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527 lines
22 KiB
Rust
527 lines
22 KiB
Rust
// -*- mode: rust; coding: utf-8; -*-
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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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//! Field arithmetic for ℤ/(2²⁵⁵-19), using 32-bit arithmetic with
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//! 64-bit products.
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//!
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//! This code was originally derived from Adam Langley's
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//! curve25519-donna and (Golang) ed25519 implementations.
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//!
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//! This implementation is intended for platforms that can multiply
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//! 32-bit inputs to produce 64-bit outputs.
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//!
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//! This implementation is not preferred for use on x86_64, since the
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//! 64-bit implementation is both much simpler and much faster.
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//! However, that implementation requires Rust's `u128`, which is not
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//! yet stable.
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use core::fmt::Debug;
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use core::ops::{Add, AddAssign};
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use core::ops::{Sub, SubAssign};
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use core::ops::{Mul, MulAssign};
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use core::ops::Neg;
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use subtle::ConditionallyAssignable;
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/// A `FieldElement32` represents an element of the field GF(2^255 - 19).
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///
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/// In the 32-bit implementation, a `FieldElement32` is represented in
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/// radix 2^25.5 as ten `u32`s, so that an element t, entries
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/// t[0],...,t[9], represents `sum(t[i]*2^ceil(i*51/2))`.
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///
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/// The coefficients t[i] are allowed to grow between multiplications.
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///
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/// XXX document by how much
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///
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/// # Warning
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///
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/// You almost certainly do not want to use `FieldElement32` directly. Consider
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/// using `curve25519_dalek::field::FieldElement`, which will automatically
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/// select between `FieldElement32` and `FieldElement64` depending on whether
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/// curve25519-dalek was compiled with `--features="nightly"`.
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///
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/// This implementation, `FieldElement32`, is intended for platforms that can
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/// multiply 32-bit inputs to produce 64-bit outputs, and is not preferred for
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/// use on x86_64, since the 64-bit implementation is both much simpler and much
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/// faster. However, the `FieldElement64` implementation requires Rust's
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/// `u128`, which is not yet stable.
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#[derive(Copy, Clone)]
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pub struct FieldElement32(pub (crate) [u32; 10]);
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impl Debug for FieldElement32 {
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fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
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write!(f, "FieldElement32({:?})", &self.0[..])
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}
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}
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impl<'b> AddAssign<&'b FieldElement32> for FieldElement32 {
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fn add_assign(&mut self, _rhs: &'b FieldElement32) {
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for i in 0..10 {
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self.0[i] += _rhs.0[i];
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}
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}
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}
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impl<'a, 'b> Add<&'b FieldElement32> for &'a FieldElement32 {
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type Output = FieldElement32;
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fn add(self, _rhs: &'b FieldElement32) -> FieldElement32 {
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let mut output = *self;
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output += _rhs;
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output
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}
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}
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impl<'b> SubAssign<&'b FieldElement32> for FieldElement32 {
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fn sub_assign(&mut self, _rhs: &'b FieldElement32) {
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// See comment in FieldElement64::Sub
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//
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// Compute a - b as ((a + 2^4 * p) - b) to avoid underflow.
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let b = &_rhs.0;
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self.0 = FieldElement32::reduce([
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((self.0[0] + (0x3ffffed << 4)) - b[0]) as u64,
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((self.0[1] + (0x1ffffff << 4)) - b[1]) as u64,
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((self.0[2] + (0x3ffffff << 4)) - b[2]) as u64,
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((self.0[3] + (0x1ffffff << 4)) - b[3]) as u64,
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((self.0[4] + (0x3ffffff << 4)) - b[4]) as u64,
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((self.0[5] + (0x1ffffff << 4)) - b[5]) as u64,
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((self.0[6] + (0x3ffffff << 4)) - b[6]) as u64,
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((self.0[7] + (0x1ffffff << 4)) - b[7]) as u64,
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((self.0[8] + (0x3ffffff << 4)) - b[8]) as u64,
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((self.0[9] + (0x1ffffff << 4)) - b[9]) as u64,
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]).0;
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}
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}
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impl<'a, 'b> Sub<&'b FieldElement32> for &'a FieldElement32 {
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type Output = FieldElement32;
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fn sub(self, _rhs: &'b FieldElement32) -> FieldElement32 {
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let mut output = *self;
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output -= _rhs;
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output
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}
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}
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impl<'b> MulAssign<&'b FieldElement32> for FieldElement32 {
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fn mul_assign(&mut self, _rhs: &'b FieldElement32) {
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let result = (self as &FieldElement32) * _rhs;
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self.0 = result.0;
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}
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}
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impl<'a, 'b> Mul<&'b FieldElement32> for &'a FieldElement32 {
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type Output = FieldElement32;
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fn mul(self, _rhs: &'b FieldElement32) -> FieldElement32 {
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/// Helper function to multiply two 32-bit integers with 64 bits
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/// of output.
