echidna

Echidna is a weird creature that eats bugs and is highly electrosensitive (with apologies to Jacob Stanley)

More seriously, it's a Haskell library designed for fuzzing/property based testing of EVM code. Currently it is quite alpha, and the API isn't guaranteed to be functional, let alone stable. It supports relatively sophisticated grammar-based fuzzing campaigns to falsify a variety of predicates.

Installation

stack is highly recommended to install echidna. If you are a particularly opinionated experienced Haskell user, cabal or hpack should work, but they are neither officially supported nor tested.

Before starting with it, make sure you have libgmp-dev installed otherwise ghc will fail to compile. Also, libbz2 and libreadline are required by some packages. For instance, in Ubuntu/Debian you can execute:

# apt-get install libgmp-dev libbz2-dev libreadline-dev

solc is another echidna dependency not handled via stack. It is technically optional, but working with solidity source will fail without it. Run npm install -g solc to install it.

Once solc is installed, installing stack (brew install haskell-stack) and running

stack upgrade
stack setup
stack install

from inside the echidna directory should be all that's needed.

If you have weird problems involving readline on MacOS, try:

brew install readline
brew link readline --force
export LDFLAGS=-L/usr/local/opt/readline/lib
export CPPFLAGS=-I/usr/local/opt/readline/include
stack install readline --extra-include-dirs=/usr/local/opt/readline/include --extra-lib-dirs=/usr/local/opt/readline/lib
stack install

Notably, if you are using stack, stack ghci will set up a REPL with all functions in scope. This can be quite useful for playing around with the library.

Usage (as an executable)

Echidna builds an executable, echidna-test that can be used from the command line to fuzz solidity code. It expects unit tests in the form of functions with names starting with echidna_ that take no arguments and return a bool indicating success or failure. For each unit test it finds, it will execute a fuzzing campaign to try and find a set of calls such that executing that call sequence, then the test either returns false or results in a VM failure.

An example contract with tests can be found solidity/cli.sol echidna-test solidity/cli.sol should find a call sequence such that echidna_sometimesfalse fails, but be unable to do so for echidna_alwaystrue.

Usage (as a library)

Echidna is actively being developed with relatively little regard for stability. As a result of this, there is a lack of extensive documentation at the present time. Nevertheless, we provide a short working example that should be relatively instructional:

module Main where

import Hedgehog hiding            (checkParallel)
import Hedgehog.Internal.Property (GroupName(..), PropertyName(..))

import Echidna.Exec
import Echidna.Solidity

main :: IO ()
main = do (v,a,ts) <- loadSolidity "test.sol" Nothing
          let prop t = (PropertyName $ show t, ePropertySeq v a (`checkETest` t) 10)
          _ <- checkParallel . Group (GroupName "test.sol") $ map prop ts
          return ()

This example can be used to test this small solidity contract:

pragma solidity ^0.4.16;

contract Test {
  uint private counter=1;
  uint private last_counter=counter;

  function inc(uint val){
    last_counter = counter;
    counter += val;
  }

  function skip() {
    return;
  }

  function echidna_check_counter() returns (bool) {
    if (last_counter > counter) {
      selfdestruct(0);
    }
    return true;
  }
}

Then, we can use echidna to find a counterexample:

━━━ test.sol ━━━
  ✗ "echidna_check_counter" failed after 14 tests and 132 shrinks.
  
      │ Call sequence: inc(111022932527598298683654135258804814522795708541881158271458983003743791605633);
      │                inc(4769156709717896739916849749883093330474276123759405767998601004169338034302);
  
  ✗ 1 failed.

Echidna.ABI

This module provides Hedgehog generators for most of the EVM ABI. It can be used without any other module to provide random "ASTs" (e.g. a random dynamic array of static arrays of 16 248-bit unsigned ints) or calldata (EVM-encoded function calls with these arguments).

Whenever possible, it tries to copy the convention of hevm.

Echidna.Exec

This module provides functionality for executing fuzzing campaigns. It's highly recommended to use ePropertySeq and checkParallel to do this. The standalone executable is a pretty good example of recommended usage. fuzz can also be used, but if the predicate doesn't need IO, it's not recommended.

It also provides some convenience functions for writing custom campaigns (checkETest, cleanUp, eCommand, execCall.) The state machine example is a pretty good example of how to use these.

Echidna.Solidity

This module provides loadSolidity, which takes a solidity source file and provides a VM with the first contract therein loaded as well as a fuzz-compatible ABI definition. At the moment, it is only compatible with single-contract solidity files, though if given a multi-contract file it will load the first and print a warning.

Questions/complaints/etc.

Join us in #ethereum on the Empire Hacking Slack, or email JP Smith (the lead author) directly.