fn optional_multiscalar_mul()

in src/traits.rs [163:226]

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

    fn optional_multiscalar_mul<I, J>(scalars: I, points: J) -> Option<Self::Point>
    where
        I: IntoIterator,
        I::Item: Borrow<Scalar>,
        J: IntoIterator<Item = Option<Self::Point>>;

    /// Given an iterator of public scalars and an iterator of
    /// public points, compute
    /// $$
    /// Q = c\_1 P\_1 + \cdots + c\_n P\_n,
    /// $$
    /// using variable-time operations.
    ///
    /// It is an error to call this function with two iterators of different lengths.
    ///
    /// # Examples
    ///
    /// The trait bound aims for maximum flexibility: the inputs must be
    /// convertable to iterators (`I: IntoIter`), and the iterator's items
    /// must be `Borrow<Scalar>` (or `Borrow<Point>`), to allow
    /// iterators returning either `Scalar`s or `&Scalar`s.
    ///
    /// ```
    /// use curve25519_dalek_fiat::constants;
    /// use curve25519_dalek_fiat::traits::VartimeMultiscalarMul;
    /// use curve25519_dalek_fiat::ristretto::RistrettoPoint;
    /// use curve25519_dalek_fiat::scalar::Scalar;
    ///
    /// // Some scalars
    /// let a = Scalar::from(87329482u64);
    /// let b = Scalar::from(37264829u64);
    /// let c = Scalar::from(98098098u64);
    ///
    /// // Some points
    /// let P = constants::RISTRETTO_BASEPOINT_POINT;
    /// let Q = P + P;
    /// let R = P + Q;
    ///
    /// // A1 = a*P + b*Q + c*R
    /// let abc = [a,b,c];
    /// let A1 = RistrettoPoint::vartime_multiscalar_mul(&abc, &[P,Q,R]);
    /// // Note: (&abc).into_iter(): Iterator<Item=&Scalar>
    ///
    /// // A2 = (-a)*P + (-b)*Q + (-c)*R
    /// let minus_abc = abc.iter().map(|x| -x);
    /// let A2 = RistrettoPoint::vartime_multiscalar_mul(minus_abc, &[P,Q,R]);
    /// // Note: minus_abc.into_iter(): Iterator<Item=Scalar>
    ///
    /// assert_eq!(A1.compress(), (-A2).compress());
    /// ```
    fn vartime_multiscalar_mul<I, J>(scalars: I, points: J) -> Self::Point
    where
        I: IntoIterator,
        I::Item: Borrow<Scalar>,
        J: IntoIterator,
        J::Item: Borrow<Self::Point>,
        Self::Point: Clone,
    {
        Self::optional_multiscalar_mul(
            scalars,
            points.into_iter().map(|P| Some(P.borrow().clone())),
        )
        .unwrap()
    }