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#[inline(always)]
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fn m(x: u32, y: u32) -> u64 { (x as u64) * (y as u64) }
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// Alias self, _rhs for more readable formulas
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let x: &[u32;10] = &self.0; let y: &[u32;10] = &_rhs.0;
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// We assume that the input limbs x[i], y[i] are bounded by:
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//
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// x[i], y[i] < 2^(26 + b) if i even
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// x[i], y[i] < 2^(25 + b) if i odd
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//
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// where b is a (real) parameter representing the excess bits of
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// the limbs. We track the bitsizes of all variables through
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// the computation and solve at the end for the allowable
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// headroom bitsize b (which determines how many additions we
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// can perform between reductions or multiplications).
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let y1_19 = 19 * y[1]; // This fits in a u32
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let y2_19 = 19 * y[2]; // iff 26 + b + lg(19) < 32
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let y3_19 = 19 * y[3]; // if b < 32 - 26 - 4.248 = 1.752
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let y4_19 = 19 * y[4];
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let y5_19 = 19 * y[5]; // below, b<2.5: this is a bottleneck,
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let y6_19 = 19 * y[6]; // could be avoided by promoting to
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let y7_19 = 19 * y[7]; // u64 here instead of in m()
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let y8_19 = 19 * y[8];
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let y9_19 = 19 * y[9];
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// What happens when we multiply x[i] with y[j] and place the
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// result into the (i+j)-th limb?
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//
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// x[i] represents the value x[i]*2^ceil(i*51/2)
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// y[j] represents the value y[j]*2^ceil(j*51/2)
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// z[i+j] represents the value z[i+j]*2^ceil((i+j)*51/2)
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// x[i]*y[j] represents the value x[i]*y[i]*2^(ceil(i*51/2)+ceil(j*51/2))
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//
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// Since the radix is already accounted for, the result placed
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// into the (i+j)-th limb should be
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//
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// x[i]*y[i]*2^(ceil(i*51/2)+ceil(j*51/2) - ceil((i+j)*51/2)).
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//
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// The value of ceil(i*51/2)+ceil(j*51/2) - ceil((i+j)*51/2) is
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// 1 when both i and j are odd, and 0 otherwise. So we add
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//
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// x[i]*y[j] if either i or j is even
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// 2*x[i]*y[j] if i and j are both odd
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//
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// by using precomputed multiples of x[i] for odd i:
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let x1_2 = 2 * x[1]; // This fits in a u32 iff 25 + b + 1 < 32
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let x3_2 = 2 * x[3]; // iff b < 6
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let x5_2 = 2 * x[5];
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let x7_2 = 2 * x[7];
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let x9_2 = 2 * x[9];
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let z0 = m(x[0],y[0]) + m(x1_2,y9_19) + m(x[2],y8_19) + m(x3_2,y7_19) + m(x[4],y6_19) + m(x5_2,y5_19) + m(x[6],y4_19) + m(x7_2,y3_19) + m(x[8],y2_19) + m(x9_2,y1_19);
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let z1 = m(x[0],y[1]) + m(x[1],y[0]) + m(x[2],y9_19) + m(x[3],y8_19) + m(x[4],y7_19) + m(x[5],y6_19) + m(x[6],y5_19) + m(x[7],y4_19) + m(x[8],y3_19) + m(x[9],y2_19);
