curve25519-dalek-source/src/backend/vector/scalar_mul/straus.rs

// -*- mode: rust; -*-
//
// This file is part of curve25519-dalek.
// Copyright (c) 2016-2021 isis lovecruft
// Copyright (c) 2016-2019 Henry de Valence
// See LICENSE for licensing information.
//
// Authors:
// - isis agora lovecruft <isis@patternsinthevoid.net>
// - Henry de Valence <hdevalence@hdevalence.ca>

#![allow(non_snake_case)]

use core::borrow::Borrow;
use core::cmp::Ordering;

use zeroize::Zeroizing;

use crate::backend::vector::{CachedPoint, ExtendedPoint};
use crate::edwards::EdwardsPoint;
use crate::scalar::Scalar;
use crate::traits::{Identity, MultiscalarMul, VartimeMultiscalarMul};
use crate::window::{LookupTable, NafLookupTable5};

#[allow(unused_imports)]
use crate::prelude::*;

/// Multiscalar multiplication using interleaved window / Straus'
/// method.  See the `Straus` struct in the serial backend for more
/// details.
///
/// This exists as a seperate implementation from that one because the
/// AVX2 code uses different curve models (it does not pass between
/// multiple models during scalar mul), and it has to convert the
/// point representation on the fly.
pub struct Straus {}

impl MultiscalarMul for Straus {
    type Point = EdwardsPoint;

    fn multiscalar_mul<I, J>(scalars: I, points: J) -> EdwardsPoint
    where
        I: IntoIterator,
        I::Item: Borrow<Scalar>,
        J: IntoIterator,
        J::Item: Borrow<EdwardsPoint>,
    {
        // Construct a lookup table of [P,2P,3P,4P,5P,6P,7P,8P]
        // for each input point P
        let lookup_tables: Vec<_> = points
            .into_iter()
            .map(|point| LookupTable::<CachedPoint>::from(point.borrow()))
            .collect();

        let scalar_digits_vec: Vec<_> = scalars
            .into_iter()
            .map(|s| s.borrow().as_radix_16())
            .collect();
        // Pass ownership to a `Zeroizing` wrapper
        let scalar_digits = Zeroizing::new(scalar_digits_vec);

        let mut Q = ExtendedPoint::identity();
        for j in (0..64).rev() {
            Q = Q.mul_by_pow_2(4);
            let it = scalar_digits.iter().zip(lookup_tables.iter());
            for (s_i, lookup_table_i) in it {
                // Q = Q + s_{i,j} * P_i
                Q = &Q + &lookup_table_i.select(s_i[j]);
            }
        }
        Q.into()
    }
}

impl VartimeMultiscalarMul for Straus {
    type Point = EdwardsPoint;

    fn optional_multiscalar_mul<I, J>(scalars: I, points: J) -> Option<EdwardsPoint>
    where
        I: IntoIterator,
        I::Item: Borrow<Scalar>,
        J: IntoIterator<Item = Option<EdwardsPoint>>,
    {
        let nafs: Vec<_> = scalars
            .into_iter()
            .map(|c| c.borrow().non_adjacent_form(5))
            .collect();
        let lookup_tables: Vec<_> = points
            .into_iter()
            .map(|P_opt| P_opt.map(|P| NafLookupTable5::<CachedPoint>::from(&P)))
            .collect::<Option<Vec<_>>>()?;

        let mut Q = ExtendedPoint::identity();

        for i in (0..256).rev() {
            Q = Q.double();

            for (naf, lookup_table) in nafs.iter().zip(lookup_tables.iter()) {
                match naf[i].cmp(&0) {
                    Ordering::Greater => {
                        Q = &Q + &lookup_table.select(naf[i] as usize);
                    }
                    Ordering::Less => {
                        Q = &Q - &lookup_table.select(-naf[i] as usize);
                    }
                    Ordering::Equal => {}
                }
            }
        }

        Some(Q.into())
    }
}