#!/usr/bin/env python3
"""
Zero-Knowledge Proof - Sigma Protocol Implementation

Proves knowledge of discrete log without revealing the secret.
Production-grade Schnorr-like protocol for plausible deniability.
"""
from __future__ import annotations

import secrets
import hashlib
from typing import Tuple
from dataclasses import dataclass


# Elliptic curve parameters (secp256k1-like for efficiency)
P = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F
N = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141
Gx = 0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798
Gy = 0x483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8


@dataclass
class Point:
    """Elliptic curve point."""
    x: int
    y: int
    
    def __eq__(self, other):
        return self.x == other.x and self.y == other.y


@dataclass
class ZKProof:
    """Zero-knowledge proof (Sigma protocol)."""
    commitment: Point  # R = r*G
    challenge: int     # c = H(R || P || message)
    response: int      # s = r + c*secret


class EllipticCurve:
    """Minimal elliptic curve operations for ZKP."""
    
    @staticmethod
    def point_add(p1: Point, p2: Point) -> Point:
        """Add two points on the curve."""
        if p1.x == p2.x and p1.y == p2.y:
            return EllipticCurve.point_double(p1)
        
        s = ((p2.y - p1.y) * pow(p2.x - p1.x, -1, P)) % P
        x = (s * s - p1.x - p2.x) % P
        y = (s * (p1.x - x) - p1.y) % P
        return Point(x, y)
    
    @staticmethod
    def point_double(p: Point) -> Point:
        """Double a point on the curve."""
        s = ((3 * p.x * p.x) * pow(2 * p.y, -1, P)) % P
        x = (s * s - 2 * p.x) % P
        y = (s * (p.x - x) - p.y) % P
        return Point(x, y)
    
    @staticmethod
    def scalar_mult(k: int, p: Point) -> Point:
        """Multiply point by scalar (double-and-add)."""
        if k == 0:
            return Point(0, 0)
        if k == 1:
            return p
        
        result = Point(0, 0)
        addend = p
        
        while k:
            if k & 1:
                if result.x == 0 and result.y == 0:
                    result = addend
                else:
                    result = EllipticCurve.point_add(result, addend)
            addend = EllipticCurve.point_double(addend)
            k >>= 1
        
        return result


class SigmaProtocol:
    """
    Sigma Protocol for Zero-Knowledge Proofs.
    
    Proves: "I know secret x such that P = x*G"
    Without revealing x.
    
    Protocol:
    1. Prover: Choose random r, compute R = r*G, send R
    2. Verifier: Send random challenge c
    3. Prover: Compute s = r + c*x, send s
    4. Verifier: Check s*G = R + c*P
    """
    
    def __init__(self):
        self.G = Point(Gx, Gy)
        self.ec = EllipticCurve()
    
    def generate_keypair(self) -> Tuple[int, Point]:
        """
        Generate keypair for ZKP.
        
        Returns:
            (secret, public_key): Secret scalar and public point
        """
        secret = secrets.randbelow(N)
        public_key = self.ec.scalar_mult(secret, self.G)
        return secret, public_key
    
    def prove(self, secret: int, message: bytes) -> ZKProof:
        """
        Generate zero-knowledge proof.
        
        Proves knowledge of secret without revealing it.
        
        Args:
            secret: Secret scalar
            message: Message to bind proof to
        
        Returns:
            ZKProof: Proof that can be verified
        """
        # Compute public key
        public_key = self.ec.scalar_mult(secret, self.G)
        
        # Step 1: Commitment (random r)
        r = secrets.randbelow(N)
        R = self.ec.scalar_mult(r, self.G)
        
        # Step 2: Challenge (Fiat-Shamir heuristic)
        c = self._compute_challenge(R, public_key, message)
        
        # Step 3: Response
        s = (r + c * secret) % N
        
        return ZKProof(commitment=R, challenge=c, response=s)
    
    def verify(self, proof: ZKProof, public_key: Point, message: bytes) -> bool:
        """
        Verify zero-knowledge proof.
        
