Spike/zk nullifier claim

docs/spikes/zk-nullifier-claim.md

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+# ZK Nullifier Claim Verification on Soroban — Feasibility Spike
+
+## Executive Summary
+
+This spike evaluates whether Groth16 zero-knowledge proofs can be verified
+inside a Soroban WASM smart contract for privacy-preserving escrow claims.
+**Conclusion: On-chain Groth16 is infeasible on Soroban today.** The
+recommended architecture is a hybrid: off-chain ZK proving (Noir +
+Barretenberg) with an on-chain nullifier registry that enforces
+double-spending via hash-based commitment checks, gated by an optional
+off-chain oracle attestation for high-value claims.
+
+---
+
+## 1. Soroban WASM Resource Analysis
+
+### Contract Size Limit
+
+| Limit | Value | Implication for ark-groth16 |
+|-------|-------|---------------------------|
+| WASM binary | 100 KB | ark-ec + ark-ff + ark-poly + ark-serialize → 500 KB+ — **blows limit** |
+| WASM modules | 1 per tx | Cannot split verification across contracts |
+
+Soroban v28 (future) may raise the limit, but no current timeline.
+
+### CPU Instruction Budget
+
+| Operation | Approx. Cost | Count Needed | Total |
+|-----------|-------------|-------------|-------|
+| BN254 pairing check (Miller loop) | 12–18M | 3 (Groth16) | 36–54M |
+| Pippenger MSM (multi-exp) | 8–12M | 1 | 8–12M |
+| SHA-256 (64 B) | 4k | 2 | 8k |
+| Storage read/write | 20k | 2 | 40k |
+
+The base budget is ~40M CPU ops (Soroban v27). A single pairing check
+consumes nearly half. **Groth16 verification exceeds the default budget.**
+Users could increase `soroban_invoke_contract` fees to raise the limit, but
+typical RPC providers cap this.
+
+### Memory
+
+WASM linear memory starts at 64 pages (4 MB). BN254 scalar multiplication
+tables need ~2–3 MB. Combined with the proving key material, memory is tight.
+
+### Crypto Primitives Available
+
+| Primitive | Soroban HostFn |
+|-----------|---------------|
+| SHA-256 | `env.crypto().sha256()` |
+| Keccak-256 | `env.crypto().keccak256()` |
+| Ed25519 verify | `env.crypto().ed25519_verify()` |
+| Secp256k1 ecdsa | `env.crypto().secp256k1_ecdsa_verify()` |
+| **BN254 pairing** | **Not available** |
+| **BLS12-381 pairing** | **Not available** |
+
+**No elliptic-curve pairing precompile exists.** A custom WASM
+implementation would add 200 KB+ to the binary and still need pairings
+implemented in WASM, which is 10–100× slower than native.
+
+---
+
+## 2. Nullifier Storage Schema
+
+The existing `contracts/zk-credential` contract already implements a
+gas-efficient nullifier registry. The pattern generalises to any claim flow:
+
+```rust
+#[contracttype]
+pub enum DataKey {
+    Nullifier(BytesN<32>),   // key   = nullifier_hash
+                             // value = bool (spent)
+}
+
+// ── Record ──
+env.storage().persistent().set(&DataKey::Nullifier(hash), &true);
+
+// ── Check ──
+if env.storage().persistent().has(&DataKey::Nullifier(hash)) {
+    return Err(Error::NullifierAlreadySpent);
+}
+```
+
+### Gas Efficiency
+
+| Storage Type | Rent (per entry) | Expiry |
+|-------------|-----------------|--------|
+| `persistent()` | ~0.5 XLM / entry | Permanent (with rent) |
+| `instance()` | ~1 XLM / contract | Contract lifetime |
+
+For nullifiers, `persistent()` is the natural fit: each nullifier is a
+32-byte key mapping to a `bool`. The footprint is minimal (~80 bytes per
+nullifier), and the rent cost is paid by the claim initiator.
+
+### Anti-Replay Guarantee
+
+- **Binding**: Nullifier hash is `Poseidon(secret, domain_sep)` in Noir
+  circuits, or `SHA256(secret || domain)` on-chain. Using the same domain
+  on both sides guarantees consistency.
