vendor/ring-0.16.20/src/limb.rs - toolchain/rustc - Git at Google

 // Copyright 2016 David Judd.
 // Copyright 2016 Brian Smith.
 //
 // Permission to use, copy, modify, and/or distribute this software for any
 // purpose with or without fee is hereby granted, provided that the above
 // copyright notice and this permission notice appear in all copies.
 //
 // THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
 // WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 // MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
 // SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 // WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
 // OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
 // CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

 //! Unsigned multi-precision integer arithmetic.
 //!
 //! Limbs ordered least-significant-limb to most-significant-limb. The bits
 //! limbs use the native endianness.

 use crate::{c, error};

 #[cfg(feature = "alloc")]
 use crate::bits;

 #[cfg(feature = "alloc")]
 use core::num::Wrapping;

 // XXX: Not correct for x32 ABIs.
 #[cfg(target_pointer_width = "64")]
 pub type Limb = u64;
 #[cfg(target_pointer_width = "32")]
 pub type Limb = u32;
 #[cfg(target_pointer_width = "64")]
 pub const LIMB_BITS: usize = 64;
 #[cfg(target_pointer_width = "32")]
 pub const LIMB_BITS: usize = 32;

 #[cfg(target_pointer_width = "64")]
 #[derive(Debug, PartialEq)]
 #[repr(u64)]
 pub enum LimbMask {
     True = 0xffff_ffff_ffff_ffff,
     False = 0,
 }

 #[cfg(target_pointer_width = "32")]
 #[derive(Debug, PartialEq)]
 #[repr(u32)]
 pub enum LimbMask {
     True = 0xffff_ffff,
     False = 0,
 }

 pub const LIMB_BYTES: usize = (LIMB_BITS + 7) / 8;

 #[inline]
 pub fn limbs_equal_limbs_consttime(a: &[Limb], b: &[Limb]) -> LimbMask {
     extern "C" {
         fn LIMBS_equal(a: *const Limb, b: *const Limb, num_limbs: c::size_t) -> LimbMask;
     }

     assert_eq!(a.len(), b.len());
     unsafe { LIMBS_equal(a.as_ptr(), b.as_ptr(), a.len()) }
 }

 #[inline]
 pub fn limbs_less_than_limbs_consttime(a: &[Limb], b: &[Limb]) -> LimbMask {
     assert_eq!(a.len(), b.len());
     unsafe { LIMBS_less_than(a.as_ptr(), b.as_ptr(), b.len()) }
 }

 #[inline]
 pub fn limbs_less_than_limbs_vartime(a: &[Limb], b: &[Limb]) -> bool {
     limbs_less_than_limbs_consttime(a, b) == LimbMask::True
 }

 #[inline]
 #[cfg(feature = "alloc")]
 pub fn limbs_less_than_limb_constant_time(a: &[Limb], b: Limb) -> LimbMask {
     unsafe { LIMBS_less_than_limb(a.as_ptr(), b, a.len()) }
 }

 #[inline]
 pub fn limbs_are_zero_constant_time(limbs: &[Limb]) -> LimbMask {
     unsafe { LIMBS_are_zero(limbs.as_ptr(), limbs.len()) }
 }

 #[cfg(feature = "alloc")]
 #[inline]
 pub fn limbs_are_even_constant_time(limbs: &[Limb]) -> LimbMask {
     unsafe { LIMBS_are_even(limbs.as_ptr(), limbs.len()) }
 }

 #[cfg(feature = "alloc")]
 #[inline]
 pub fn limbs_equal_limb_constant_time(a: &[Limb], b: Limb) -> LimbMask {
     unsafe { LIMBS_equal_limb(a.as_ptr(), b, a.len()) }
 }

 /// Returns the number of bits in `a`.
 //
 // This strives to be constant-time with respect to the values of all bits
 // except the most significant bit. This does not attempt to be constant-time
 // with respect to `a.len()` or the value of the result or the value of the
 // most significant bit (It's 1, unless the input is zero, in which case it's
 // zero.)
 #[cfg(feature = "alloc")]
 pub fn limbs_minimal_bits(a: &[Limb]) -> bits::BitLength {
     for num_limbs in (1..=a.len()).rev() {
         let high_limb = a[num_limbs - 1];

