How to use Echidna to test smart contracts

Installation

Echidna can be installed through docker or using the pre-compiled binary.

Echidna through docker

docker pull trailofbits/eth-security-toolbox
docker run -it -v "$PWD":/home/training trailofbits/eth-security-toolbox

The last command runs eth-security-toolbox in a docker that has access to your current directory. You can change the files from your host, and run the tools on the files from the docker

Inside docker, run :

solc-select 0.5.11
cd /home/training

Binary

https://github.com/crytic/echidna/releases/tag/v1.4.0.0(opens in a new tab)

Introduction to property-based fuzzing

Echidna is a property-based fuzzer, we described in our previous blogposts (1(opens in a new tab), 2(opens in a new tab), 3(opens in a new tab)).

Fuzzing

Fuzzing(opens in a new tab) is a well-known technique in the security community. It consists of generating inputs that are more or less random to find bugs in the program. Fuzzers for traditional software (such as AFL(opens in a new tab) or LibFuzzer(opens in a new tab)) are known to be efficient tools to find bugs.

Beyond the purely random generation of inputs, there are many techniques and strategies to generate good inputs, including:

• Obtain feedback from each execution and guide generation using it. For example, if a newly generated input leads to the discovery of a new path, it can make sense to generate new inputs closes to it.
• Generating the input respecting a structural constraint. For example, if your input contains a header with a checksum, it will make sense to let the fuzzer generates input validating the checksum.
• Using known inputs to generate new inputs: if you have access to a large dataset of valid input, your fuzzer can generate new inputs from them, rather than starting from scratch its generation. These are usually called seeds.

Property-based fuzzing

Echidna belongs to a specific family of fuzzer: property-based fuzzing heavily inspired by QuickCheck(opens in a new tab). In contrast to classic fuzzer that will try to find crashes, Echidna will try to break user-defined invariants.

In smart contracts, invariants are Solidity functions, that can represent any incorrect or invalid state that the contract can reach, including:

• Incorrect access control: the attacker became the owner of the contract.
• Incorrect state machine: the tokens can be transferred while the contract is paused.
• Incorrect arithmetic: the user can underflow its balance and get unlimited free tokens.

Testing a property with Echidna

We will see how to test a smart contract with Echidna. The target is the following smart contract token.sol(opens in a new tab):

1contract Token{
2  mapping(address => uint) public balances;
3  function airdrop() public{
4      balances[msg.sender] = 1000;
5  }
6  function consume() public{
7      require(balances[msg.sender]>0);
8      balances[msg.sender] -= 1;
9  }
10  function backdoor() public{
11      balances[msg.sender] += 1;
12  }
13}
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We will make the assumption that this token must have the following properties:

• Anyone can have at maximum 1000 tokens
• The token cannot be transferred (it is not an ERC20 token)

Write a property

Echidna properties are Solidity functions. A property must:

• Have no argument
• Return true if it is successful
• Have its name starting with echidna

Echidna will:

• Automatically generate arbitrary transactions to test the property.
• Report any transactions leading a property to return false or throw an error.
• Discard side-effect when calling a property (i.e. if the property changes a state variable, it is discarded after the test)

The following property checks that the caller has no more than 1000 tokens:

1function echidna_balance_under_1000() public view returns(bool){
2      return balances[msg.sender] <= 1000;
3}
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Use inheritance to separate your contract from your properties:

1contract TestToken is Token{
2      function echidna_balance_under_1000() public view returns(bool){
3            return balances[msg.sender] <= 1000;
4      }
5  }
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token.sol(opens in a new tab) implements the property and inherits from the token.

Initiate a contract

Echidna needs a constructor without argument. If your contract needs a specific initialization, you need to do it in the constructor.

There are some specific addresses in Echidna:

• 0x00a329c0648769A73afAc7F9381E08FB43dBEA72 which calls the constructor.
• 0x10000, 0x20000, and 0x00a329C0648769a73afAC7F9381e08fb43DBEA70 which randomly calls the other functions.

We do not need any particular initialization in our current example, as a result our constructor is empty.