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let z2 = m(x[0],y[2]) + m(x1_2,y[1]) + m(x[2],y[0]) + m(x3_2,y9_19) + m(x[4],y8_19) + m(x5_2,y7_19) + m(x[6],y6_19) + m(x7_2,y5_19) + m(x[8],y4_19) + m(x9_2,y3_19);
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let z3 = m(x[0],y[3]) + m(x[1],y[2]) + m(x[2],y[1]) + m(x[3],y[0]) + m(x[4],y9_19) + m(x[5],y8_19) + m(x[6],y7_19) + m(x[7],y6_19) + m(x[8],y5_19) + m(x[9],y4_19);
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let z4 = m(x[0],y[4]) + m(x1_2,y[3]) + m(x[2],y[2]) + m(x3_2,y[1]) + m(x[4],y[0]) + m(x5_2,y9_19) + m(x[6],y8_19) + m(x7_2,y7_19) + m(x[8],y6_19) + m(x9_2,y5_19);
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let z5 = m(x[0],y[5]) + m(x[1],y[4]) + m(x[2],y[3]) + m(x[3],y[2]) + m(x[4],y[1]) + m(x[5],y[0]) + m(x[6],y9_19) + m(x[7],y8_19) + m(x[8],y7_19) + m(x[9],y6_19);
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let z6 = m(x[0],y[6]) + m(x1_2,y[5]) + m(x[2],y[4]) + m(x3_2,y[3]) + m(x[4],y[2]) + m(x5_2,y[1]) + m(x[6],y[0]) + m(x7_2,y9_19) + m(x[8],y8_19) + m(x9_2,y7_19);
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let z7 = m(x[0],y[7]) + m(x[1],y[6]) + m(x[2],y[5]) + m(x[3],y[4]) + m(x[4],y[3]) + m(x[5],y[2]) + m(x[6],y[1]) + m(x[7],y[0]) + m(x[8],y9_19) + m(x[9],y8_19);
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let z8 = m(x[0],y[8]) + m(x1_2,y[7]) + m(x[2],y[6]) + m(x3_2,y[5]) + m(x[4],y[4]) + m(x5_2,y[3]) + m(x[6],y[2]) + m(x7_2,y[1]) + m(x[8],y[0]) + m(x9_2,y9_19);
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let z9 = m(x[0],y[9]) + m(x[1],y[8]) + m(x[2],y[7]) + m(x[3],y[6]) + m(x[4],y[5]) + m(x[5],y[4]) + m(x[6],y[3]) + m(x[7],y[2]) + m(x[8],y[1]) + m(x[9],y[0]);
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// How big is the contribution to z[i+j] from x[i], y[j]?
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//
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// Using the bounds above, we get:
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//
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// i even, j even: x[i]*y[j] < 2^(26+b)*2^(26+b) = 2*2^(51+2*b)
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// i odd, j even: x[i]*y[j] < 2^(25+b)*2^(26+b) = 1*2^(51+2*b)
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// i even, j odd: x[i]*y[j] < 2^(26+b)*2^(25+b) = 1*2^(51+2*b)
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// i odd, j odd: 2*x[i]*y[j] < 2*2^(25+b)*2^(25+b) = 1*2^(51+2*b)
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//
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// We perform inline reduction mod p by replacing 2^255 by 19
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// (since 2^255 - 19 = 0 mod p). This adds a factor of 19, so
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// we get the bounds (z0 is the biggest one, but calculated for
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// posterity here in case finer estimation is needed later):
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//
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// z0 < ( 2 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 )*2^(51 + 2b) = 249*2^(51 + 2*b)
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// z1 < ( 1 + 1 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 )*2^(51 + 2b) = 154*2^(51 + 2*b)
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// z2 < ( 2 + 1 + 2 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 )*2^(51 + 2b) = 195*2^(51 + 2*b)
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// z3 < ( 1 + 1 + 1 + 1 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 + 1*19 )*2^(51 + 2b) = 118*2^(51 + 2*b)
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// z4 < ( 2 + 1 + 2 + 1 + 2 + 1*19 + 2*19 + 1*19 + 2*19 + 1*19 )*2^(51 + 2b) = 141*2^(51 + 2*b)
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// z5 < ( 1 + 1 + 1 + 1 + 1 + 1 + 1*19 + 1*19 + 1*19 + 1*19 )*2^(51 + 2b) = 82*2^(51 + 2*b)
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// z6 < ( 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1*19 + 2*19 + 1*19 )*2^(51 + 2b) = 87*2^(51 + 2*b)
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// z7 < ( 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1*19 + 1*19 )*2^(51 + 2b) = 46*2^(51 + 2*b)
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// z6 < ( 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1 + 2 + 1*19 )*2^(51 + 2b) = 33*2^(51 + 2*b)
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// z7 < ( 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 )*2^(51 + 2b) = 10*2^(51 + 2*b)
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//
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// So z[0] fits into a u64 if 51 + 2*b + lg(249) < 64
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// if b < 2.5.
