fn compute_tweaked_key()

in src/poprf.rs [576:609]

• 29 lines of code
• 5 McCabe index (conditional complexity)

fn compute_tweaked_key<CS: CipherSuite>(
    pk: <CS::Group as Group>::Elem,
    info: Option<&[u8]>,
) -> Result<<CS::Group as Group>::Elem>
where
    <CS::Hash as OutputSizeUser>::OutputSize:
        IsLess<U256> + IsLessOrEqual<<CS::Hash as BlockSizeUser>::BlockSize>,
{
    // None for info is treated the same as empty bytes
    let info = info.unwrap_or_default();

    // framedInfo = "Info" || I2OSP(len(info), 2) || info
    // m = G.HashToScalar(framedInfo)
    // T = G.ScalarBaseMult(m)
    // tweakedKey = T + pkS
    // if tweakedKey == G.Identity():
    //   raise InvalidInputError
    let info_len = i2osp_2(info.len()).map_err(|_| Error::Info)?;
    let framed_info = [STR_INFO.as_slice(), &info_len, info];

    let dst =
        GenericArray::from(STR_HASH_TO_SCALAR).concat(create_context_string::<CS>(Mode::Poprf));
    // This can't fail, the size of the `input` is known.
    let m = CS::Group::hash_to_scalar::<CS>(&framed_info, &dst).unwrap();

    let t = CS::Group::base_elem() * &m;
    let tweaked_key = t + &pk;

    // Check if resulting element
    match bool::from(CS::Group::is_identity_elem(tweaked_key)) {
        true => Err(Error::Protocol),
        false => Ok(tweaked_key),
    }
}