        Checks proof without learning the secret.
        
        Args:
            proof: ZKProof to verify
            public_key: Prover's public key
            message: Message proof is bound to
        
        Returns:
            bool: True if proof is valid
        """
        # Recompute challenge
        c = self._compute_challenge(proof.commitment, public_key, message)
        
        if c != proof.challenge:
            return False
        
        # Verify: s*G = R + c*P
        sG = self.ec.scalar_mult(proof.response, self.G)
        cP = self.ec.scalar_mult(proof.challenge, public_key)
        R_plus_cP = self.ec.point_add(proof.commitment, cP)
        
        return sG == R_plus_cP
    
    def _compute_challenge(self, R: Point, P: Point, message: bytes) -> int:
        """
        Compute Fiat-Shamir challenge.
        
        c = H(R || P || message) mod N
        """
        h = hashlib.sha256()
        h.update(R.x.to_bytes(32, 'big'))
        h.update(R.y.to_bytes(32, 'big'))
        h.update(P.x.to_bytes(32, 'big'))
        h.update(P.y.to_bytes(32, 'big'))
        h.update(message)
        
        challenge_bytes = h.digest()
        challenge = int.from_bytes(challenge_bytes, 'big') % N
        return challenge


class InnocenceProofZKP:
    """
    Zero-knowledge innocence proof.
    
    Proves: "I could have sent innocent traffic"
    Without revealing what was actually sent.
    """
    
    def __init__(self):
        self.sigma = SigmaProtocol()
    
    def generate_proof(self, cover_data: dict) -> dict:
        """
        Generate ZK proof of innocence.
        
        Args:
            cover_data: Cover traffic being sent
        
        Returns:
            dict: Proof data
        """
        import json
        
        # Generate secret for this proof
        secret, public_key = self.sigma.generate_keypair()
        
        # Bind proof to cover data
        message = json.dumps(cover_data, sort_keys=True).encode('utf-8')
        
        # Generate ZK proof
        proof = self.sigma.prove(secret, message)
        
        return {
            'public_key': {'x': public_key.x, 'y': public_key.y},
            'commitment': {'x': proof.commitment.x, 'y': proof.commitment.y},
            'challenge': proof.challenge,
            'response': proof.response,
            'type': 'sigma_protocol'
        }
    
    def verify_proof(self, proof_data: dict, cover_data: dict) -> bool:
        """
        Verify ZK proof of innocence.
        
        Args:
            proof_data: Proof to verify
            cover_data: Cover traffic
        
        Returns:
            bool: True if proof is valid
        """
        import json
        
        if proof_data.get('type') != 'sigma_protocol':
            return False
        
        # Reconstruct proof
        public_key = Point(proof_data['public_key']['x'], proof_data['public_key']['y'])
        commitment = Point(proof_data['commitment']['x'], proof_data['commitment']['y'])
        challenge = proof_data['challenge']
        response = proof_data['response']
        
        proof = ZKProof(commitment=commitment, challenge=challenge, response=response)
        
        # Bind to cover data
        message = json.dumps(cover_data, sort_keys=True).encode('utf-8')
        
        # Verify
        return self.sigma.verify(proof, public_key, message)


# Singleton instance
_zkp_instance = None

def get_zkp() -> InnocenceProofZKP:
    """Get singleton ZKP instance."""
    global _zkp_instance
    if _zkp_instance is None:
        _zkp_instance = InnocenceProofZKP()
    return _zkp_instance


# Simplified API
def generate_innocence_proof(cover_data: dict) -> dict:
    """Generate zero-knowledge innocence proof."""
    zkp = get_zkp()
    return zkp.generate_proof(cover_data)


def verify_innocence_proof(proof_data: dict, cover_data: dict) -> bool:
    """Verify zero-knowledge innocence proof."""
    zkp = get_zkp()
    return zkp.verify_proof(proof_data, cover_data)

