crytic / tayt

StarkNet smart contract fuzzer

Tayt

Tayt is a StarkNet smart contract fuzzer.

Installation

We recommend using a Python virtual environment.

git clone https://github.com/crytic/tayt.git && cd tayt
python setup.py install

If you don't have cairo-lang already installed and you are on MacOS you may have an error about a missing gmp.h file even if you executed brew install gmp. The following command can be used to solve it.

CFLAGS=-I`brew --prefix gmp`/include LDFLAGS=-L`brew --prefix gmp`/lib pip install ecdsa fastecdsa sympy

If the above command doesn't work you can find more solutions here.

Usage

Run with default options.

tayt tests/flags.cairo

When starting you will see the properties to be checked and the external functions used to generate a sequence of transactions.

Fuzzing the following properties:
	tayt_flag1
External functions:
	set0
	set1

Eventually if a property is violated a call sequence will be presented with the order of functions to be called, the respective arguments passed, the caller address, and the events emitted represented by a starting E.

[!] tayt_flag1 violated
Call sequence:
	set0[0] from 1
	 E set_flag0[0]
	set1[97066683862585213645535248899637309600] from 0
	 E set_flag1[97066683862585213645535248899637309600]

The full help menu is:

usage: tayt [-h] [--seq-len SEQ_LEN] [--blacklist-function BLACKLIST_FUNCTION [BLACKLIST_FUNCTION ...]]
            [--psender PSENDER] [--sender SENDER [SENDER ...]] [--cairo-path CAIRO_PATH [CAIRO_PATH ...]]
            [--coverage] [--no-shrink] [--get-class-hash] [--declare DECLARE [DECLARE ...]]
            filename

StarkNet smart contract fuzzer.

positional arguments:
  filename              Cairo file to analyze.

optional arguments:
  -h, --help            show this help message and exit
  --seq-len SEQ_LEN     Number of transactions to generate during testing. (default: 10)
  --blacklist-function BLACKLIST_FUNCTION [BLACKLIST_FUNCTION ...]
                        Function name (space separated) to blacklist from execution.
  --psender PSENDER     Address of the sender for property transactions. (default: 1)
  --sender SENDER [SENDER ...]
                        Addresses (space separated) to use for the transactions sent during testing.
                        (default: [0, 1, 2])
  --cairo-path CAIRO_PATH [CAIRO_PATH ...]
                        A list of directories, separated by space to resolve import paths.
  --coverage            Output a coverage file.
  --no-shrink           Avoid shrinking failing sequences.
  --get-class-hash      Get the class hash to use with a deploy function.
  --declare DECLARE [DECLARE ...]
                        A list of contracts that will be declared.

Writing invariants

Invariants are StarkNet view functions with names that begin with tayt_, have no arguments, and return a felt. An invariant is considered failed when it returns 0.

@view
func tayt_flag{
        range_check_ptr,
        syscall_ptr: felt*,
        pedersen_ptr: HashBuiltin*
    }() -> (res: felt):
    let (flag_result) = flag.read()
    if flag_result == 1:
        return (0)
    end
    return (1)
end

If the flag storage variable is set to 1 the invariant will fail.

How to test a contract that deploys other contracts

We will use test/deploy.cairo as an example of a contract that deploys other contracts. First we have to get the class hash of the contracts we want to deploy:

In our case we will deploy test/flags.cairo.

tayt --get-class-hash tests/flags.cairo

We will get the class hash to use in the deploy function.

Class hash for tests/flags.cairo
2024779828085525422431444182955849544076259995530386260630136607064428821244

Finally we can test deploy.cairo, the --declare option takes a list of contracts to declare in the fuzzing state.

tayt tests/deploy.cairo --declare tests/flags.cairo

Coverage

When the --coverage option is enabled, a file named covered.{time}.txt which contains the source code with coverage annotations will be saved. A line starting with * has been executed at least once.

Example with tests/flags.cairo:

@external
*func set1{
*        syscall_ptr: felt*,
*        pedersen_ptr: HashBuiltin*,
*        range_check_ptr,
*        ecdsa_ptr: SignatureBuiltin*
*    }(val: felt):
*    let (res, remainder) = unsigned_div_rem(val, 10)
*    if remainder == 0:
*        let (flag_0) = flag0.read()
*        if flag_0 == 1:
*            flag1.write(1)
*            set_flag1.emit(val)
*            return ()
        end
*        return ()
    end
*    return()    
end