Security

Untrusted Values

While Rue's type system aims to provide the means to comprehensively check the type of values at runtime, the types of values passed into the main function are currently assumed to be correct by default.

This means that a program like this is actually incorrect:

fn main(
    parent_coin_id: Bytes32,
    puzzle_hash: Bytes32,
    amount: Int,
) -> Bytes32 {
    sha256(parent_coin_id + puzzle_hash + amount as Bytes)
}

Because the length of the untrusted values parent_coin_id and puzzle_hash are unchecked, they may be manipulated in a way that increases the length of amount without affecting the concatenated value.

For example, if parent_coin_id was 28 bytes, puzzle_hash was shifted 4 bytes to the left, and the value of amount grew by 4 bytes, it would suddenly be much larger than it was before without changing the hash.

This is what led to the CAT1 vulnerability.

The solution is to use a more general type and check the invariants you care about at runtime:

fn main(
    parent_coin_id: Bytes,
    puzzle_hash: Bytes,
    amount: Int,
) -> Bytes32 {
    assert parent_coin_id is Bytes32;
    assert puzzle_hash is Bytes32;

    sha256(parent_coin_id + puzzle_hash + amount as Bytes)
}

Or you can use the builtin calculate_coin_id function:

calculate_coin_id(parent_coin_id, puzzle_hash, amount)

Now, if someone tried to play tricks with the lengths, the assertions would fail and the program would raise an error.