+- **Finality**: Once `persistent().set()` commits, the nullifier is
+  permanently spent. No expiry is needed for nullifier entries (unlike
+  Merkle roots).
+
+---
+
+## 3. Architectural Comparison
+
+| Criterion | On-Chain WASM | Off-Chain Oracle |
+|-----------|--------------|------------------|
+| **ZK proof verification** | Infeasible (pairing precompiles missing; budget blown) | Feasible (Barretenberg runs natively on verifier node) |
+| **Proof size** | ~200 B (Groth16) — fits | ~200 B (Groth16) — fits in tx calldata |
+| **Latency** | Immediate (~5 s finality) | ~1–2 rounds (prover submits, oracle attestation settles) |
+| **Trust model** | Trustless (Soroban consensus) | Trusted oracle set (threshold signature or attestation) |
+| **Nullifier recording** | On-chain `persistent()` | On-chain `persistent()` (oracle attests and records) |
+| **Upgradeability** | Hard (contract must be replaced) | Easy (oracle software updates) |
+| **Cost per claim** | ~0.01 XLM (storage + invocation) | ~0.02 XLM (oracle fee + storage) |
+| **Privacy level** | Full privacy (ZK proof verified, no wallet revealed) | Conditional privacy (oracle sees proof metadata unless encrypted) |
+
+### Decision Matrix
+
+| Use Case | Verdict | Rationale |
+|----------|---------|-----------|
+| Low-value private claims (< 100 USDC) | **On-chain hash commitment** | Don't need full ZK; SHA256(root \|\| nullifier) is sufficient to bind proof |
+| Medium-value claims (100–10k USDC) | **Off-chain ZK + on-chain nullifier** | Prove off-chain, record nullifier on-chain via oracle attestation |
+| High-value claims (> 10k USDC) | **Off-chain ZK + oracle attestation + dispute period** | Full ZK proof verified by multiple oracles with slashing |
+| Cross-chain claims (Stellar → EVM) | **Off-chain ZK + relayer** | Proof verified off-chain, relayer submits lightweight attestation |
+
+---
+
+## 4. Recommended Architecture
+
+```
+┌──────────────┐     Groth16 proof      ┌───────────────┐
+│   Claimant   │ ──────────────────────▶ │ ZK Verifier   │
+│  (Noir Prover)│                        │   Node        │
+└──────────────┘                        │ (Barretenberg)│
+                                         └───────┬───────┘
+                                                 │
+                                          attestation sig
+                                                 │
+                                                 ▼
+                                   ┌─────────────────────────┐
+                                   │  Soroban: Escrow Claim   │
+                                   │  + Nullifier Registry    │
+                                   └─────────────────────────┘
+```
+
+### Key Components
+
+1. **Claimant (Noir Prover)**: Generates Groth16 proof using Noir
+   `credential_verifier` circuit. Submits `(root, nullifier_hash, proof)` to
+   the ZK Verifier Node.
+
+2. **ZK Verifier Node**: Runs Barretenberg to verify the Groth16 proof. On
+   success, signs an attestation containing `(nullifier_hash, claim_id,
+   timestamp)` with its Ed25519 key.
+
+3. **Soroban Escrow Contract**: Receives the attestation, verifies the
+   oracle's signature via `env.crypto().ed25519_verify()`, checks the
+   nullifier hasn't been spent, records it, and releases funds.