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FieldElement32::reduce([z0, z1, z2, z3, z4, z5, z6, z7, z8, z9])
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}
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}
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impl<'a> Neg for &'a FieldElement32 {
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type Output = FieldElement32;
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fn neg(self) -> FieldElement32 {
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let mut output = *self;
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output.negate();
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output
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}
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}
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impl ConditionallyAssignable for FieldElement32 {
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fn conditional_assign(&mut self, f: &FieldElement32, choice: u8) {
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let mask = (-(choice as i32)) as u32;
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for i in 0..10 {
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self.0[i] ^= mask & (self.0[i] ^ f.0[i]);
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}
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}
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}
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impl FieldElement32 {
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/// Invert the sign of this field element
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pub fn negate(&mut self) {
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// Compute -b as ((2^4 * p) - b) to avoid underflow.
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let neg = FieldElement32::reduce([
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((0x3ffffed << 4) - self.0[0]) as u64,
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((0x1ffffff << 4) - self.0[1]) as u64,
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((0x3ffffff << 4) - self.0[2]) as u64,
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((0x1ffffff << 4) - self.0[3]) as u64,
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((0x3ffffff << 4) - self.0[4]) as u64,
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((0x1ffffff << 4) - self.0[5]) as u64,
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((0x3ffffff << 4) - self.0[6]) as u64,
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((0x1ffffff << 4) - self.0[7]) as u64,
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((0x3ffffff << 4) - self.0[8]) as u64,
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((0x1ffffff << 4) - self.0[9]) as u64,
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]);
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self.0 = neg.0;
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}
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/// Construct zero.
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pub fn zero() -> FieldElement32 {
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FieldElement32([ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ])
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}
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/// Construct one.
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pub fn one() -> FieldElement32 {
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FieldElement32([ 1, 0, 0, 0, 0, 0, 0, 0, 0, 0 ])
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}
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/// Construct -1.
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pub fn minus_one() -> FieldElement32 {
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FieldElement32([
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0x3ffffec, 0x1ffffff, 0x3ffffff, 0x1ffffff, 0x3ffffff,
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0x1ffffff, 0x3ffffff, 0x1ffffff, 0x3ffffff, 0x1ffffff,
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])
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}
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/// Given unreduced coefficients `z[0], ..., z[9]` of any size,
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/// carry and reduce them mod p to obtain a `FieldElement32`
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/// whose coefficients have excess `b < 0.007`.
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///
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/// In other words, each coefficient of the result is bounded by
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/// either `2^(25 + 0.007)` or `2^(26 + 0.007)`, as appropriate.
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fn reduce(mut z: [u64; 10]) -> FieldElement32 {
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const LOW_25_BITS: u64 = (1 << 25) - 1;
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const LOW_26_BITS: u64 = (1 << 26) - 1;
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/// Carry the value from limb i = 0..8 to limb i+1
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#[inline(always)]
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fn carry(z: &mut [u64; 10], i: usize) {
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debug_assert!(i < 9);
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if i % 2 == 0 {
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// Even limbs have 26 bits
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z[i+1] += z[i] >> 26;
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z[i] &= LOW_26_BITS;
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} else {
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// Odd limbs have 25 bits
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z[i+1] += z[i] >> 25;
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z[i] &= LOW_25_BITS;
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}
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}
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// Perform two halves of the carry chain in parallel.
