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Merge remote-tracking branch 'briansmith/addition-chain' into develop

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Henry de Valence 2017-09-15 16:16:11 -07:00
commit e27bcd0057
1 changed files with 53 additions and 6 deletions
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59
src/scalar.rs
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@ -2,11 +2,13 @@
// //
// This file is part of curve25519-dalek. // This file is part of curve25519-dalek.
// Copyright (c) 2016-2017 Isis Lovecruft, Henry de Valence // Copyright (c) 2016-2017 Isis Lovecruft, Henry de Valence
// Portions Copyright 2017 Brian Smith
// See LICENSE for licensing information. // See LICENSE for licensing information.
// //
// Authors: // Authors:
// - Isis Agora Lovecruft <isis@patternsinthevoid.net> // - Isis Agora Lovecruft <isis@patternsinthevoid.net>
// - Henry de Valence <hdevalence@hdevalence.ca> // - Henry de Valence <hdevalence@hdevalence.ca>
// - Brian Smith <brian@briansmith.org>
//! Arithmetic for scalar multiplication. //! Arithmetic for scalar multiplication.
//! //!
@ -565,14 +567,59 @@ impl UnpackedScalar {
/// Compute the multiplicative inverse of this scalar. /// Compute the multiplicative inverse of this scalar.
pub fn invert(&self) -> UnpackedScalar { pub fn invert(&self) -> UnpackedScalar {
let mut y = UnpackedScalar::one(); // This is a direct transliteration of the addition chain from
// Run through bits of l-2 from highest to least // https://briansmith.org/ecc-inversion-addition-chains-01#curve25519_scalar_inversion
for bit in constants::l_minus_2.bits().iter().rev() { // as it was published on 2017-09-03.
y = y.square();
if *bit == 1 { let _1 = *self;
y = UnpackedScalar::multiply_add(&y, self, &UnpackedScalar::zero()); let _10 = _1.square();
let _100 = _10.square();
let _11 = UnpackedScalar::multiply_add(&_10, &_1, &UnpackedScalar::zero());
let _101 = UnpackedScalar::multiply_add(&_10, &_11, &UnpackedScalar::zero());
let _111 = UnpackedScalar::multiply_add(&_10, &_101, &UnpackedScalar::zero());
let _1001 = UnpackedScalar::multiply_add(&_10, &_111, &UnpackedScalar::zero());
let _1011 = UnpackedScalar::multiply_add(&_10, &_1001, &UnpackedScalar::zero());
let _1111 = UnpackedScalar::multiply_add(&_100, &_1011, &UnpackedScalar::zero());
// _10000
let mut y = UnpackedScalar::multiply_add(&_1111, &_1, &UnpackedScalar::zero());
#[inline]
fn square_multiply(y: &mut UnpackedScalar, squarings: usize, x: &UnpackedScalar) {
for _ in 0..squarings {
*y = y.square();
} }
*y = UnpackedScalar::multiply_add(y, x, &UnpackedScalar::zero());
} }
square_multiply(&mut y, 123 + 3, &_101);
square_multiply(&mut y, 2 + 2, &_11);
square_multiply(&mut y, 1 + 4, &_1111);
square_multiply(&mut y, 1 + 4, &_1111);
square_multiply(&mut y, 4, &_1001);
square_multiply(&mut y, 2, &_11);
square_multiply(&mut y, 1 + 4, &_1111);
square_multiply(&mut y, 1 + 3, &_101);
square_multiply(&mut y, 3 + 3, &_101);
square_multiply(&mut y, 3, &_111);
square_multiply(&mut y, 1 + 4, &_1111);
square_multiply(&mut y, 2 + 3, &_111);
square_multiply(&mut y, 2 + 2, &_11);
square_multiply(&mut y, 1 + 4, &_1011);
square_multiply(&mut y, 2 + 4, &_1011);
square_multiply(&mut y, 6 + 4, &_1001);
square_multiply(&mut y, 2 + 2, &_11);
square_multiply(&mut y, 3 + 2, &_11);
square_multiply(&mut y, 3 + 2, &_11);
square_multiply(&mut y, 1 + 4, &_1001);
square_multiply(&mut y, 1 + 3, &_111);
square_multiply(&mut y, 2 + 4, &_1111);
square_multiply(&mut y, 1 + 4, &_1011);
square_multiply(&mut y, 3, &_101);
square_multiply(&mut y, 2 + 4, &_1111);
square_multiply(&mut y, 3, &_101);
square_multiply(&mut y, 1 + 2, &_11);
y y
} }