Echidna Fuzzer

Echidna Fuzzing Skill

Property-based testing and fuzzing for smart contracts using Echidna, the premier smart contract fuzzer from Trail of Bits.

Capabilities

• Property Tests: Write Echidna-compatible property tests
• Configuration: Customize fuzzing parameters
• Invariant Testing: Define and verify contract invariants
• Coverage Analysis: Analyze fuzzing coverage
• Corpus Management: Handle and minimize test cases
• Extended Campaigns: Run long fuzzing campaigns
• CI Integration: Automate fuzzing in pipelines

Installation

# Install via docker (recommended)
docker pull ghcr.io/crytic/echidna/echidna:latest

# Or download binary
curl -LO https://github.com/crytic/echidna/releases/latest/download/echidna-Linux
chmod +x echidna-Linux
mv echidna-Linux /usr/local/bin/echidna

# Verify
echidna --version

Property Testing

Basic Properties

// contracts/Token.sol
contract Token {
    mapping(address => uint256) public balances;
    uint256 public totalSupply;

    function transfer(address to, uint256 amount) external {
        require(balances[msg.sender] >= amount);
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}

// contracts/TokenTest.sol
contract TokenTest is Token {
    constructor() {
        balances[msg.sender] = 10000;
        totalSupply = 10000;
    }

    // Echidna property: name starts with echidna_
    function echidna_totalSupply_constant() public view returns (bool) {
        return totalSupply == 10000;
    }

    function echidna_balance_under_total() public view returns (bool) {
        return balances[msg.sender] <= totalSupply;
    }
}

Assertion Mode

contract TokenAssertions is Token {
    function transfer(address to, uint256 amount) external override {
        uint256 balanceBefore = balances[msg.sender] + balances[to];

        super.transfer(to, amount);

        uint256 balanceAfter = balances[msg.sender] + balances[to];

        // Assertion: conservation of tokens
        assert(balanceBefore == balanceAfter);
    }
}

Configuration

echidna.yaml

# Test configuration
testMode: property  # property, assertion, exploration, overflow
testLimit: 50000
seqLen: 100
shrinkLimit: 5000

# Contract configuration
deployer: "0x10000"
sender: ["0x10000", "0x20000", "0x30000"]
psender: "0x10000"

# Corpus configuration
corpusDir: "corpus"
coverage: true
coverageFormats: ["html", "lcov"]

# Filtering
filterBlacklist: true
filterFunctions: ["excludedFunction"]

# Advanced
codeSize: 0xffff
gasLimit: 10000000
prefix: "echidna_"

# Assertion mode specific
checkAsserts: true

# Workers
workers: 4

Run Commands

# Basic run
echidna contracts/TokenTest.sol --contract TokenTest

# With config
echidna contracts/TokenTest.sol --contract TokenTest --config echidna.yaml

# Assertion mode
echidna contracts/TokenTest.sol --contract TokenTest --test-mode assertion

# Multi-ABI mode (test multiple contracts)
echidna . --contract TokenTest --crytic-args "--compile-all"

Advanced Patterns

Time-Based Properties

contract TimeBased {
    uint256 public startTime;
    uint256 public lockedUntil;

    constructor() {
        startTime = block.timestamp;
        lockedUntil = block.timestamp + 1 days;
    }

    function withdraw() external {
        require(block.timestamp >= lockedUntil);
        // withdraw logic
    }

    function echidna_locked_before_time() public view returns (bool) {
        // Echidna can manipulate block.timestamp
        return block.timestamp < lockedUntil || true; // simplified
    }
}

Multi-Contract Testing

contract EchidnaTest {
    Token token;
    Staking staking;

    constructor() {
        token = new Token();
        staking = new Staking(address(token));
    }

    function stake(uint256 amount) public {
        token.approve(address(staking), amount);
        staking.stake(amount);
    }

    function echidna_staking_invariant() public view returns (bool) {
        return staking.totalStaked() <= token.totalSupply();
    }
}

DeFi Invariants

contract AMMTest is AMM {
    function echidna_constant_product() public view returns (bool) {
        // k = x * y should be constant (or increase)
        uint256 currentK = reserveX * reserveY;
        return currentK >= initialK;
    }

    function echidna_no_free_tokens() public view returns (bool) {
        // Total tokens in pool >= total LP tokens value
        return reserveX + reserveY >= totalLPTokens;
    }

    function echidna_price_bounds() public view returns (bool) {
        // Price should be within reasonable bounds
        uint256 price = (reserveX * 1e18) / reserveY;
        return price > 0 && price < type(uint256).max / 1e18;
    }
}

Coverage Analysis

Generate Coverage

# echidna.yaml
coverage: true
coverageFormats: ["html", "lcov", "txt"]
corpusDir: "corpus"

# Run with coverage
echidna contracts/Test.sol --contract Test --config echidna.yaml

# View HTML coverage
open corpus/covered.html

Interpret Coverage

• Green: Fully covered lines
• Yellow: Partially covered (some branches)
• Red: Uncovered code
• Gray: Non-executable

Corpus Management

Corpus Structure

corpus/
├── coverage/
│   ├── covered.txt
│   └── covered.html
├── reproducers/
│   └── failing_test.txt
└── corpus/
    └── sequence_1234.txt

Replay Corpus

# Replay a failing sequence
echidna contracts/Test.sol --contract Test --corpus-dir corpus --replay

CI/CD Integration

GitHub Actions

name: Echidna Fuzzing
on: [push, pull_request]

jobs:
  echidna:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      - name: Install Foundry
        uses: foundry-rs/foundry-toolchain@v1

      - name: Compile contracts
        run: forge build

      - name: Run Echidna
        uses: crytic/echidna-action@v2
        with:
          files: contracts/Test.sol
          contract: Test
          config: echidna.yaml
          test-limit: 10000

Extended Fuzzing

# echidna-extended.yaml
testLimit: 1000000
timeout: 86400  # 24 hours
workers: 8

# Run extended campaign
echidna . --contract Test --config echidna-extended.yaml

Process Integration

Best Practices

1. Start with simple properties
2. Use assertion mode for internal checks
3. Test invariants that should NEVER break
4. Run extended campaigns before deployments
5. Maintain corpus between runs
6. Review coverage to improve tests
7. Combine with Slither and Mythril

Troubleshooting

Out of Gas

gasLimit: 100000000  # Increase limit

Compilation Issues

# Use crytic-compile directly
echidna . --crytic-args "--foundry-compile-all"

Slow Fuzzing

workers: 8  # Increase parallelism
shrinkLimit: 1000  # Reduce shrinking

See Also

• skills/slither-analysis/SKILL.md - Static analysis
• skills/mythril-symbolic/SKILL.md - Symbolic execution
• skills/foundry-framework/SKILL.md - Forge invariant testing
• agents/solidity-auditor/AGENT.md - Security auditor
• Echidna Documentation