         // Find the number of set bits in |high_limb| by a linear scan from the
         // most significant bit to the least significant bit. This works great
         // for the most common inputs because usually the most significant bit
         // it set.
         for high_limb_num_bits in (1..=LIMB_BITS).rev() {
             let shifted = unsafe { LIMB_shr(high_limb, high_limb_num_bits - 1) };
             if shifted != 0 {
                 return bits::BitLength::from_usize_bits(
                     ((num_limbs - 1) * LIMB_BITS) + high_limb_num_bits,
                 );
             }
         }
     }

     // No bits were set.
     bits::BitLength::from_usize_bits(0)
 }

 /// Equivalent to `if (r >= m) { r -= m; }`
 #[inline]
 pub fn limbs_reduce_once_constant_time(r: &mut [Limb], m: &[Limb]) {
     assert_eq!(r.len(), m.len());
     unsafe { LIMBS_reduce_once(r.as_mut_ptr(), m.as_ptr(), m.len()) };
 }

 #[derive(Clone, Copy, PartialEq)]
 pub enum AllowZero {
     No,
     Yes,
 }

 /// Parses `input` into `result`, reducing it via conditional subtraction
 /// (mod `m`). Assuming 2**((self.num_limbs * LIMB_BITS) - 1) < m and
 /// m < 2**(self.num_limbs * LIMB_BITS), the value will be reduced mod `m` in
 /// constant time so that the result is in the range [0, m) if `allow_zero` is
 /// `AllowZero::Yes`, or [1, m) if `allow_zero` is `AllowZero::No`. `result` is
 /// padded with zeros to its length.
 pub fn parse_big_endian_in_range_partially_reduced_and_pad_consttime(
     input: untrusted::Input,
     allow_zero: AllowZero,
     m: &[Limb],
     result: &mut [Limb],
 ) -> Result<(), error::Unspecified> {
     parse_big_endian_and_pad_consttime(input, result)?;
     limbs_reduce_once_constant_time(result, m);
     if allow_zero != AllowZero::Yes {
         if limbs_are_zero_constant_time(&result) != LimbMask::False {
             return Err(error::Unspecified);
         }
     }
     Ok(())
 }

 /// Parses `input` into `result`, verifies that the value is less than
 /// `max_exclusive`, and pads `result` with zeros to its length. If `allow_zero`
 /// is not `AllowZero::Yes`, zero values are rejected.
 ///
 /// This attempts to be constant-time with respect to the actual value *only if*
 /// the value is actually in range. In other words, this won't leak anything
 /// about a valid value, but it might leak small amounts of information about an
 /// invalid value (which constraint it failed).
 pub fn parse_big_endian_in_range_and_pad_consttime(
     input: untrusted::Input,
     allow_zero: AllowZero,
     max_exclusive: &[Limb],
     result: &mut [Limb],
 ) -> Result<(), error::Unspecified> {
     parse_big_endian_and_pad_consttime(input, result)?;
     if limbs_less_than_limbs_consttime(&result, max_exclusive) != LimbMask::True {
         return Err(error::Unspecified);
     }
     if allow_zero != AllowZero::Yes {
         if limbs_are_zero_constant_time(&result) != LimbMask::False {
             return Err(error::Unspecified);
         }
     }
     Ok(())
 }

 /// Parses `input` into `result`, padding `result` with zeros to its length.
 /// This attempts to be constant-time with respect to the value but not with
 /// respect to the length; it is assumed that the length is public knowledge.
 pub fn parse_big_endian_and_pad_consttime(
     input: untrusted::Input,
     result: &mut [Limb],
 ) -> Result<(), error::Unspecified> {
     if input.is_empty() {
         return Err(error::Unspecified);
     }

