oxify_authz/

zkp.rs

//! # Zero-Knowledge Proofs for Privacy-Preserving Authorization
//!
//! Prove permissions without revealing relation tuples.
//!
//! This module provides zero-knowledge proof (ZKP) primitives for authorization:
//! - **zkSNARKs**: Succinct Non-Interactive Arguments of Knowledge
//! - **Bulletproofs**: Range proofs for attribute-based access control
//! - **Privacy-Preserving Checks**: Prove permission without revealing tuple details
//!
//! ## Use Cases
//!
//! 1. **Confidential Authorization**: Prove access rights without exposing sensitive relationships
//! 2. **Compliance**: Demonstrate authorization without revealing user identities
//! 3. **Multi-Party Authorization**: Aggregate proofs from multiple parties
//! 4. **Audit Privacy**: Prove compliance without exposing full audit trail
//!
//! ## Example
//!
//! ```no_run
//! use oxify_authz::zkp::*;
//!
//! # async fn example() -> Result<(), Box<dyn std::error::Error>> {
//! // Create ZKP prover
//! let mut prover = ZkProver::new();
//!
//! // Generate proof that alice can view document:123
//! let proof = prover.prove_permission(
//!     "alice",
//!     "document:123",
//!     "viewer",
//!     &["owner", "editor", "viewer"], // Permission hierarchy
//! )?;
//!
//! // Verify proof without knowing which specific permission alice has
//! let mut verifier = ZkVerifier::new();
//! assert!(verifier.verify_permission_proof(&proof)?);
//! # Ok(())
//! # }
//! ```
//!
//! ## Security Considerations
//!
//! - **Trusted Setup**: Some zkSNARK schemes require trusted setup ceremony
//! - **Performance**: ZKP verification is ~1-10ms (acceptable for authorization)
//! - **Proof Size**: ~200-2000 bytes depending on scheme
//!
//! ## Implementation Status
//!
//! This is a **research implementation** providing the framework for ZKP integration.
//! Production deployment requires:
//! - Integration with `ark-snark` or `bellman` for zkSNARKs
//! - Trusted setup ceremony for production circuits
//! - Performance benchmarking for authorization workloads

use crate::{AuthzError, RelationTuple, Result};
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::time::SystemTime;

/// Zero-knowledge proof scheme selection
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum ZkProofScheme {
    /// Groth16: Fast verification, requires trusted setup
    /// Proof size: ~200 bytes, Verify time: ~2ms
    Groth16,

    /// PLONK: Universal trusted setup, slightly slower
    /// Proof size: ~400 bytes, Verify time: ~5ms
    Plonk,

    /// Bulletproofs: No trusted setup, larger proofs
    /// Proof size: ~1500 bytes, Verify time: ~10ms
    Bulletproofs,

    /// STARKs: No trusted setup, quantum-resistant
    /// Proof size: ~100KB, Verify time: ~50ms
    Stark,
}

/// Zero-knowledge proof for permission check
///
/// Proves: "Subject S has relation R on object O" without revealing the specific tuple
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct PermissionProof {
    /// Proof scheme used
    pub scheme: ZkProofScheme,

    /// The actual zero-knowledge proof bytes
    pub proof: Vec<u8>,

    /// Public inputs (non-secret): object_id, relation
    pub public_inputs: ZkPublicInputs,

    /// Proof generation timestamp
    pub created_at: SystemTime,

    /// Optional proof metadata (e.g., circuit ID)
    #[serde(skip_serializing_if = "Option::is_none")]
    pub metadata: Option<HashMap<String, String>>,
}

/// Public inputs to the zero-knowledge proof
///
/// These are revealed during verification but don't compromise privacy
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ZkPublicInputs {
    /// Hash of the object being accessed (not the object itself)
    pub object_hash: Vec<u8>,

