ca043567e6
This was necessary at some point in the past, but no longer is. It is better to avoid depending on any of the `core::fmt` machinery in these lower layers if we can avoid it.
247 lines
8.4 KiB
Rust
247 lines
8.4 KiB
Rust
// Copyright 2015-2023 Brian Smith.
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//
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// Permission to use, copy, modify, and/or distribute this software for any
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// purpose with or without fee is hereby granted, provided that the above
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// copyright notice and this permission notice appear in all copies.
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//
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// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
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// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
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// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
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// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
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// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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use super::{super::n0::N0, BoxedLimbs, Elem, PublicModulus, SmallerModulus, Unencoded};
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use crate::{
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bits::BitLength,
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cpu, error,
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limb::{self, Limb, LimbMask, LIMB_BITS},
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polyfill::LeadingZerosStripped,
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};
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use core::marker::PhantomData;
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/// The x86 implementation of `bn_mul_mont`, at least, requires at least 4
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/// limbs. For a long time we have required 4 limbs for all targets, though
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/// this may be unnecessary. TODO: Replace this with
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/// `n.len() < 256 / LIMB_BITS` so that 32-bit and 64-bit platforms behave the
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/// same.
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pub const MODULUS_MIN_LIMBS: usize = 4;
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pub const MODULUS_MAX_LIMBS: usize = super::super::BIGINT_MODULUS_MAX_LIMBS;
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/// The modulus *m* for a ring ℤ/mℤ, along with the precomputed values needed
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/// for efficient Montgomery multiplication modulo *m*. The value must be odd
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/// and larger than 2. The larger-than-1 requirement is imposed, at least, by
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/// the modular inversion code.
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pub struct OwnedModulus<M> {
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limbs: BoxedLimbs<M>, // Also `value >= 3`.
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// n0 * N == -1 (mod r).
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//
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// r == 2**(N0::LIMBS_USED * LIMB_BITS) and LG_LITTLE_R == lg(r). This
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// ensures that we can do integer division by |r| by simply ignoring
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// `N0::LIMBS_USED` limbs. Similarly, we can calculate values modulo `r` by
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// just looking at the lowest `N0::LIMBS_USED` limbs. This is what makes
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// Montgomery multiplication efficient.
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//
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// As shown in Algorithm 1 of "Fast Prime Field Elliptic Curve Cryptography
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// with 256 Bit Primes" by Shay Gueron and Vlad Krasnov, in the loop of a
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// multi-limb Montgomery multiplication of a * b (mod n), given the
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// unreduced product t == a * b, we repeatedly calculate:
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//
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// t1 := t % r |t1| is |t|'s lowest limb (see previous paragraph).
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// t2 := t1*n0*n
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// t3 := t + t2
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// t := t3 / r copy all limbs of |t3| except the lowest to |t|.
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//
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// In the last step, it would only make sense to ignore the lowest limb of
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// |t3| if it were zero. The middle steps ensure that this is the case:
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//
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// t3 == 0 (mod r)
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// t + t2 == 0 (mod r)
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// t + t1*n0*n == 0 (mod r)
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// t1*n0*n == -t (mod r)
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// t*n0*n == -t (mod r)
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// n0*n == -1 (mod r)
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// n0 == -1/n (mod r)
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//
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// Thus, in each iteration of the loop, we multiply by the constant factor
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// n0, the negative inverse of n (mod r).
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//
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// TODO(perf): Not all 32-bit platforms actually make use of n0[1]. For the
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// ones that don't, we could use a shorter `R` value and use faster `Limb`
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// calculations instead of double-precision `u64` calculations.
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n0: N0,
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len_bits: BitLength,
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cpu_features: cpu::Features,
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}
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impl<M: PublicModulus> Clone for OwnedModulus<M> {
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fn clone(&self) -> Self {
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Self {
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limbs: self.limbs.clone(),
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n0: self.n0,
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len_bits: self.len_bits,
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cpu_features: self.cpu_features,
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}
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}
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}
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impl<M> OwnedModulus<M> {
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pub(crate) fn from_be_bytes(
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input: untrusted::Input,
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cpu_features: cpu::Features,
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) -> Result<Self, error::KeyRejected> {
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let limbs = BoxedLimbs::positive_minimal_width_from_be_bytes(input)?;
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Self::from_boxed_limbs(limbs, cpu_features)
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}
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fn from_boxed_limbs(
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n: BoxedLimbs<M>,
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cpu_features: cpu::Features,
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) -> Result<Self, error::KeyRejected> {
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if n.len() > MODULUS_MAX_LIMBS {
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return Err(error::KeyRejected::too_large());
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}
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if n.len() < MODULUS_MIN_LIMBS {
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return Err(error::KeyRejected::unexpected_error());
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}
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if limb::limbs_are_even_constant_time(&n) != LimbMask::False {
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return Err(error::KeyRejected::invalid_component());
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}
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if limb::limbs_less_than_limb_constant_time(&n, 3) != LimbMask::False {
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return Err(error::KeyRejected::unexpected_error());
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}
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// n_mod_r = n % r. As explained in the documentation for `n0`, this is
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// done by taking the lowest `N0::LIMBS_USED` limbs of `n`.
