Is the function from_hardened_idx correctly implemented?

[ywiyogo]

In https://github.com/rust-bitcoin/rust-bitcoin/blob/master/bitcoin/src/bip32.rs#L149, I see that the if check is as same as the from_normal_idx. Shouldn't the implementation like this:

        if index & (1 << 31) == 1 {
            Ok(ChildNumber::Hardened { index })
        } else {
            Err(Error::InvalidChildNumber(index))
        }       

[tcharding]

The ChildNumber enum maintains the invariant that each value does not have the top bit set. In other words the inner values of ChildNumber::Hardened and ChildNumber::Normal do not use the top bit. The from_hardened_idx and from_normal_idx expect the input value to not use the top bit (to be within the range [0, 2^31 - 1]). There is also the From<u32> impl that accepts a u32 with the top bit specifying hardened/normal

impl From<u32> for ChildNumber {
    fn from(number: u32) -> Self {
        if number & (1 << 31) != 0 {
            ChildNumber::Hardened { index: number ^ (1 << 31) }
        } else {
            ChildNumber::Normal { index: number }
        }
    }
}

[apoelstra]

Yep, this is correct. Though it could perhaps be commented better.

[ywiyogo]

Thanks for the clarification!