     // `bytes_in_current_limb` is the number of bytes in the current limb.
     // It will be `LIMB_BYTES` for all limbs except maybe the highest-order
     // limb.
     let mut bytes_in_current_limb = input.len() % LIMB_BYTES;
     if bytes_in_current_limb == 0 {
         bytes_in_current_limb = LIMB_BYTES;
     }

     let num_encoded_limbs = (input.len() / LIMB_BYTES)
         + (if bytes_in_current_limb == LIMB_BYTES {
             0
         } else {
             1
         });
     if num_encoded_limbs > result.len() {
         return Err(error::Unspecified);
     }

     for r in &mut result[..] {
         *r = 0;
     }

     // XXX: Questionable as far as constant-timedness is concerned.
     // TODO: Improve this.
     input.read_all(error::Unspecified, |input| {
         for i in 0..num_encoded_limbs {
             let mut limb: Limb = 0;
             for _ in 0..bytes_in_current_limb {
                 let b: Limb = input.read_byte()?.into();
                 limb = (limb << 8) | b;
             }
             result[num_encoded_limbs - i - 1] = limb;
             bytes_in_current_limb = LIMB_BYTES;
         }
         Ok(())
     })
 }

 pub fn big_endian_from_limbs(limbs: &[Limb], out: &mut [u8]) {
     let num_limbs = limbs.len();
     let out_len = out.len();
     assert_eq!(out_len, num_limbs * LIMB_BYTES);
     for i in 0..num_limbs {
         let mut limb = limbs[i];
         for j in 0..LIMB_BYTES {
             out[((num_limbs - i - 1) * LIMB_BYTES) + (LIMB_BYTES - j - 1)] = (limb & 0xff) as u8;
             limb >>= 8;
         }
     }
 }

 #[cfg(feature = "alloc")]
 pub type Window = Limb;

 /// Processes `limbs` as a sequence of 5-bit windows, folding the windows from
 /// most significant to least significant and returning the accumulated result.
 /// The first window will be mapped by `init` to produce the initial value for
 /// the accumulator. Then `f` will be called to fold the accumulator and the
 /// next window until all windows are processed. When the input's bit length
 /// isn't divisible by 5, the window passed to `init` will be partial; all
 /// windows passed to `fold` will be full.
 ///
 /// This is designed to avoid leaking the contents of `limbs` through side
 /// channels as long as `init` and `fold` are side-channel free.
 ///
 /// Panics if `limbs` is empty.
 #[cfg(feature = "alloc")]
 pub fn fold_5_bit_windows<R, I: FnOnce(Window) -> R, F: Fn(R, Window) -> R>(
     limbs: &[Limb],
     init: I,
     fold: F,
 ) -> R {
     #[derive(Clone, Copy)]
     #[repr(transparent)]
     struct BitIndex(Wrapping<c::size_t>);

     const WINDOW_BITS: Wrapping<c::size_t> = Wrapping(5);

     extern "C" {
         fn LIMBS_window5_split_window(
             lower_limb: Limb,
             higher_limb: Limb,
             index_within_word: BitIndex,
         ) -> Window;
         fn LIMBS_window5_unsplit_window(limb: Limb, index_within_word: BitIndex) -> Window;
     }

     let num_limbs = limbs.len();
     let mut window_low_bit = {
         let num_whole_windows = (num_limbs * LIMB_BITS) / 5;
         let mut leading_bits = (num_limbs * LIMB_BITS) - (num_whole_windows * 5);
         if leading_bits == 0 {
             leading_bits = WINDOW_BITS.0;
         }
         BitIndex(Wrapping(LIMB_BITS - leading_bits))
     };

     let initial_value = {
         let leading_partial_window =
             unsafe { LIMBS_window5_split_window(*limbs.last().unwrap(), 0, window_low_bit) };
         window_low_bit.0 -= WINDOW_BITS;
         init(leading_partial_window)
     };

     let mut low_limb = 0;
     limbs
         .iter()
         .rev()
         .fold(initial_value, |mut acc, current_limb| {
             let higher_limb = low_limb;
             low_limb = *current_limb;

             if window_low_bit.0 > Wrapping(LIMB_BITS) - WINDOW_BITS {
                 let window =
                     unsafe { LIMBS_window5_split_window(low_limb, higher_limb, window_low_bit) };
                 window_low_bit.0 -= WINDOW_BITS;
                 acc = fold(acc, window);
             };
             while window_low_bit.0 < Wrapping(LIMB_BITS) {
                 let window = unsafe { LIMBS_window5_unsplit_window(low_limb, window_low_bit) };
                 // The loop exits when this subtraction underflows, causing `window_low_bit` to
                 // wrap around to a very large value.
                 window_low_bit.0 -= WINDOW_BITS;
                 acc = fold(acc, window);
             }
             window_low_bit.0 += Wrapping(LIMB_BITS); // "Fix" the underflow.

