}

    def pool_stats(self) -> dict:
        return {
            'total_deposits': self.total_deposited,
            'total_withdrawals': self.total_withdrawn,
            'pool_balance': self.total_deposited - self.total_withdrawn,
            'n_commitments': len(self.tree.leaves),
            'n_nullifiers': len(self.nullifiers),
            'anonymity_set_size': len(self.tree.leaves),
            'merkle_root': self.tree.root().hex()[:16],
        }


# ══════════════════════════════════════════════════════════════════
# DEMO
# ══════════════════════════════════════════════════════════════════

if __name__ == "__main__":
    print("=" * 70)
    print("SHIELDPAY: Shielded Agent-to-Agent Payments (ZK Simulation)")
    print("Authors: Zamora-Tehozol EA (ORCID:0000-0002-7888-3961), DNAI")
    print("=" * 70)

    pool = ShieldPool()

    # ── Deposits ──
    print("\n── SHIELDED DEPOSITS ──")
    d1 = pool.deposit(1000000, 'Hospital-IMSS')  # 1 USDC (6 decimals)
    print(f"  Deposit 1: 1.0 USDC → note {d1['note_id']}")
    d2 = pool.deposit(5000000, 'ResearchDAO')
    print(f"  Deposit 2: 5.0 USDC → note {d2['note_id']}")
    d3 = pool.deposit(500000, 'Clinic-UNAM')
    print(f"  Deposit 3: 0.5 USDC → note {d3['note_id']}")

    # ── Pool Privacy ──
    print("\n── POOL STATE (public view) ──")
    stats = pool.pool_stats()
    print(f"  Commitments: {stats['n_commitments']}")
    print(f"  Anonymity set: {stats['anonymity_set_size']}")
    print(f"  Merkle root: {stats['merkle_root']}")
    print(f"  Note: amounts and depositors are NOT visible on-chain")

    # ── Withdrawal with ZK proof ──
    print("\n── SHIELDED WITHDRAWAL ──")
    w1 = pool.withdraw(d1['note_id'], 'RheumaScore', 1000000)
    print(f"  Withdrawal: {w1['status']}, proof_verified={w1['proof_verified']}")
    print(f"  Nullifier: {w1['nullifier']} (prevents double-spend)")

    # ── Double-spend attempt ──
    print("\n── DOUBLE-SPEND ATTEMPT ──")
    w1_dup = pool.withdraw(d1['note_id'], 'Attacker', 1000000)
    print(f"  Result: {w1_dup.get('error', 'SHOULD NOT SUCCEED')}")

    # ── MPP 402 Authorization ──
    print("\n── MPP 402 AUTHORIZATION ──")
    auth = pool.verify_payment(w1['nullifier'][:8], 500000)
    print(f"  Verify against provider: authorized={auth['authorized']}")
    auth_fail = pool.verify_payment('deadbeef', 500000)
    print(f"  Invalid proof: authorized={auth_fail['authorized']}")

    # ── Final stats ──
    print("\n── FINAL POOL STATE ──")
    final = pool.pool_stats()
    print(f"  Total deposited: {final['total_deposits']} units")
    print(f"  Total withdrawn: {final['total_withdrawals']} units")
    print(f"  Pool balance: {final['pool_balance']} units")
    print(f"  Spent nullifiers: {final['n_nullifiers']}")

    # Privacy guarantees
    print("\n── PRIVACY GUARANTEES ──")
    print("  ✓ Sender identity hidden (commitment anonymity set)")
    print("  ✓ Amount hidden (Pedersen commitment)")
    print("  ✓ Receiver identity hidden (only nullifier revealed)")
    print("  ✓ Double-spend prevented (nullifier set)")
    print("  ✓ MPP 402 compatible (proof-based authorization)")

    print(f"\n── LIMITATIONS ──")
    print("  • Cryptographic primitives are SIMULATED (SHA-256 not Poseidon, no real pairings)")
    print("  • Real deployment requires circom/snarkjs Groth16 circuits")
    print("  • Merkle tree is in-memory only (real: on-chain contract storage)")
    print("  • No actual Ethereum/Base L2 integration")
    print("  • Anonymity set is small in demo (real: needs 1000+ deposits)")
    print("  • No range proofs on amounts (needed to prevent negative values)")
    print("  • Regulatory compliance mode (audit trail) not implemented")
    print(f"\n{'='*70}")
    print("END — ShieldPay Skill v1.0")

Demo Output

3 shielded deposits, withdrawal verified
Double-spend prevented, MPP 402 authorized
Pool balance: 5500000 units, 1 spent nullifier