+
+### Why This Works Within Soroban Limits
+
+| Constraint | How We Stay Within It |
+|-----------|----------------------|
+| 100 KB WASM | Ed25519 verify + SHA-256 + storage — under 40 KB binary |
+| 40M CPU budget | Ed25519 verify (~200k) + SHA-256 (~4k) + storage (~20k) — under 1M total |
+| No pairing precompile | Pairings happen off-chain in Barretenberg |
+
+---
+
+## 5. Implementation Roadmap
+
+### Phase 1 (Current — zk-credential exists)
+- Nullifier storage via `persistent()`
+- Hash-based commitment binding (SHA-256)
+- Merkle root registry
+
+### Phase 2 (Next)
+- Deploy standalone ZK Verifier Node (Rust + Barretenberg)
+- Add Ed25519 oracle signature verification to escrow contract
+- Wire nullifier check into `claim()` flow
+
+### Phase 3 (Future)
+- Explore Soroban v28 precompile additions for BN254
+- Replace oracle with on-chain Groth16 if precompiles land
+- Multi-party oracle for high-value dispute resolution
+
+---
+
+## References
+
+- Existing contract: `contracts/zk-credential/src/lib.rs`
+- Noir circuits: `circuits/credential_verifier/`, `circuits/provider_reputation/`
+- Soroban SDK v27 docs: https://soroban.stellar.org/docs
+- ark-groth16: https://github.com/arkworks-rs/groth16

examples/zk_nullifier_poc.rs

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+//! POC: ZK nullifier claim verification on Soroban.
+//!
+//! Demonstrates the hybrid architecture:
+//!   1. Off-chain ZK proof is verified by a ZK Verifier Node (Barretenberg).
+//!   2. The node signs an attestation with its Ed25519 key.
+//!   3. This Soroban contract verifies the attestation, checks the nullifier,
+//!      records it, and releases escrow funds.
+
+#![no_std]
+
+use soroban_sdk::{
+    contract, contracterror, contractimpl, contracttype, symbol_short, Address, Bytes, BytesN, Env,
+};
+
+#[contracttype]
+#[derive(Clone)]
+pub enum DataKey {
+    OracleKey,
+    Nullifier(BytesN<32>),
+}
+
+#[contracterror]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum Error {
+    NotInitialized = 1,
+    AlreadyInitialized = 2,
+    NullifierAlreadySpent = 3,
+    InvalidAttestation = 4,
+}
+
+#[contract]
+pub struct ZKNullifierClaim;
+
+#[contractimpl]
+impl ZKNullifierClaim {
+    pub fn initialize(env: Env, oracle_pubkey: BytesN<32>) -> Result<(), Error> {
+        if env.storage().instance().has(&DataKey::OracleKey) {
+            return Err(Error::AlreadyInitialized);
+        }
+        env.storage()
+            .instance()
+            .set(&DataKey::OracleKey, &oracle_pubkey);
+        Ok(())
+    }
+
+    /// Verify an oracle-signed ZK attestation and record the nullifier.
+    ///
+    /// `attestation_payload` = nullifier_hash (32 B) || claim_id (32 B) || timestamp (8 B)
+    /// `signature` = Ed25519 signature over the payload.
+    pub fn claim_with_zk_attestation(
+        env: Env,
+        claimant: Address,
+        attestation_payload: Bytes,
+        signature: BytesN<64>,
+    ) -> Result<(), Error> {
+        let oracle_key: BytesN<32> = env
+            .storage()
+            .instance()
+            .get(&DataKey::OracleKey)
+            .ok_or(Error::NotInitialized)?;
+
+        claimant.require_auth();
+
+        // Extract nullifier hash from payload (first 32 bytes)
+        let nullifier_hash: BytesN<32> = attestation_payload
+            .slice(0..32)
+            .try_into()
+            .unwrap_or(BytesN::from_array(&env, &[0u8; 32]));
+
+        // Reject if already spent
+        if env
+            .storage()
+            .persistent()
+            .has(&DataKey::Nullifier(nullifier_hash.clone()))
+        {
+            return Err(Error::NullifierAlreadySpent);
+        }
+
+        // Verify oracle Ed25519 signature over the payload
+        let valid = env
+            .crypto()
+            .ed25519_verify(&oracle_key, &attestation_payload, &signature);
+        if !valid {
+            return Err(Error::InvalidAttestation);
+        }
+
+        // Record nullifier as spent
+        env.storage()
+            .persistent()
+            .set(&DataKey::Nullifier(nullifier_hash.clone()), &true);
+
+        env.events()
+            .publish((symbol_short!("claim"), claimant), (nullifier_hash,));
+
+        Ok(())
+    }
+
+    pub fn is_nullifier_spent(env: Env, nullifier_hash: BytesN<32>) -> bool {
+        env.storage()
+            .persistent()
+            .has(&DataKey::Nullifier(nullifier_hash))
+    }
+}