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carry(&mut z, 0); carry(&mut z, 4);
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carry(&mut z, 1); carry(&mut z, 5);
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carry(&mut z, 2); carry(&mut z, 6);
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carry(&mut z, 3); carry(&mut z, 7);
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// Since z[3] < 2^64, c < 2^(64-25) = 2^39,
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// so z[4] < 2^26 + 2^39 < 2^39.0002
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carry(&mut z, 4); carry(&mut z, 8);
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// Now z[4] < 2^26
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// and z[5] < 2^25 + 2^13.0002 < 2^25.0004 (good enough)
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// Last carry has a multiplication by 19:
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z[0] += 19*(z[9] >> 25);
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z[9] &= LOW_25_BITS;
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// Since z[9] < 2^64, c < 2^(64-25) = 2^39,
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// so z[0] + 19*c < 2^26 + 2^43.248 < 2^43.249.
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carry(&mut z, 0);
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// Now z[1] < 2^25 - 2^(43.249 - 26)
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// < 2^25.007 (good enough)
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// and we're done.
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FieldElement32([
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z[0] as u32, z[1] as u32, z[2] as u32, z[3] as u32, z[4] as u32,
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z[5] as u32, z[6] as u32, z[7] as u32, z[8] as u32, z[9] as u32,
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])
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}
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/// Load a `FieldElement64` from the low 255 bits of a 256-bit
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/// input.
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///
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/// # Warning
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///
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/// This function does not check that the input used the canonical
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/// representative. It masks the high bit, but it will happily
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/// decode 2^255 - 18 to 1. Applications that require a canonical
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/// encoding of every field element should decode, re-encode to
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/// the canonical encoding, and check that the input was
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/// canonical.
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pub fn from_bytes(data: &[u8; 32]) -> FieldElement32 { //FeFromBytes
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#[inline]
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fn load3(b: &[u8]) -> u64 {
|
||
(b[0] as u64) | ((b[1] as u64) << 8) | ((b[2] as u64) << 16)
|
||
}
|
||
|
||
#[inline]
|
||
fn load4(b: &[u8]) -> u64 {
|
||
(b[0] as u64) | ((b[1] as u64) << 8) | ((b[2] as u64) << 16) | ((b[3] as u64) << 24)
|
||
}
|
||
|
||
let mut h = [0u64;10];
|
||
const LOW_23_BITS: u64 = (1 << 23) - 1;
|
||
h[0] = load4(&data[ 0..]);
|
||
h[1] = load3(&data[ 4..]) << 6;
|
||
h[2] = load3(&data[ 7..]) << 5;
|
||
h[3] = load3(&data[10..]) << 3;
|
||
h[4] = load3(&data[13..]) << 2;
|
||
h[5] = load4(&data[16..]);
|
||
h[6] = load3(&data[20..]) << 7;
|
||
h[7] = load3(&data[23..]) << 5;
|
||
h[8] = load3(&data[26..]) << 4;
|
||
h[9] = (load3(&data[29..]) & LOW_23_BITS) << 2;
|
||
|
||
FieldElement32::reduce(h)
|
||
}
|
||
|
||
/// Serialize this `FieldElement64` to a 32-byte array. The
|
||
/// encoding is canonical.
|
||
pub fn to_bytes(&self) -> [u8; 32] {
|
||
|
||
let inp = &self.0;
|
||
// Reduce the value represented by `in` to the range [0,2*p)
|
||
let mut h: [u32; 10] = FieldElement32::reduce([
|
||
// XXX this cast is annoying
|
||
inp[0] as u64, inp[1] as u64, inp[2] as u64, inp[3] as u64, inp[4] as u64,
|
||
inp[5] as u64, inp[6] as u64, inp[7] as u64, inp[8] as u64, inp[9] as u64,
|
||
]).0;
|
||
|
||
// Let h be the value to encode.
|
||
//
|
||
// Write h = pq + r with 0 <= r < p. We want to compute r = h mod p.
|
||
//
|
||
// Since h < 2*p, q = 0 or 1, with q = 0 when h < p and q = 1 when h >= p.
|
||
//
|
||
// Notice that h >= p <==> h + 19 >= p + 19 <==> h + 19 >= 2^255.
|
||
// Therefore q can be computed as the carry bit of h + 19.