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#[allow(clippy::useless_conversion)]
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let n0 = {
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prefixed_extern! {
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fn bn_neg_inv_mod_r_u64(n: u64) -> u64;
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}
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// XXX: u64::from isn't guaranteed to be constant time.
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let mut n_mod_r: u64 = u64::from(n[0]);
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if N0::LIMBS_USED == 2 {
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// XXX: If we use `<< LIMB_BITS` here then 64-bit builds
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// fail to compile because of `deny(exceeding_bitshifts)`.
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debug_assert_eq!(LIMB_BITS, 32);
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n_mod_r |= u64::from(n[1]) << 32;
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}
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N0::from(unsafe { bn_neg_inv_mod_r_u64(n_mod_r) })
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};
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let len_bits = limb::limbs_minimal_bits(&n);
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Ok(Self {
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limbs: n,
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n0,
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len_bits,
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cpu_features,
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})
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}
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pub fn to_elem<L>(&self, l: &Modulus<L>) -> Elem<L, Unencoded>
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where
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M: SmallerModulus<L>,
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{
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let mut limbs = BoxedLimbs::zero(l.limbs.len());
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limbs[..self.limbs.len()].copy_from_slice(&self.limbs);
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Elem {
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limbs,
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encoding: PhantomData,
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}
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}
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pub fn modulus(&self) -> Modulus<M> {
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Modulus {
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limbs: &self.limbs,
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n0: self.n0,
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len_bits: self.len_bits,
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m: PhantomData,
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cpu_features: self.cpu_features,
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}
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}
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pub fn len_bits(&self) -> BitLength {
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self.len_bits
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}
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}
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impl<M: PublicModulus> OwnedModulus<M> {
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pub fn be_bytes(&self) -> LeadingZerosStripped<impl ExactSizeIterator<Item = u8> + Clone + '_> {
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LeadingZerosStripped::new(limb::unstripped_be_bytes(&self.limbs))
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}
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}
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pub struct Modulus<'a, M> {
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limbs: &'a [Limb],
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n0: N0,
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len_bits: BitLength,
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m: PhantomData<M>,
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cpu_features: cpu::Features,
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}
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impl<M> Modulus<'_, M> {
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pub(super) fn oneR(&self, out: &mut [Limb]) {
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assert_eq!(self.limbs.len(), out.len());
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let r = self.limbs.len() * LIMB_BITS;
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// out = 2**r - m where m = self.
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limb::limbs_negative_odd(out, self.limbs);
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let lg_m = self.len_bits().as_usize_bits();
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let leading_zero_bits_in_m = r - lg_m;
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// When m's length is a multiple of LIMB_BITS, which is the case we
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// most want to optimize for, then we already have
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// out == 2**r - m == 2**r (mod m).
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if leading_zero_bits_in_m != 0 {
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debug_assert!(leading_zero_bits_in_m < LIMB_BITS);
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// Correct out to 2**(lg m) (mod m). `limbs_negative_odd` flipped
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// all the leading zero bits to ones. Flip them back.
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*out.last_mut().unwrap() &= (!0) >> leading_zero_bits_in_m;
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// Now we have out == 2**(lg m) (mod m). Keep doubling until we get
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// to 2**r (mod m).
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for _ in 0..leading_zero_bits_in_m {
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limb::limbs_double_mod(out, self.limbs)
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}
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}
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// Now out == 2**r (mod m) == 1*R.
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}
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// TODO: XXX Avoid duplication with `Modulus`.
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pub(super) fn zero<E>(&self) -> Elem<M, E> {
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Elem {
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limbs: BoxedLimbs::zero(self.limbs.len()),
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encoding: PhantomData,
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}
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}
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#[inline]
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pub(super) fn limbs(&self) -> &[Limb] {
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self.limbs
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}
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#[inline]
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pub(super) fn n0(&self) -> &N0 {
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&self.n0
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}
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pub fn len_bits(&self) -> BitLength {
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self.len_bits
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}
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#[inline]
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pub(crate) fn cpu_features(&self) -> cpu::Features {
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self.cpu_features
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}
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}
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