    /// Relation being checked (e.g., "viewer")
    pub relation: String,

    /// Namespace (e.g., "document")
    pub namespace: String,

    /// Challenge nonce to prevent replay attacks
    pub nonce: u64,
}

/// Zero-knowledge prover for generating permission proofs
#[derive(Debug)]
pub struct ZkProver {
    /// Proof scheme to use
    scheme: ZkProofScheme,

    /// Circuit proving key (in production, loaded from trusted setup)
    #[allow(dead_code)]
    proving_key: Vec<u8>,

    /// Nonce counter for replay protection
    nonce_counter: u64,
}

impl ZkProver {
    /// Create a new ZK prover with default scheme (Groth16)
    pub fn new() -> Self {
        Self::with_scheme(ZkProofScheme::Groth16)
    }

    /// Create prover with specific proof scheme
    pub fn with_scheme(scheme: ZkProofScheme) -> Self {
        Self {
            scheme,
            proving_key: vec![0u8; 1024], // Placeholder for proving key
            nonce_counter: 0,
        }
    }

    /// Generate a proof that subject has permission on object
    ///
    /// # Arguments
    ///
    /// * `subject` - Subject claiming permission (kept private in proof)
    /// * `object_id` - Object being accessed
    /// * `relation` - Relation being checked
    /// * `permitted_relations` - All relations that grant access (for hierarchy)
    ///
    /// # Privacy Guarantees
    ///
    /// The proof reveals:
    /// - Object hash (not object ID itself)
    /// - Relation being checked
    /// - Namespace
    ///
    /// The proof **does not** reveal:
    /// - Subject identity
    /// - Actual tuple that grants permission
    /// - Parent relationships in hierarchy
    #[allow(clippy::too_many_arguments)]
    pub fn prove_permission(
        &mut self,
        subject: &str,
        object_id: &str,
        relation: &str,
        permitted_relations: &[&str],
    ) -> Result<PermissionProof> {
        // In production, this would:
        // 1. Build circuit constraint system for permission check
        // 2. Generate witness (private inputs: subject, tuple, hierarchy)
        // 3. Compute zkSNARK proof using proving key
        // 4. Output proof + public inputs

        // Hash the object ID (public input)
        let object_hash = Self::hash_object(object_id);

        // Generate nonce for replay protection
        self.nonce_counter += 1;
        let nonce = self.nonce_counter;

        let public_inputs = ZkPublicInputs {
            object_hash: object_hash.clone(),
            relation: relation.to_string(),
            namespace: "document".to_string(), // In production, extract from context
            nonce,
        };

        // Simulate proof generation
        // In production: proof = groth16::create_proof(circuit, witness, proving_key)
        let proof = self.generate_simulated_proof(
            subject,
            object_id,
            relation,
            permitted_relations,
            &public_inputs,
        );

        Ok(PermissionProof {
            scheme: self.scheme,
            proof,
            public_inputs,
            created_at: SystemTime::now(),
            metadata: None,
        })
    }

    /// Generate a proof for a specific relation tuple
    pub fn prove_tuple(&mut self, tuple: &RelationTuple) -> Result<PermissionProof> {
        let subject_str = tuple.subject.to_string();
        self.prove_permission(
            &subject_str,
            &tuple.object_id,
            &tuple.relation,
            &[&tuple.relation],
        )
    }

    /// Hash an object ID for use in public inputs
    fn hash_object(object_id: &str) -> Vec<u8> {
        // In production, use cryptographic hash (SHA-256, Blake2)
        // For now, simple placeholder
        let mut hash = object_id.as_bytes().to_vec();
        hash.extend_from_slice(b"_hashed");
        hash
    }

    /// Simulate proof generation (placeholder for actual zkSNARK)
    fn generate_simulated_proof(
        &self,
        subject: &str,
        object_id: &str,
        relation: &str,
        permitted_relations: &[&str],
        public_inputs: &ZkPublicInputs,
    ) -> Vec<u8> {
        // In production, this would be the actual zkSNARK proof bytes
        // For simulation, create deterministic proof based on inputs

        let mut proof = Vec::new();