Run Echidna

Echidna is launched with:

echidna-test contract.sol

If contract.sol contains multiple contracts, you can specify the target:

echidna-test contract.sol --contract MyContract

Summary: Testing a property

The following summarizes the run of echidna on our example:

1contract TestToken is Token{
2    constructor() public {}
3        function echidna_balance_under_1000() public view returns(bool){
4          return balances[msg.sender] <= 1000;
5        }
6  }
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echidna-test testtoken.sol --contract TestToken
...
echidna_balance_under_1000: failed!💥
  Call sequence, shrinking (1205/5000):
    airdrop()
    backdoor()
...
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Echidna found that the property is violated if backdoor is called.

Filtering functions to call during a fuzzing campaign

We will see how to filter the functions to be fuzzed. The target is the following smart contract:

1contract C {
2  bool state1 = false;
3  bool state2 = false;
4  bool state3 = false;
5  bool state4 = false;
6
7  function f(uint x) public {
8    require(x == 12);
9    state1 = true;
10  }
11
12  function g(uint x) public {
13    require(state1);
14    require(x == 8);
15    state2 = true;
16  }
17
18  function h(uint x) public {
19    require(state2);
20    require(x == 42);
21    state3 = true;
22  }
23
24 function i() public {
25    require(state3);
26    state4 = true;
27  }
28
29  function reset1() public {
30    state1 = false;
31    state2 = false;
32    state3 = false;
33    return;
34  }
35
36  function reset2() public {
37    state1 = false;
38    state2 = false;
39    state3 = false;
40    return;
41  }
42
43  function echidna_state4() public returns (bool) {
44    return (!state4);
45  }
46}
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This small example forces Echidna to find a certain sequence of transactions to change a state variable. This is hard for a fuzzer (it is recommended to use a symbolic execution tool like Manticore(opens in a new tab)). We can run Echidna to verify this:

echidna-test multi.sol
...
echidna_state4: passed! 🎉
Seed: -3684648582249875403

Filtering functions

Echidna has trouble finding the correct sequence to test this contract because the two reset functions (reset1 and reset2) will set all the state variables to false. However, we can use a special Echidna feature to either blacklist the reset function or to whitelist only the f, g, h and i functions.

To blacklist functions, we can use this configuration file:

1filterBlacklist: true
2filterFunctions: ["reset1", "reset2"]

Another approach to filter functions is to list the whitelisted functions. To do that, we can use this configuration file:

1filterBlacklist: false
2filterFunctions: ["f", "g", "h", "i"]

• filterBlacklist is true by default.
• Filtering will be performed by name only (without parameters). If you have f() and f(uint256), the filter "f" will match both functions.

Run Echidna

To run Echidna with a configuration file blacklist.yaml:

echidna-test multi.sol --config blacklist.yaml
...
echidna_state4: failed!💥
  Call sequence:
    f(12)
    g(8)
    h(42)
    i()

Echidna will find the sequence of transactions to falsify the property almost immediately.

Summary: Filtering functions

Echidna can either blacklist or whitelist functions to call during a fuzzing campaign using:

1filterBlacklist: true
2filterFunctions: ["f1", "f2", "f3"]

echidna-test contract.sol --config config.yaml
...

Echidna starts a fuzzing campaign either blacklisting f1, f2 and f3 or only calling these, according to the value of the filterBlacklist boolean.

How to test Solidity's assert with Echidna

In this short tutorial, we are going to show how to use Echidna to test assertion checking in contracts. Let's suppose we have a contract like this one:

1contract Incrementor {
2  uint private counter = 2**200;
3
4  function inc(uint val) public returns (uint){
5    uint tmp = counter;
6    counter += val;
7    // tmp <= counter
8    return (counter - tmp);
9  }
10}
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Write an assertion

We want to make sure that tmp is less or equal than counter after returning its difference. We could write an Echidna property, but we will need to store the tmp value somewhere. Instead, we could use an assertion like this one:

1contract Incrementor {
2  uint private counter = 2**200;
3
4  function inc(uint val) public returns (uint){
5    uint tmp = counter;
6    counter += val;
7    assert (tmp <= counter);
8    return (counter - tmp);
9  }
10}
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Run Echidna

To enable the assertion failure testing, create an Echidna configuration file(opens in a new tab) config.yaml:

1checkAsserts: true

When we run this contract in Echidna, we obtain the expected results:

echidna-test assert.sol --config config.yaml
Analyzing contract: assert.sol:Incrementor
assertion in inc: failed!💥
  Call sequence, shrinking (2596/5000):
    inc(21711016731996786641919559689128982722488122124807605757398297001483711807488)
    inc(7237005577332262213973186563042994240829374041602535252466099000494570602496)
    inc(86844066927987146567678238756515930889952488499230423029593188005934847229952)
Seed: 1806480648350826486
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As you can see, Echidna reports some assertion failure in the inc function. Adding more than one assertion per function is possible, but Echidna cannot tell which assertion failed.

