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.
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.
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:
Echidna should find a a call sequence that falisfies `echidna_sometimesfalse` and should be unable to find a falsifying input for `echidna_alwaystrue`.
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):
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).
Echidna exports an API to build powerful fuzzing systems, and has a multitude of configuration options. Unfortunately, these parts of the codebase change quickly and are thus poorly documented. The [examples/api directory](examples/api) or [Trail of Bits blog](https://blog.trailofbits.com/2018/05/03/state-machine-testing-with-echidna/) are excellent references, or use the references below to get in touch with us directly.
If you want to quickly test Echidna in Linux, we offer a statically linked binary release of v1.0.0.0 to download [here](https://github.com/crytic/echidna/releases/tag/1.0.0.0).
Otherwise, to install the latest revision of Echidna, we recommend to use [docker](https://www.docker.com/):
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](.travis/install-libff.sh) used in our CI tests)