        // Encode proof scheme (1 byte)
        proof.push(self.scheme as u8);

        // Encode witness commitment (private)
        let witness = format!("{}:{}:{}", subject, object_id, relation);
        proof.extend_from_slice(witness.as_bytes());

        // Encode permitted relations (shows hierarchy without revealing path)
        for rel in permitted_relations {
            proof.extend_from_slice(rel.as_bytes());
        }

        // Include nonce
        proof.extend_from_slice(&public_inputs.nonce.to_le_bytes());

        // Add padding to simulate realistic proof size
        let target_size = match self.scheme {
            ZkProofScheme::Groth16 => 200,
            ZkProofScheme::Plonk => 400,
            ZkProofScheme::Bulletproofs => 1500,
            ZkProofScheme::Stark => 100_000,
        };

        while proof.len() < target_size {
            proof.push(0);
        }

        proof
    }
}

impl Default for ZkProver {
    fn default() -> Self {
        Self::new()
    }
}

/// Zero-knowledge verifier for checking permission proofs
#[derive(Debug)]
pub struct ZkVerifier {
    /// Verification key (public, derived from trusted setup)
    #[allow(dead_code)]
    verification_key: Vec<u8>,

    /// Nonce cache to prevent replay attacks
    used_nonces: HashMap<u64, SystemTime>,
}

impl ZkVerifier {
    /// Create a new ZK verifier
    pub fn new() -> Self {
        Self {
            verification_key: vec![0u8; 512], // Placeholder for verification key
            used_nonces: HashMap::new(),
        }
    }

    /// Verify a permission proof
    ///
    /// # Security Checks
    ///
    /// 1. Cryptographic proof verification (zkSNARK/Bulletproof/STARK)
    /// 2. Public inputs match claimed values
    /// 3. Nonce hasn't been used (replay protection)
    /// 4. Proof is recent (within validity window)
    pub fn verify_permission_proof(&mut self, proof: &PermissionProof) -> Result<bool> {
        // Check nonce for replay attacks
        if self.used_nonces.contains_key(&proof.public_inputs.nonce) {
            return Err(AuthzError::PermissionDenied(
                "Replay attack detected: nonce already used".to_string(),
            ));
        }

        // Check proof age (valid for 5 minutes)
        let age = SystemTime::now()
            .duration_since(proof.created_at)
            .map_err(|e| AuthzError::InvalidTuple(format!("Invalid timestamp: {}", e)))?;

        if age.as_secs() > 300 {
            return Err(AuthzError::PermissionDenied(
                "Proof expired (>5 minutes old)".to_string(),
            ));
        }

        // In production: verify zkSNARK using verification key
        // let valid = match proof.scheme {
        //     ZkProofScheme::Groth16 => groth16::verify(&proof.proof, &public_inputs, &vk),
        //     ZkProofScheme::Plonk => plonk::verify(&proof.proof, &public_inputs, &vk),
        //     ZkProofScheme::Bulletproofs => bulletproofs::verify(&proof.proof, &public_inputs),
        //     ZkProofScheme::Stark => stark::verify(&proof.proof, &public_inputs),
        // };

        // Simulated verification
        let valid = self.verify_simulated_proof(proof);

        if valid {
            // Mark nonce as used
            self.used_nonces
                .insert(proof.public_inputs.nonce, SystemTime::now());

            // Clean up old nonces (older than 10 minutes)
            self.cleanup_old_nonces();
        }

        Ok(valid)
    }

    /// Batch verify multiple proofs (more efficient)
    ///
    /// Some zkSNARK schemes support batch verification with significant speedup
    pub fn batch_verify(&mut self, proofs: &[PermissionProof]) -> Result<Vec<bool>> {
        // In production, use actual batch verification:
        // - Groth16: ~30% faster for batches
        // - Bulletproofs: ~50% faster for batches

        proofs
            .iter()
            .map(|p| self.verify_permission_proof(p))
            .collect()
    }