             acc
         })
 }

 extern "C" {
     #[cfg(feature = "alloc")]
     fn LIMB_shr(a: Limb, shift: c::size_t) -> Limb;

     #[cfg(feature = "alloc")]
     fn LIMBS_are_even(a: *const Limb, num_limbs: c::size_t) -> LimbMask;
     fn LIMBS_are_zero(a: *const Limb, num_limbs: c::size_t) -> LimbMask;
     #[cfg(feature = "alloc")]
     fn LIMBS_equal_limb(a: *const Limb, b: Limb, num_limbs: c::size_t) -> LimbMask;
     fn LIMBS_less_than(a: *const Limb, b: *const Limb, num_limbs: c::size_t) -> LimbMask;
     #[cfg(feature = "alloc")]
     fn LIMBS_less_than_limb(a: *const Limb, b: Limb, num_limbs: c::size_t) -> LimbMask;
     fn LIMBS_reduce_once(r: *mut Limb, m: *const Limb, num_limbs: c::size_t);
 }

 #[cfg(test)]
 mod tests {
     use super::*;

     const MAX: Limb = LimbMask::True as Limb;

     #[test]
     fn test_limbs_are_even() {
         static EVENS: &[&[Limb]] = &[
             &[],
             &[0],
             &[2],
             &[0, 0],
             &[2, 0],
             &[0, 1],
             &[0, 2],
             &[0, 3],
             &[0, 0, 0, 0, MAX],
         ];
         for even in EVENS {
             assert_eq!(limbs_are_even_constant_time(even), LimbMask::True);
         }
         static ODDS: &[&[Limb]] = &[
             &[1],
             &[3],
             &[1, 0],
             &[3, 0],
             &[1, 1],
             &[1, 2],
             &[1, 3],
             &[1, 0, 0, 0, MAX],
         ];
         for odd in ODDS {
             assert_eq!(limbs_are_even_constant_time(odd), LimbMask::False);
         }
     }

     static ZEROES: &[&[Limb]] = &[
         &[],
         &[0],
         &[0, 0],
         &[0, 0, 0],
         &[0, 0, 0, 0],
         &[0, 0, 0, 0, 0],
         &[0, 0, 0, 0, 0, 0, 0],
         &[0, 0, 0, 0, 0, 0, 0, 0],
         &[0, 0, 0, 0, 0, 0, 0, 0, 0],
     ];

     static NONZEROES: &[&[Limb]] = &[
         &[1],
         &[0, 1],
         &[1, 1],
         &[1, 0, 0, 0],
         &[0, 1, 0, 0],
         &[0, 0, 1, 0],
         &[0, 0, 0, 1],
     ];

     #[test]
     fn test_limbs_are_zero() {
         for zero in ZEROES {
             assert_eq!(limbs_are_zero_constant_time(zero), LimbMask::True);
         }
         for nonzero in NONZEROES {
             assert_eq!(limbs_are_zero_constant_time(nonzero), LimbMask::False);
         }
     }

     #[test]
     fn test_limbs_equal_limb() {
         for zero in ZEROES {
             assert_eq!(limbs_equal_limb_constant_time(zero, 0), LimbMask::True);
         }
         for nonzero in NONZEROES {
             assert_eq!(limbs_equal_limb_constant_time(nonzero, 0), LimbMask::False);
         }
         static EQUAL: &[(&[Limb], Limb)] = &[
             (&[1], 1),
             (&[MAX], MAX),
             (&[1, 0], 1),
             (&[MAX, 0, 0], MAX),
             (&[0b100], 0b100),
             (&[0b100, 0], 0b100),
         ];
         for &(a, b) in EQUAL {
             assert_eq!(limbs_equal_limb_constant_time(a, b), LimbMask::True);
         }
         static UNEQUAL: &[(&[Limb], Limb)] = &[
             (&[0], 1),
             (&[2], 1),
             (&[3], 1),
             (&[1, 1], 1),
             (&[0b100, 0b100], 0b100),
             (&[1, 0, 0b100, 0, 0, 0, 0, 0], 1),
             (&[1, 0, 0, 0, 0, 0, 0, 0b100], 1),
             (&[MAX, MAX], MAX),
             (&[MAX, 1], MAX),
         ];
         for &(a, b) in UNEQUAL {
             assert_eq!(limbs_equal_limb_constant_time(a, b), LimbMask::False);
         }
     }