|
||
|
||
let mut q: u32 = (h[0] + 19) >> 26;
|
||
q = (h[1] + q) >> 25;
|
||
q = (h[2] + q) >> 26;
|
||
q = (h[3] + q) >> 25;
|
||
q = (h[4] + q) >> 26;
|
||
q = (h[5] + q) >> 25;
|
||
q = (h[6] + q) >> 26;
|
||
q = (h[7] + q) >> 25;
|
||
q = (h[8] + q) >> 26;
|
||
q = (h[9] + q) >> 25;
|
||
|
||
debug_assert!( q == 0 || q == 1 );
|
||
|
||
// Now we can compute r as r = h - pq = r - (2^255-19)q = r + 19q - 2^255q
|
||
|
||
const LOW_25_BITS: u32 = (1 << 25) - 1;
|
||
const LOW_26_BITS: u32 = (1 << 26) - 1;
|
||
|
||
h[0] += 19*q;
|
||
|
||
// Now carry the result to compute r + 19q...
|
||
h[1] += h[0] >> 26;
|
||
h[0] = h[0] & LOW_26_BITS;
|
||
h[2] += h[1] >> 25;
|
||
h[1] = h[1] & LOW_25_BITS;
|
||
h[3] += h[2] >> 26;
|
||
h[2] = h[2] & LOW_26_BITS;
|
||
h[4] += h[3] >> 25;
|
||
h[3] = h[3] & LOW_25_BITS;
|
||
h[5] += h[4] >> 26;
|
||
h[4] = h[4] & LOW_26_BITS;
|
||
h[6] += h[5] >> 25;
|
||
h[5] = h[5] & LOW_25_BITS;
|
||
h[7] += h[6] >> 26;
|
||
h[6] = h[6] & LOW_26_BITS;
|
||
h[8] += h[7] >> 25;
|
||
h[7] = h[7] & LOW_25_BITS;
|
||
h[9] += h[8] >> 26;
|
||
h[8] = h[8] & LOW_26_BITS;
|
||
|
||
// ... but instead of carrying the value
|
||
// (h[9] >> 25) = q*2^255 into another limb,
|
||
// discard it, subtracting the value from h.
|
||
debug_assert!( (h[9] >> 25) == 0 || (h[9] >> 25) == 1);
|
||
h[9] = h[9] & LOW_25_BITS;
|
||
|
||
let mut s = [0u8; 32];
|
||
s[0] = (h[0] >> 0) as u8;
|
||
s[1] = (h[0] >> 8) as u8;
|
||
s[2] = (h[0] >> 16) as u8;
|
||
s[3] = ((h[0] >> 24) | (h[1] << 2)) as u8;
|
||
s[4] = (h[1] >> 6) as u8;
|
||
s[5] = (h[1] >> 14) as u8;
|
||
s[6] = ((h[1] >> 22) | (h[2] << 3)) as u8;
|
||
s[7] = (h[2] >> 5) as u8;
|
||
s[8] = (h[2] >> 13) as u8;
|
||
s[9] = ((h[2] >> 21) | (h[3] << 5)) as u8;
|
||
s[10] = (h[3] >> 3) as u8;
|
||
s[11] = (h[3] >> 11) as u8;
|
||
s[12] = ((h[3] >> 19) | (h[4] << 6)) as u8;
|
||
s[13] = (h[4] >> 2) as u8;
|
||
s[14] = (h[4] >> 10) as u8;
|
||
s[15] = (h[4] >> 18) as u8;
|
||
s[16] = (h[5] >> 0) as u8;
|
||
s[17] = (h[5] >> 8) as u8;
|
||
s[18] = (h[5] >> 16) as u8;
|
||
s[19] = ((h[5] >> 24) | (h[6] << 1)) as u8;
|
||
s[20] = (h[6] >> 7) as u8;
|
||
s[21] = (h[6] >> 15) as u8;
|
||
s[22] = ((h[6] >> 23) | (h[7] << 3)) as u8;
|
||
s[23] = (h[7] >> 5) as u8;
|
||
s[24] = (h[7] >> 13) as u8;
|
||
s[25] = ((h[7] >> 21) | (h[8] << 4)) as u8;
|
||
s[26] = (h[8] >> 4) as u8;
|
||
s[27] = (h[8] >> 12) as u8;
|
||
s[28] = ((h[8] >> 20) | (h[9] << 6)) as u8;
|
||
s[29] = (h[9] >> 2) as u8;
|
||
s[30] = (h[9] >> 10) as u8;
|
||
s[31] = (h[9] >> 18) as u8;
|
||
|
||
// Check that high bit is cleared
|
||
debug_assert!((s[31] & 0b1000_0000u8) == 0u8);
|
||
|
||
s
|
||
}
|
||
|
||
fn square_inner(&self) -> [u64; 10] {
|
||
// Optimized version of multiplication for the case of squaring.
