echidna/README.md

# echidna

![Build Status](https://github.com/crytic/echidna/workflows/CI/badge.svg)
![echidna logo](echidna.png)

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

More seriously, Echidna is a Haskell library designed for fuzzing/property-based testing of EVM code. It supports relatively sophisticated grammar-based fuzzing campaigns to falsify a variety of predicates.

## Features

* Generates inputs tailored to your actual code
* Optional coverage guidance to find deeper bugs
* Automatic testcase minimization for quick triage
* Seamless integration into the development workflow
* Fast
* Beautiful logo

## Usage

### Executing the test runner

The core Echidna functionality is an executable called `echidna-test`. `echidna-test` takes a contract and a list of invariants (properties that should always remain true) as input. For each invariant, it generates random sequences of calls to the contract and checks if the invariant holds. If it can find some way to falsify the invariant, it prints the call sequence that does so. If it can't, you have some assurance the contract is safe.

### Writing invariants

Invariants are expressed as Solidity functions with names that begin with `echidna_`, have no arguments, and return a boolean. For example, if you have some `balance` variable that should never go below `20`, you can write an extra function in your contract like this one:

```solidity
function echidna_check_balance() public returns (bool) {
    return(balance >= 20);
}
```

To check these invariants, run:

```
$ echidna-test myContract.sol
```

An example contract with tests can be found [examples/solidity/basic/flags.sol](examples/solidity/basic/flags.sol). To run it, you should execute:
```
$ echidna-test examples/solidity/basic/flags.sol
```

Echidna should find a a call sequence that falisfies `echidna_sometimesfalse` and should be unable to find a falsifying input for `echidna_alwaystrue`.

### Truffle integration

Echidna can be used to test contracts compiled with [Truffle](https://truffleframework.com/) using [crytic-compile](https://github.com/crytic/crytic-compile). For instance,
we can uncover an integer overflow in the [Metacoin Truffle box](https://github.com/truffle-box/metacoin-box) using a
[contract with Echidna properties to test](examples/solidity/truffle/metacoin/contracts/MetaCoinEchidna.sol):

```
$ cd examples/solidity/truffle/metacoin
$ echidna-test . --contract TEST
...
echidna_convert: failed!💥
  Call sequence:
    mint(57896044618658097711785492504343953926634992332820282019728792003956564819968)
```

### Configuration options

Echidna's CLI can be used to choose the contract to test and load a configuration file.

```
$ echidna-test contract.sol --contract TEST --config="config.yaml"
```

The configuration file allows users to choose EVM and test generation
parameters. An example of a complete and annotated config file with the default options can be found at [examples/solidity/basic/default.yaml](examples/solidity/basic/default.yaml). More detailed documentation on the configuration options is available in our [wiki](https://github.com/trailofbits/echidna/wiki/Config).

## Installation

If you want to quickly test Echidna in Linux, we offer a statically linked binary release of v1.4.0.1 to download [here](https://github.com/crytic/echidna/releases/tag/v1.4.0.1).

Otherwise, to install the latest revision of Echidna, we recommend to use [docker](https://www.docker.com/):

```
$ docker build -t echidna .
```

for example

```
$ docker run -it -v `pwd`:/src echidna echidna-test /src/examples/solidity/basic/flags.sol
```

Nix users can install the lastest Echidna with:
```
nix-env -i -f https://github.com/crytic/echidna/tarball/master
```

If you'd prefer to build from source, use [Stack](https://docs.haskellstack.org/en/stable/README/).
`stack install` should build and compile `echidna-test` in `~/.local/bin`.
You will need to link against libreadline and libsecp256k1 (built with recovery enabled), which should be installed with the package manager of your choosing.
Additionally, you need to install the latest release of [libff](https://github.com/scipr-lab/libff), you can take a look to [this script](.github/scripts/install-libff.sh) used in our CI tests.
Some linux distributions do not ship static libraries for certain things that Haskell needs, e.g. archlinux, which will cause `stack build` to fail with linking errors because we use the `-static` flag. Removing these from `package.yaml` should get everything to build if you are not looking for a static build.

If you're getting errors building related to linking, try tinkering with `--extra-include-dirs` and `--extra-lib-dirs`.

## Development

It is possible to devlop Echidna with Cabal inside `nix-shell`. Nix will automatically
install all the dependencies required for development including `crytic-compile` and `solc`.
A quick way to get GHCi with Echidna ready for work:
```
$ git clone https://github.com/crytic/echidna
$ cd echidna
$ nix-shell
[nix-shell]$ cabal new-repl
```

## Getting help

Feel free to stop by our #ethereum slack channel in [Empire Hacking](https://empireslacking.herokuapp.com/) for help using or extending Echidna.

* Get started by reviewing these simple [Echidna invariants](examples/solidity/basic/flags.sol)

* Review the [Solidity examples](examples/solidity) directory for more extensive Echidna use cases

* Considering [emailing](mailto:echidna-dev@trailofbits.com) the Echidna development team directly for more detailed questions