When and how use assertions

Assertions can be used as alternatives to explicit properties, specially if the conditions to check are directly related with the correct use of some operation f. Adding assertions after some code will enforce that the check will happen immediately after it was executed:

1function f(..) public {
2    // some complex code
3    ...
4    assert (condition);
5    ...
6}
7
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On the contrary, using an explicit echidna property will randomly execution transactions and there is no easy way to enforce exactly when it will be checked. It is still possible to do this workaround:

1function echidna_assert_after_f() public returns (bool) {
2    f(..);
3    return(condition);
4}
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However, there are some issues:

• It fails if f is declared as internal or external.
• It is unclear which arguments should be used to call f.
• If f reverts, the property will fail.

In general, we recommend following John Regehr's recommendation(opens in a new tab) on how to use assertions:

• Do not force any side effect during the assertion checking. For instance: assert(ChangeStateAndReturn() == 1)
• Do not assert obvious statements. For instance assert(var >= 0) where var is declared as uint.

Finally, please do not use require instead of assert, since Echidna will not be able to detect it (but the contract will revert anyway).

Summary: Assertion Checking

The following summarizes the run of echidna on our example:

1contract Incrementor {
2  uint private counter = 2**200;
3
4  function inc(uint val) public returns (uint){
5    uint tmp = counter;
6    counter += val;
7    assert (tmp <= counter);
8    return (counter - tmp);
9  }
10}
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echidna-test assert.sol --config config.yaml
Analyzing contract: assert.sol:Incrementor
assertion in inc: failed!💥
  Call sequence, shrinking (2596/5000):
    inc(21711016731996786641919559689128982722488122124807605757398297001483711807488)
    inc(7237005577332262213973186563042994240829374041602535252466099000494570602496)
    inc(86844066927987146567678238756515930889952488499230423029593188005934847229952)
Seed: 1806480648350826486
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Echidna found that the assertion in inc can fail if this function is called multiple times with large arguments.

Collecting and modifying an Echidna corpus

We will see how to collect and use a corpus of transactions with Echidna. The target is the following smart contract magic.sol(opens in a new tab):

1contract C {
2  bool value_found = false;
3  function magic(uint magic_1, uint magic_2, uint magic_3, uint magic_4) public {
4    require(magic_1 == 42);
5    require(magic_2 == 129);
6    require(magic_3 == magic_4+333);
7    value_found = true;
8    return;
9  }
10
11  function echidna_magic_values() public returns (bool) {
12    return !value_found;
13  }
14
15}
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This small example forces Echidna to find certain values to change a state variable. This is hard for a fuzzer (it is recommended to use a symbolic execution tool like Manticore(opens in a new tab)). We can run Echidna to verify this:

echidna-test magic.sol
...
echidna_magic_values: passed! 🎉
Seed: 2221503356319272685

However, we can still use Echidna to collect corpus when running this fuzzing campaign.

Collecting a corpus

To enable the corpus collection, create a corpus directory:

mkdir corpus-magic

And an Echidna configuration file(opens in a new tab) config.yaml:

1coverage: true
2corpusDir: "corpus-magic"

Now we can run our tool and check the collected corpus:

echidna-test magic.sol --config config.yaml

Echidna still cannot find the correct magic values, but we can take look to the corpus it collected. For instance, one of these files was:

1[
2  {
3    "_gas'": "0xffffffff",
4    "_delay": ["0x13647", "0xccf6"],
5    "_src": "00a329c0648769a73afac7f9381e08fb43dbea70",
6    "_dst": "00a329c0648769a73afac7f9381e08fb43dbea72",
7    "_value": "0x0",
8    "_call": {
9      "tag": "SolCall",
10      "contents": [
11        "magic",
12        [
13          {
14            "contents": [
15              256,
16              "93723985220345906694500679277863898678726808528711107336895287282192244575836"
17            ],
18            "tag": "AbiUInt"
19          },
20          {
21            "contents": [256, "334"],
22            "tag": "AbiUInt"
23          },
24          {
25            "contents": [
26              256,
27              "68093943901352437066264791224433559271778087297543421781073458233697135179558"
28            ],
29            "tag": "AbiUInt"
30          },
31          {
32            "tag": "AbiUInt",
33            "contents": [256, "332"]
34          }
35        ]
36      ]
37    },
38    "_gasprice'": "0xa904461f1"
39  }
40]
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Clearly, this input will not trigger the failure in our property. However, in the next step, we will see how to modify it for that.

