fn compute_tweak()

in src/poprf.rs [615:646]

• 20 lines of code
• 1 McCabe index (conditional complexity)

fn compute_tweak<CS: CipherSuite>(
    sk: <CS::Group as Group>::Scalar,
    info: Option<&[u8]>,
) -> Result<<CS::Group as Group>::Scalar>
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 = skS + m
    // if t == 0:
    //   raise InverseError
    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 = sk + &m;

    // Check if resulting element is equal to zero
    match bool::from(CS::Group::is_zero_scalar(t)) {
        true => Err(Error::Protocol),
        false => Ok(t),
    }
}