curve25519_dalek/backend/serial/scalar_mul/
precomputed_straus.rs

1// -*- mode: rust; -*-
2//
3// This file is part of curve25519-dalek.
4// Copyright (c) 2019 Henry de Valence.
5// See LICENSE for licensing information.
6//
7// Authors:
8// - Henry de Valence <hdevalence@hdevalence.ca>
9
10//! Precomputation for Straus's method.
11
12#![allow(non_snake_case)]
13
14use alloc::vec::Vec;
15
16use core::borrow::Borrow;
17use core::cmp::Ordering;
18
19use crate::backend::serial::curve_models::{
20    AffineNielsPoint, CompletedPoint, ProjectiveNielsPoint, ProjectivePoint,
21};
22use crate::edwards::EdwardsPoint;
23use crate::scalar::Scalar;
24use crate::traits::Identity;
25use crate::traits::VartimePrecomputedMultiscalarMul;
26use crate::window::{NafLookupTable5, NafLookupTable8};
27
28#[allow(missing_docs)]
29pub struct VartimePrecomputedStraus {
30    static_lookup_tables: Vec<NafLookupTable8<AffineNielsPoint>>,
31}
32
33impl VartimePrecomputedMultiscalarMul for VartimePrecomputedStraus {
34    type Point = EdwardsPoint;
35
36    fn new<I>(static_points: I) -> Self
37    where
38        I: IntoIterator,
39        I::Item: Borrow<Self::Point>,
40    {
41        Self {
42            static_lookup_tables: static_points
43                .into_iter()
44                .map(|P| NafLookupTable8::<AffineNielsPoint>::from(P.borrow()))
45                .collect(),
46        }
47    }
48
49    fn optional_mixed_multiscalar_mul<I, J, K>(
50        &self,
51        static_scalars: I,
52        dynamic_scalars: J,
53        dynamic_points: K,
54    ) -> Option<Self::Point>
55    where
56        I: IntoIterator,
57        I::Item: Borrow<Scalar>,
58        J: IntoIterator,
59        J::Item: Borrow<Scalar>,
60        K: IntoIterator<Item = Option<Self::Point>>,
61    {
62        let static_nafs = static_scalars
63            .into_iter()
64            .map(|c| c.borrow().non_adjacent_form(5))
65            .collect::<Vec<_>>();
66        let dynamic_nafs: Vec<_> = dynamic_scalars
67            .into_iter()
68            .map(|c| c.borrow().non_adjacent_form(5))
69            .collect::<Vec<_>>();
70
71        let dynamic_lookup_tables = dynamic_points
72            .into_iter()
73            .map(|P_opt| P_opt.map(|P| NafLookupTable5::<ProjectiveNielsPoint>::from(&P)))
74            .collect::<Option<Vec<_>>>()?;
75
76        let sp = self.static_lookup_tables.len();
77        let dp = dynamic_lookup_tables.len();
78        assert_eq!(sp, static_nafs.len());
79        assert_eq!(dp, dynamic_nafs.len());
80
81        // We could save some doublings by looking for the highest
82        // nonzero NAF coefficient, but since we might have a lot of
83        // them to search, it's not clear it's worthwhile to check.
84        let mut S = ProjectivePoint::identity();
85        for j in (0..256).rev() {
86            let mut R: CompletedPoint = S.double();
87
88            for i in 0..dp {
89                let t_ij = dynamic_nafs[i][j];
90                match t_ij.cmp(&0) {
91                    Ordering::Greater => {
92                        R = &R.as_extended() + &dynamic_lookup_tables[i].select(t_ij as usize)
93                    }
94                    Ordering::Less => {
95                        R = &R.as_extended() - &dynamic_lookup_tables[i].select(-t_ij as usize)
96                    }
97                    Ordering::Equal => {}
98                }
99            }
100
101            #[allow(clippy::needless_range_loop)]
102            for i in 0..sp {
103                let t_ij = static_nafs[i][j];
104                match t_ij.cmp(&0) {
105                    Ordering::Greater => {
106                        R = &R.as_extended() + &self.static_lookup_tables[i].select(t_ij as usize)
107                    }
108                    Ordering::Less => {
109                        R = &R.as_extended() - &self.static_lookup_tables[i].select(-t_ij as usize)
110                    }
111                    Ordering::Equal => {}
112                }
113            }
114
115            S = R.as_projective();
116        }
117
118        Some(S.as_extended())
119    }
120}
curve25519_dalek/backend/serial/scalar_mul/precomputed_straus.rs

curve25519_dalek/backend/serial/scalar_mul/
precomputed_straus.rs