    /// Simulated proof verification (placeholder)
    fn verify_simulated_proof(&self, proof: &PermissionProof) -> bool {
        // Basic sanity checks
        if proof.proof.is_empty() {
            return false;
        }

        // Check proof size matches scheme
        let expected_size = match proof.scheme {
            ZkProofScheme::Groth16 => 200,
            ZkProofScheme::Plonk => 400,
            ZkProofScheme::Bulletproofs => 1500,
            ZkProofScheme::Stark => 100_000,
        };

        proof.proof.len() >= expected_size
    }

    /// Remove nonces older than 10 minutes
    fn cleanup_old_nonces(&mut self) {
        let cutoff = SystemTime::now() - std::time::Duration::from_secs(600);
        self.used_nonces
            .retain(|_, &mut timestamp| timestamp > cutoff);
    }
}

impl Default for ZkVerifier {
    fn default() -> Self {
        Self::new()
    }
}

/// Aggregate multiple permission proofs into a single proof
///
/// Useful for "user has access to ANY of [doc1, doc2, doc3]" without revealing which one
#[derive(Debug)]
pub struct AggregateProof {
    /// Individual proofs being aggregated
    pub proofs: Vec<PermissionProof>,

    /// Aggregated proof (smaller than sum of individual proofs)
    pub aggregate: Vec<u8>,
}

impl AggregateProof {
    /// Create an aggregate proof from multiple permission proofs
    pub fn aggregate(proofs: Vec<PermissionProof>) -> Result<Self> {
        if proofs.is_empty() {
            return Err(AuthzError::InvalidTuple(
                "Cannot aggregate empty proof set".to_string(),
            ));
        }

        // Check all proofs use same scheme
        let scheme = proofs[0].scheme;
        if !proofs.iter().all(|p| p.scheme == scheme) {
            return Err(AuthzError::InvalidTuple(
                "Cannot aggregate proofs with different schemes".to_string(),
            ));
        }

        // In production: Use proof aggregation techniques
        // - Groth16: Aggregate via pairing operations
        // - Bulletproofs: Native aggregation support
        // - STARKs: FRI-based aggregation

        // Simulated aggregation
        let aggregate = proofs.iter().flat_map(|p| p.proof.clone()).collect();

        Ok(Self { proofs, aggregate })
    }

    /// Verify the aggregate proof
    pub fn verify(&self, verifier: &mut ZkVerifier) -> Result<bool> {
        // In production: Single verification of aggregate
        // For now: Verify each proof individually
        for proof in &self.proofs {
            if !verifier.verify_permission_proof(proof)? {
                return Ok(false);
            }
        }
        Ok(true)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::Subject;

    #[test]
    fn test_permission_proof_generation() {
        let mut prover = ZkProver::new();

        let proof = prover
            .prove_permission("alice", "doc123", "viewer", &["owner", "editor", "viewer"])
            .unwrap();

        assert_eq!(proof.scheme, ZkProofScheme::Groth16);
        assert!(proof.proof.len() >= 200);
        assert_eq!(proof.public_inputs.relation, "viewer");
    }

    #[test]
    fn test_proof_verification() {
        let mut prover = ZkProver::new();
        let mut verifier = ZkVerifier::new();

        let proof = prover
            .prove_permission("bob", "doc456", "editor", &["editor"])
            .unwrap();

        assert!(verifier.verify_permission_proof(&proof).unwrap());
    }

    #[test]
    fn test_replay_protection() {
        let mut prover = ZkProver::new();
        let mut verifier = ZkVerifier::new();

        let proof = prover
            .prove_permission("alice", "doc123", "viewer", &["viewer"])
            .unwrap();

        // First verification succeeds
        assert!(verifier.verify_permission_proof(&proof).unwrap());

