README.md

Incorrect Felt Comparison

In Cairo, there are two methods for the less than or equal to comparison operator: assert_le and assert_nn_le:

• assert_le asserts that a number a is less than or equal to b, regardless of the size of a
• assert_nn_le additionally asserts that a is non-negative, i.e. not greater than or equal to the RANGE_CHECK_BOUND value of 2^128.

assert_nn_le works to compare unsigned integers but with a value less than 2^128 (e.g. an Uint256 field). To compare felts as unsigned integer over the entire range (0, P], assert_le_felt should be used. Note these functions exist also with the is_ prefix where they return 1 (TRUE) or 0 (FALSE).

Due to the complexity of these assertions, a common mistake is to use assert_le when assert_nn_le should be used.

Example

Suppose that a codebase uses the following checks regarding a hypothetical ERC20 token. In the first function, it may be possible that value is in fact greater than max_supply, yet because the function does not verify value >= 0 the assertion will incorrectly pass. The second function, however, asserts that 0 <= value <= max_supply, which will correctly not let an incorrect value go through the assertion.

@storage_var
func max_supply() -> (res: felt) {
}

@external
func bad_comparison{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}() {
    let (value: felt) = ERC20.total_supply();
    assert_le{range_check_ptr=range_check_ptr}(value, max_supply.read());

    // do something...

    return ();
}

@external
func better_comparison{syscall_ptr : felt*, pedersen_ptr : HashBuiltin*, range_check_ptr}() {
    let (value: felt) = ERC20.total_supply();
    assert_nn_le{range_check_ptr=range_check_ptr}(value, max_supply.read());

    // do something...

    return ();

    
}

Mitigations

• Review all felt comparisons closely.
• Determine what sort of behavior the comparison should have, and if assert_nn_le or assert_le_felt is more appropriate.
• Use assert_le if you explicitly want to make comparison between signed integers - otherwise explicitely document why it is used over assert_nn_le