# [−][src]Trait curve25519_dalek::traits::MultiscalarMul

pub trait MultiscalarMul {
type Point;
fn multiscalar_mul<I, J>(scalars: I, points: J) -> Self::Point    where        I: IntoIterator,        I::Item: Borrow<Scalar>,        J: IntoIterator,        J::Item: Borrow<Self::Point>;
}

A trait for constant-time multiscalar multiplication without precomputation.

## Associated Types

### type Point

The type of point being multiplied, e.g., RistrettoPoint.

## Required methods

### fn multiscalar_mul<I, J>(scalars: I, points: J) -> Self::Point where    I: IntoIterator,    I::Item: Borrow<Scalar>,    J: IntoIterator,    J::Item: Borrow<Self::Point>,

Given an iterator of (possibly secret) scalars and an iterator of public points, compute $$Q = c_1 P_1 + \cdots + c_n P_n.$$

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 Scalars or &Scalars.

use curve25519_dalek::constants;
use curve25519_dalek::traits::MultiscalarMul;
use curve25519_dalek::ristretto::RistrettoPoint;
use curve25519_dalek::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::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::multiscalar_mul(minus_abc, &[P,Q,R]);
// Note: minus_abc.into_iter(): Iterator<Item=Scalar>

assert_eq!(A1.compress(), (-A2).compress());