Aztec Scanner

---
id: AZTEC-SCAN
title: Aztec Network Security Scanner
category: chain-scanner
trigger: "Audit Aztec|Noir|private contract"
last_updated: 2026-02-24
description: >-
  Use when the user wants to audit Aztec Network smart contracts written in
  Noir, scan for privacy-specific vulnerabilities including state leakage, note
  handling, or nullifier collisions, review private DeFi protocols for
  information disclosure, or analyze encrypted computation and zero-knowledge
  proof circuits.
---

# Aztec Network Security Scanner

Security scanner for Aztec Network smart contracts written in Noir. Aztec is a privacy-focused L2 on Ethereum with a unique dual public/private state model.

---

## Language & Runtime

| Attribute | Value |
|-----------|-------|
| Chain | Aztec Network (Ethereum L2) |
| Language | Noir (Rust-inspired, ZK-circuit language) |
| Proof System | Ultra-PLONK (client-side proving) |
| State Model | Dual: Private (UTXO/notes) + Public (storage slots) |
| Privacy | Private functions hide inputs, outputs, and who called them |
| Execution | Private: client-side → Public: sequencer-side |
| Token Standard | Private tokens via note encryption |

---

## Architecture: Public vs Private

```
┌─────────────────────────────────────────────────────────┐
│  Aztec Contract                                         │
├──────────────────────────┬──────────────────────────────┤
│  Private Functions          │  Public Functions               │
│  (Client-side execution)    │  (Sequencer-side execution)     │
│  - Hidden inputs/outputs    │  - Visible state changes        │
│  - Note creation/nullifying │  - Storage slot reads/writes    │
│  - ZK proof generation      │  - Similar to Solidity          │
├──────────────────────────┴──────────────────────────────┤
│  Private State: Notes (UTXO-like, encrypted)             │
│  Public State: Storage slots (like Solidity)             │
│  Shared: Nullifier tree, note hash tree, L1-L2 messages  │
└─────────────────────────────────────────────────────────┘
```

### Private State: Notes

Aztec uses a UTXO-like model for private state:

1. **Notes** are encrypted data stored in the note hash tree
2. To "read" a note, the owner decrypts it client-side
3. To "spend" a note, a **nullifier** is emitted (marks the note as consumed)
4. To "create" a note, a new note hash is added to the tree
5. Notes are **immutable** — to update, nullify the old note and create a new one

### Execution Order

Private functions execute FIRST (client-side), then public functions execute SECOND (sequencer-side). This ordering has security implications:
- Private can enqueue calls to public, but not vice versa
- Private computations can't read current public state (stale reads)
- Public functions see the effects of private functions

---

## Detection Capabilities

| Category | Detection | Severity |
|----------|-----------|----------|
| **Privacy** | Private data exposed via public state or function args | Critical |
| **Privacy** | Note content leaked through observable behavior (timing, gas) | High |
| **Privacy** | Encryption key mismanagement (wrong recipient) | Critical |
| **Nullifier** | Nullifier collision (two notes produce same nullifier) | Critical |
| **Nullifier** | Nullifier not emitted when note consumed (double-spend) | Critical |
| **Nullifier** | Nullifier predictable (allows front-running) | High |
| **State Sync** | Public/private state inconsistency | High |
| **State Sync** | Private function reads stale public state | Medium |
| **Circuit** | Under-constrained circuit (invalid proofs accepted) | Critical |
| **Circuit** | Over-constrained circuit (valid transactions rejected) | Medium |
| **Oracle** | Untrusted oracle data used in circuit constraints | High |
| **Access** | Private function callable without proper auth | High |
| **Notes** | Note not encrypted for correct recipient | Critical |
| **Notes** | Note discovery failure (recipient can't find their notes) | High |