|
||
// Pre- and post- conditions identical to multiplication function.
|
||
let x = &self.0;
|
||
let x0_2 = 2 * x[0];
|
||
let x1_2 = 2 * x[1];
|
||
let x2_2 = 2 * x[2];
|
||
let x3_2 = 2 * x[3];
|
||
let x4_2 = 2 * x[4];
|
||
let x5_2 = 2 * x[5];
|
||
let x6_2 = 2 * x[6];
|
||
let x7_2 = 2 * x[7];
|
||
let x5_19 = 19 * x[5];
|
||
let x6_19 = 19 * x[6];
|
||
let x7_19 = 19 * x[7];
|
||
let x8_19 = 19 * x[8];
|
||
let x9_19 = 19 * x[9];
|
||
|
||
/// Helper function to multiply two 32-bit integers with 64 bits
|
||
/// of output.
|
||
#[inline(always)]
|
||
fn m(x: u32, y: u32) -> u64 { (x as u64) * (y as u64) }
|
||
|
||
// This block is rearranged so that instead of doing a 32-bit multiplication by 38, we do a
|
||
// 64-bit multiplication by 2 on the results. This is because lg(38) is too big: we would
|
||
// have less than 1 bit of headroom left, which is too little.
|
||
let mut z = [0u64;10];
|
||
z[0] = m(x[0],x[0]) + m(x2_2,x8_19) + m(x4_2,x6_19) + (m(x1_2,x9_19) + m(x3_2,x7_19) + m(x[5],x5_19))*2;
|
||
z[1] = m(x0_2,x[1]) + m(x3_2,x8_19) + m(x5_2,x6_19) + (m(x[2],x9_19) + m(x[4],x7_19))*2;
|
||
z[2] = m(x0_2,x[2]) + m(x1_2,x[1]) + m(x4_2,x8_19) + m(x[6],x6_19) + (m(x3_2,x9_19) + m(x5_2,x7_19))*2;
|
||
z[3] = m(x0_2,x[3]) + m(x1_2,x[2]) + m(x5_2,x8_19) + (m(x[4],x9_19) + m(x[6],x7_19))*2;
|
||
z[4] = m(x0_2,x[4]) + m(x1_2,x3_2) + m(x[2],x[2]) + m(x6_2,x8_19) + (m(x5_2,x9_19) + m(x[7],x7_19))*2;
|
||
z[5] = m(x0_2,x[5]) + m(x1_2,x[4]) + m(x2_2,x[3]) + m(x7_2,x8_19) + m(x[6],x9_19)*2;
|
||
z[6] = m(x0_2,x[6]) + m(x1_2,x5_2) + m(x2_2,x[4]) + m(x3_2,x[3]) + m(x[8],x8_19) + m(x7_2,x9_19)*2;
|
||
z[7] = m(x0_2,x[7]) + m(x1_2,x[6]) + m(x2_2,x[5]) + m(x3_2,x[4]) + m(x[8],x9_19)*2;
|
||
z[8] = m(x0_2,x[8]) + m(x1_2,x7_2) + m(x2_2,x[6]) + m(x3_2,x5_2) + m(x[4],x[4]) + m(x[9],x9_19)*2;
|
||
z[9] = m(x0_2,x[9]) + m(x1_2,x[8]) + m(x2_2,x[7]) + m(x3_2,x[6]) + m(x4_2,x[5]) ;
|
||
|
||
z
|
||
}
|
||
|
||
/// Compute `self^2`.
|
||
pub fn square(&self) -> FieldElement32 {
|
||
FieldElement32::reduce(self.square_inner())
|
||
}
|
||
|
||
/// Compute `2*self^2`.
|
||
pub fn square2(&self) -> FieldElement32 {
|
||
let mut coeffs = self.square_inner();
|
||
for i in 0..self.0.len() {
|
||
coeffs[i] += coeffs[i];
|
||
}
|
||
FieldElement32::reduce(coeffs)
|
||
}
|
||
}
|