     #[test]
     #[cfg(feature = "alloc")]
     fn test_limbs_less_than_limb_constant_time() {
         static LESSER: &[(&[Limb], Limb)] = &[
             (&[0], 1),
             (&[0, 0], 1),
             (&[1, 0], 2),
             (&[2, 0], 3),
             (&[2, 0], 3),
             (&[MAX - 1], MAX),
             (&[MAX - 1, 0], MAX),
         ];
         for &(a, b) in LESSER {
             assert_eq!(limbs_less_than_limb_constant_time(a, b), LimbMask::True);
         }
         static EQUAL: &[(&[Limb], Limb)] = &[
             (&[0], 0),
             (&[0, 0, 0, 0], 0),
             (&[1], 1),
             (&[1, 0, 0, 0, 0, 0, 0], 1),
             (&[MAX], MAX),
         ];
         static GREATER: &[(&[Limb], Limb)] = &[
             (&[1], 0),
             (&[2, 0], 1),
             (&[3, 0, 0, 0], 1),
             (&[0, 1, 0, 0], 1),
             (&[0, 0, 1, 0], 1),
             (&[0, 0, 1, 1], 1),
             (&[MAX], MAX - 1),
         ];
         for &(a, b) in EQUAL.iter().chain(GREATER.iter()) {
             assert_eq!(limbs_less_than_limb_constant_time(a, b), LimbMask::False);
         }
     }

     #[test]
     fn test_parse_big_endian_and_pad_consttime() {
         const LIMBS: usize = 4;

         {
             // Empty input.
             let inp = untrusted::Input::from(&[]);
             let mut result = [0; LIMBS];
             assert!(parse_big_endian_and_pad_consttime(inp, &mut result).is_err());
         }

         // The input is longer than will fit in the given number of limbs.
         {
             let inp = [1, 2, 3, 4, 5, 6, 7, 8, 9];
             let inp = untrusted::Input::from(&inp);
             let mut result = [0; 8 / LIMB_BYTES];
             assert!(parse_big_endian_and_pad_consttime(inp, &mut result[..]).is_err());
         }

         // Less than a full limb.
         {
             let inp = [0xfe];
             let inp = untrusted::Input::from(&inp);
             let mut result = [0; LIMBS];
             assert_eq!(
                 Ok(()),
                 parse_big_endian_and_pad_consttime(inp, &mut result[..])
             );
             assert_eq!(&[0xfe, 0, 0, 0], &result);
         }

         // A whole limb for 32-bit, half a limb for 64-bit.
         {
             let inp = [0xbe, 0xef, 0xf0, 0x0d];
             let inp = untrusted::Input::from(&inp);
             let mut result = [0; LIMBS];
             assert_eq!(Ok(()), parse_big_endian_and_pad_consttime(inp, &mut result));
             assert_eq!(&[0xbeeff00d, 0, 0, 0], &result);
         }

         // XXX: This is a weak set of tests. TODO: expand it.
     }

     #[test]
     fn test_big_endian_from_limbs_same_length() {
         #[cfg(target_pointer_width = "32")]
         let limbs = [
             0xbccddeef, 0x89900aab, 0x45566778, 0x01122334, 0xddeeff00, 0x99aabbcc, 0x55667788,
             0x11223344,
         ];

         #[cfg(target_pointer_width = "64")]
         let limbs = [
             0x8990_0aab_bccd_deef,
             0x0112_2334_4556_6778,
             0x99aa_bbcc_ddee_ff00,
             0x1122_3344_5566_7788,
         ];

         let expected = [
             0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee,
             0xff, 0x00, 0x01, 0x12, 0x23, 0x34, 0x45, 0x56, 0x67, 0x78, 0x89, 0x90, 0x0a, 0xab,
             0xbc, 0xcd, 0xde, 0xef,
         ];

         let mut out = [0xabu8; 32];
         big_endian_from_limbs(&limbs[..], &mut out);
         assert_eq!(&out[..], &expected[..]);
     }