        // Replay attempt fails
        let result = verifier.verify_permission_proof(&proof);
        assert!(result.is_err());
        assert!(result.unwrap_err().to_string().contains("Replay attack"));
    }

    #[test]
    fn test_proof_expiration() {
        let mut prover = ZkProver::new();
        let mut verifier = ZkVerifier::new();

        let mut proof = prover
            .prove_permission("alice", "doc123", "viewer", &["viewer"])
            .unwrap();

        // Set proof to be 10 minutes old
        proof.created_at = SystemTime::now() - std::time::Duration::from_secs(600);

        let result = verifier.verify_permission_proof(&proof);
        assert!(result.is_err());
        assert!(result.unwrap_err().to_string().contains("expired"));
    }

    #[test]
    fn test_batch_verification() {
        let mut prover = ZkProver::new();
        let mut verifier = ZkVerifier::new();

        let proofs = vec![
            prover
                .prove_permission("alice", "doc1", "viewer", &["viewer"])
                .unwrap(),
            prover
                .prove_permission("bob", "doc2", "editor", &["editor"])
                .unwrap(),
            prover
                .prove_permission("charlie", "doc3", "owner", &["owner"])
                .unwrap(),
        ];

        let results = verifier.batch_verify(&proofs).unwrap();
        assert_eq!(results.len(), 3);
        assert!(results.iter().all(|&r| r));
    }

    #[test]
    fn test_different_proof_schemes() {
        let schemes = vec![
            ZkProofScheme::Groth16,
            ZkProofScheme::Plonk,
            ZkProofScheme::Bulletproofs,
            ZkProofScheme::Stark,
        ];

        for scheme in schemes {
            let mut prover = ZkProver::with_scheme(scheme);
            let proof = prover
                .prove_permission("alice", "doc", "viewer", &["viewer"])
                .unwrap();

            assert_eq!(proof.scheme, scheme);

            let expected_size = match scheme {
                ZkProofScheme::Groth16 => 200,
                ZkProofScheme::Plonk => 400,
                ZkProofScheme::Bulletproofs => 1500,
                ZkProofScheme::Stark => 100_000,
            };

            assert!(proof.proof.len() >= expected_size);
        }
    }

    #[test]
    fn test_aggregate_proofs() {
        let mut prover = ZkProver::new();

        let proofs = vec![
            prover
                .prove_permission("alice", "doc1", "viewer", &["viewer"])
                .unwrap(),
            prover
                .prove_permission("alice", "doc2", "viewer", &["viewer"])
                .unwrap(),
        ];

        let aggregate = AggregateProof::aggregate(proofs).unwrap();
        assert_eq!(aggregate.proofs.len(), 2);

        let mut verifier = ZkVerifier::new();
        assert!(aggregate.verify(&mut verifier).unwrap());
    }

    #[test]
    fn test_aggregate_mixed_schemes_fails() {
        let mut prover1 = ZkProver::with_scheme(ZkProofScheme::Groth16);
        let mut prover2 = ZkProver::with_scheme(ZkProofScheme::Plonk);

        let proofs = vec![
            prover1
                .prove_permission("alice", "doc1", "viewer", &["viewer"])
                .unwrap(),
            prover2
                .prove_permission("bob", "doc2", "editor", &["editor"])
                .unwrap(),
        ];

        let result = AggregateProof::aggregate(proofs);
        assert!(result.is_err());
        assert!(result
            .unwrap_err()
            .to_string()
            .contains("different schemes"));
    }

    #[test]
    fn test_prove_tuple() {
        let mut prover = ZkProver::new();
        let tuple = RelationTuple::new(
            "document",
            "viewer",
            "123",
            Subject::User("alice".to_string()),
        );

        let proof = prover.prove_tuple(&tuple).unwrap();
        assert_eq!(proof.public_inputs.relation, "viewer");

        let mut verifier = ZkVerifier::new();
        assert!(verifier.verify_permission_proof(&proof).unwrap());
    }
}