Seeding a corpus

Echidna needs some help in order to deal with the magic function. We are going to copy and modify the input to use suitable parameters for it:

cp corpus/2712688662897926208.txt corpus/new.txt

We will modify new.txt to call magic(42,129,333,0). Now, we can re-run Echidna:

echidna-test magic.sol --config config.yaml
...
echidna_magic_values: failed!💥
  Call sequence:
    magic(42,129,333,0)
Unique instructions: 142
Unique codehashes: 1
Seed: -7293830866560616537
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This time, it found that the property is violated immediately.

Finding transactions with high gas consumption

We will see how to find the transactions with high gas consumption with Echidna. The target is the following smart contract:

1contract C {
2  uint state;
3
4  function expensive(uint8 times) internal {
5    for(uint8 i=0; i < times; i++)
6      state = state + i;
7  }
8
9  function f(uint x, uint y, uint8 times) public {
10    if (x == 42 && y == 123)
11      expensive(times);
12    else
13      state = 0;
14  }
15
16  function echidna_test() public returns (bool) {
17    return true;
18  }
19
20}
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Here expensive can have a large gas consumption.

Currently, Echidna always need a property to test: here echidna_test always returns true. We can run Echidna to verify this:

1echidna-test gas.sol
2...
3echidna_test: passed! 🎉
4
5Seed: 2320549945714142710

Measuring Gas Consumption

To enable the gas consumption with Echidna, create a configuration file config.yaml:

1estimateGas: true

In this example, we will also reduce the size of the transaction sequence to make the results easier to understand:

1seqLen: 2
2estimateGas: true

Run Echidna

Once we have the configuration file created, we can run Echidna like this:

echidna-test gas.sol --config config.yaml
...
echidna_test: passed! 🎉
f used a maximum of 1333608 gas
  Call sequence:
    f(42,123,249) Gas price: 0x10d5733f0a Time delay: 0x495e5 Block delay: 0x88b2
Unique instructions: 157
Unique codehashes: 1
Seed: -325611019680165325
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• The gas shown is an estimation provided by HEVM(opens in a new tab).

Filtering Out Gas-Reducing Calls

The tutorial on filtering functions to call during a fuzzing campaign above shows how to remove some functions from your testing.
This can be critical for getting an accurate gas estimate. Consider the following example:

1contract C {
2  address [] addrs;
3  function push(address a) public {
4    addrs.push(a);
5  }
6  function pop() public {
7    addrs.pop();
8  }
9  function clear() public{
10    addrs.length = 0;
11  }
12  function check() public{
13    for(uint256 i = 0; i < addrs.length; i++)
14      for(uint256 j = i+1; j < addrs.length; j++)
15        if (addrs[i] == addrs[j])
16          addrs[j] = address(0x0);
17  }
18  function echidna_test() public returns (bool) {
19      return true;
20  }
21}
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If Echidna can call all the functions, it won't easily find transactions with high gas cost:

1echidna-test pushpop.sol --config config.yaml
2...
3pop used a maximum of 10746 gas
4...
5check used a maximum of 23730 gas
6...
7clear used a maximum of 35916 gas
8...
9push used a maximum of 40839 gas
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That's because the cost depends on the size of addrs and random calls tend to leave the array almost empty. Blacklisting pop and clear, however, gives us much better results:

1filterBlacklist: true
2filterFunctions: ["pop", "clear"]

1echidna-test pushpop.sol --config config.yaml
2...
3push used a maximum of 40839 gas
4...
5check used a maximum of 1484472 gas

Summary: Finding transactions with high gas consumption

Echidna can find transactions with high gas consumption using the estimateGas configuration option:

1estimateGas: true

echidna-test contract.sol --config config.yaml
...

Echidna will report a sequence with the maximum gas consumption for every function, once the fuzzing campaign is over.