---

## Privacy Threat Model

| Threat | Attack Vector | Mitigation |
|--------|--------------|-------------|
| Transaction graph analysis | Linking sender/receiver via timing | Add delay, use shield/unshield |
| Amount leakage | Fixed denomination reveals nothing | Use uniform note sizes |
| Metadata leakage | Public function calls reveal intent | Minimize public function usage |
| Sequencer censorship | Sequencer refuses to include tx | Forced inclusion via L1 |
| Nullifier linking | Linking nullifiers to note creation | Nullifier derived from note secret + position |

---

## Resources
- [Aztec Patterns](resources/aztec-patterns.md)

## Workflows
- [Aztec Audit](workflows/aztec-audit.md)

## Overview
Aztec is a privacy-focused L2 with:
- Private and public state separation
- UTXO-like note model for private data
- Noir language for circuit programming
- Encrypted function arguments
- Client-side proof generation

## Error Code Reference

Common Aztec/Noir errors encountered during audits. Noir circuits fail at proof generation time, making errors harder to debug than runtime reverts.

### Noir Language Errors

| Error Type | Error Pattern | Meaning |
|-----------|--------------|----------|
| Assertion failure | `assert(condition)` failed | Constraint not satisfied — proof cannot be generated |
| Array bounds | `index out of bounds` | Array access exceeds declared length |
| Integer overflow | `Overflow on arithmetic` | Operation exceeds field/integer bounds |
| Type mismatch | `Expected type X, found Y` | Incorrect type in circuit computation |
| Unresolved variable | `Variable not found` | Reference to undefined variable in circuit |
| Division by zero | `Division by zero` | Denominator is zero in integer division |

### Aztec Protocol Errors

| Error Pattern | Source | Meaning |
|--------------|--------|----------|
| `'NOTE_NOT_FOUND'` | Note management | Note does not exist or was nullified |
| `'NULLIFIER_ALREADY_EXISTS'` | Nullifier tree | Double-spend attempt — note already consumed |
| `'INVALID_CALLER'` | Access control | Unauthorized function caller |
| `'INVALID_CONTEXT'` | Context check | Wrong execution context (private vs public) |
| `'INVALID_NOTE_TYPE'` | Note deserialization | Note type does not match expected schema |
| `'PUBLIC_CALL_FAILED'` | Public function | Public portion of transaction reverted |
| `'PRIVATE_CALL_FAILED'` | Private function | Private proof verification failed |
| `'ENCRYPTION_FAILED'` | Note encryption | Failed to encrypt note for recipient |
| `'INVALID_SELECTOR'` | Function dispatch | Function selector not found on contract |

### Privacy-Specific Audit Errors

| Issue | Error Pattern | Audit Significance |
|-------|--------------|--------------------|
| Note viewing key leak | No error — silent | If viewing keys are shared incorrectly, privacy is broken without any on-chain error |
| Nullifier predictability | No error — logical | Predictable nullifiers allow note existence tracking — check nullifier derivation |
| Public/private boundary leak | Transaction appears normal | Data flowing from private to public context may leak information — audit cross-context calls |
| Incomplete nullification | No error | Note consumed but nullifier not published — allows double-spend |
| Proof witness leakage | No on-chain error | Witness data in proof reveals private inputs — verify proof contains only public signals |

## Troubleshooting

| Issue | Likely Cause | Solution |
|-------|-------------|----------|
| Privacy leakage not detected | Scanner audits logic without modeling information flow | Trace all data paths between private and public contexts; flag any private state exposed via public functions |
| Nullifier collision risks missed | Scanner doesn't model nullifier tree | Verify nullifier derivation includes unique components (note hash, secret, index) |
| Note encryption issues not caught | Scanner doesn't analyze encryption correctness | Verify notes are encrypted to correct recipient keys; check key rotation handling |
| Cross-context reentrancy missed | Scanner treats private/public as isolated | Trace private → public → private call chains for state manipulation opportunities |
| Proof circuit constraint gaps | Scanner checks Noir syntax, not constraint completeness | Verify every private input is constrained; unconstrained inputs allow proof forgery |
| Client-side proof manipulation ignored | Scanner only checks on-chain contracts | Audit client-side proof generation logic for witness injection or tampering vectors |