mirror of
https://github.com/saymrwulf/risc0-curve25519-dalek-source.git
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123 lines
3.7 KiB
Rust
123 lines
3.7 KiB
Rust
// -*- mode: rust; -*-
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//
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// This file is part of curve25519-dalek.
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// Copyright (c) 2016-2017 Isis Lovecruft, Henry de Valence
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// See LICENSE for licensing information.
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//
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// Authors:
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// - Isis Agora Lovecruft <isis@patternsinthevoid.net>
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// - Henry de Valence <hdevalence@hdevalence.ca>
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//! Code for fixed- and sliding-window functionality
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#![allow(non_snake_case)]
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use core::fmt::Debug;
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use subtle::ConditionallyNegatable;
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use subtle::ConditionallyAssignable;
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use subtle::ConstantTimeEq;
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use subtle::Choice;
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use traits::Identity;
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/// A lookup table of precomputed multiples of a point \\(P\\), used to
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/// compute \\( xP \\) for \\( -8 \leq x \leq 8 \\).
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///
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/// The computation of \\( xP \\) is done in constant time by the `select` function.
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///
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/// Since `LookupTable` does not implement `Index`, it's more difficult
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/// to accidentally use the table directly. Unfortunately the table is
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/// only `pub(crate)` so that we can write hardcoded constants, so it's
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/// still technically possible. It would be nice to prevent direct
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/// access to the table.
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///
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/// XXX make this generic with respect to table size
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#[derive(Copy, Clone)]
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pub struct LookupTable<T>(pub(crate) [T; 8]);
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use clear_on_drop::clear::ZeroSafe;
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/// This type isn't actually zeroable (all zero bytes are not valid
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/// points), but we want to be able to use `clear_on_drop` to erase slices
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/// of `LookupTable`.
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///
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/// Since the `ZeroSafe` trait is only used by `clear_on_drop`, the only
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/// situation where this would be a problem is if code attempted to use
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/// a `ClearOnDrop` to erase a `LookupTable` and then used the table
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/// afterwards.
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///
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/// Normally this is not a problem, since the table's storage is usually
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/// dropped too.
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///
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/// XXX is this a good compromise?
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unsafe impl<T> ZeroSafe for LookupTable<T> {}
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impl<T> LookupTable<T>
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where T: Identity + ConditionallyAssignable + ConditionallyNegatable
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{
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/// Given \\(-8 \leq x \leq 8\\), return \\(xP\\) in constant time.
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pub fn select(&self, x: i8) -> T {
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debug_assert!(x >= -8); debug_assert!(x <= 8);
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// Compute xabs = |x|
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let xmask = x >> 7;
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let xabs = (x + xmask) ^ xmask;
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// Set t = 0 * P = identity
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let mut t = T::identity();
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for j in 1..9 {
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// Copy `points[j-1] == j*P` onto `t` in constant time if `|x| == j`.
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let c = (xabs as u8).ct_eq(&(j as u8));
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t.conditional_assign(&self.0[j-1], c);
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}
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// Now t == |x| * P.
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let neg_mask = Choice::from((xmask & 1) as u8);
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t.conditional_negate(neg_mask);
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// Now t == x * P.
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t
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}
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}
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impl<T: Copy + Default> Default for LookupTable<T> {
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fn default() -> LookupTable<T> {
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LookupTable([T::default(); 8])
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}
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}
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impl<T: Debug> Debug for LookupTable<T> {
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fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
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write!(f, "LookupTable({:?})", self.0)
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}
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}
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use edwards::EdwardsPoint;
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use curve_models::ProjectiveNielsPoint;
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use curve_models::AffineNielsPoint;
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impl<'a> From<&'a EdwardsPoint> for LookupTable<ProjectiveNielsPoint> {
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fn from(P: &'a EdwardsPoint) -> Self {
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let mut points = [P.to_projective_niels(); 8];
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for j in 0..7 {
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points[j+1] = (P + &points[j])
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.to_extended()
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.to_projective_niels();
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}
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LookupTable(points)
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}
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}
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impl<'a> From<&'a EdwardsPoint> for LookupTable<AffineNielsPoint> {
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fn from(P: &'a EdwardsPoint) -> Self {
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let mut points = [P.to_affine_niels(); 8];
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// XXX batch inversion would be good if perf mattered here
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for j in 0..7 {
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points[j+1] = (P + &points[j])
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.to_extended()
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.to_affine_niels()
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}
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LookupTable(points)
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}
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}
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