Constant time curve arithmetic

[axelexic]

Does arkworks plan to have a software side-channel safe implementation for ec and ff arithmetic -- at least for prime-order groups in short-Weierstrass model? Arkworks is a well written library (kudos!), and we will like to use it in production for some 90's crypto (ECDSA, ECIES, ECDH, etc.). However, the current curve arithmetic seems to have no side-channel protection. For example:

pub fn sw_double_and_add_projective<P: SWCurveConfig>(
    base: &Projective<P>,
    scalar: impl AsRef<[u64]>,
) -> Projective<P> {
    let mut res = Projective::<P>::zero();
    for b in ark_ff::BitIteratorBE::without_leading_zeros(scalar) {
        res.double_in_place();
        if b {                          <== timing leaks scalar
            res += base  
        }
    }

    res
}

[burdges]

At minimum, we could've seperate constant time curve crates where external crates already supply them, so maybe wrappers over cruve25519-dalek and bls12_381.