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Add an OddMultiples helper struct

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Henry de Valence 2017-03-13 18:08:59 -07:00
parent 6991b4264b
commit 57ebc9d7f9
1 changed files with 29 additions and 12 deletions
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41
src/curve.rs
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@ -79,7 +79,7 @@
use core::fmt::Debug;
use core::iter::Iterator;
use core::ops::{Add, Sub, Neg};
use core::ops::{Add, Sub, Neg, Index};
use constants;
use field::FieldElement;
@ -957,6 +957,30 @@ impl ExtendedPoint {
}
}
/// Holds odd multiples 1A, 3A, ..., 15A of a point A.
struct OddMultiples(pub [ProjectiveNielsPoint; 8]);
impl OddMultiples {
fn create(A: &ExtendedPoint) -> OddMultiples {
let mut Ai = [ProjectiveNielsPoint::identity(); 8];
let A2 = A.double();
Ai[0] = A.to_projective_niels();
for i in 0..7 {
Ai[i+1] = (&A2 + &Ai[i]).to_extended().to_projective_niels();
}
// Now Ai = [A, 3A, 5A, 7A, 9A, 11A, 13A, 15A]
OddMultiples(Ai)
}
}
impl Index<usize> for OddMultiples {
type Output = ProjectiveNielsPoint;
fn index<'a>(&'a self, _index: usize) -> &'a ProjectiveNielsPoint {
&(self.0[_index])
}
}
/// Given a point `A` and scalars `a` and `b`, compute the point
/// `aA+bB`, where `B` is the Ed25519 basepoint (i.e., `B = (x,4/5)`
/// with x positive).
@ -969,15 +993,6 @@ pub fn double_scalar_mult_vartime(a: &Scalar, A: &ExtendedPoint, b: &Scalar) ->
let a_naf = a.non_adjacent_form();
let b_naf = b.non_adjacent_form();
// Build a lookup table of odd multiples of A
let mut Ai = [ProjectiveNielsPoint::identity(); 8];
let A2 = A.double();
Ai[0] = A.to_projective_niels();
for i in 0..7 {
Ai[i+1] = (&A2 + &Ai[i]).to_extended().to_projective_niels();
}
// Now Ai = [A, 3A, 5A, 7A, 9A, 11A, 13A, 15A]
// Find starting index
let mut i: usize = 255;
for j in (0..255).rev() {
@ -987,14 +1002,16 @@ pub fn double_scalar_mult_vartime(a: &Scalar, A: &ExtendedPoint, b: &Scalar) ->
}
}
let odd_multiples_of_A = OddMultiples::create(A);
let mut r = ProjectivePoint::identity();
loop {
let mut t = r.double();
if a_naf[i] > 0 {
t = &t.to_extended() + &Ai[( a_naf[i]/2) as usize];
t = &t.to_extended() + &odd_multiples_of_A[( a_naf[i]/2) as usize];
} else if a_naf[i] < 0 {
t = &t.to_extended() - &Ai[(-a_naf[i]/2) as usize];
t = &t.to_extended() - &odd_multiples_of_A[(-a_naf[i]/2) as usize];
}
if b_naf[i] > 0 {