     #[should_panic]
     #[test]
     fn test_big_endian_from_limbs_fewer_limbs() {
         #[cfg(target_pointer_width = "32")]
         // Two fewer limbs.
         let limbs = [
             0xbccddeef, 0x89900aab, 0x45566778, 0x01122334, 0xddeeff00, 0x99aabbcc,
         ];

         // One fewer limb.
         #[cfg(target_pointer_width = "64")]
         let limbs = [
             0x8990_0aab_bccd_deef,
             0x0112_2334_4556_6778,
             0x99aa_bbcc_ddee_ff00,
         ];

         let mut out = [0xabu8; 32];

         big_endian_from_limbs(&limbs[..], &mut out);
     }

     #[test]
     fn test_limbs_minimal_bits() {
         const ALL_ONES: Limb = LimbMask::True as Limb;
         static CASES: &[(&[Limb], usize)] = &[
             (&[], 0),
             (&[0], 0),
             (&[ALL_ONES], LIMB_BITS),
             (&[ALL_ONES, 0], LIMB_BITS),
             (&[ALL_ONES, 1], LIMB_BITS + 1),
             (&[0, 0], 0),
             (&[1, 0], 1),
             (&[0, 1], LIMB_BITS + 1),
             (&[0, ALL_ONES], 2 * LIMB_BITS),
             (&[ALL_ONES, ALL_ONES], 2 * LIMB_BITS),
             (&[ALL_ONES, ALL_ONES >> 1], 2 * LIMB_BITS - 1),
             (&[ALL_ONES, 0b100_0000], LIMB_BITS + 7),
             (&[ALL_ONES, 0b101_0000], LIMB_BITS + 7),
             (&[ALL_ONES, ALL_ONES >> 1], LIMB_BITS + (LIMB_BITS) - 1),
         ];
         for (limbs, bits) in CASES {
             assert_eq!(limbs_minimal_bits(limbs).as_usize_bits(), *bits);
         }
     }
 }
	// Copyright 2016 David Judd.
	// Copyright 2016 Brian Smith.
	//
	// Permission to use, copy, modify, and/or distribute this software for any
	// purpose with or without fee is hereby granted, provided that the above
	// copyright notice and this permission notice appear in all copies.
	//
	// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
	// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
	// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
	// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
	// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
	// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
	// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

	//! Unsigned multi-precision integer arithmetic.
	//!
	//! Limbs ordered least-significant-limb to most-significant-limb. The bits
	//! limbs use the native endianness.

	use crate::{c, error};

	#[cfg(feature = "alloc")]
	use crate::bits;

	#[cfg(feature = "alloc")]
	use core::num::Wrapping;

	// XXX: Not correct for x32 ABIs.
	#[cfg(target_pointer_width = "64")]
	pub type Limb = u64;
	#[cfg(target_pointer_width = "32")]
	pub type Limb = u32;
	#[cfg(target_pointer_width = "64")]
	pub const LIMB_BITS: usize = 64;
	#[cfg(target_pointer_width = "32")]
	pub const LIMB_BITS: usize = 32;

	#[cfg(target_pointer_width = "64")]
	#[derive(Debug, PartialEq)]
	#[repr(u64)]
	pub enum LimbMask {
	True = 0xffff_ffff_ffff_ffff,
	False = 0,
	}

	#[cfg(target_pointer_width = "32")]
	#[derive(Debug, PartialEq)]
	#[repr(u32)]
	pub enum LimbMask {
	True = 0xffff_ffff,
	False = 0,
	}

	pub const LIMB_BYTES: usize = (LIMB_BITS + 7) / 8;

	#[inline]
	pub fn limbs_equal_limbs_consttime(a: &[Limb], b: &[Limb]) -> LimbMask {
	extern "C" {
	fn LIMBS_equal(a: const Limb, b: const Limb, num_limbs: c::size_t) -> LimbMask;
	}

	assert_eq!(a.len(), b.len());
	unsafe { LIMBS_equal(a.as_ptr(), b.as_ptr(), a.len()) }
	}

	#[inline]
	pub fn limbs_less_than_limbs_consttime(a: &[Limb], b: &[Limb]) -> LimbMask {
	assert_eq!(a.len(), b.len());
	unsafe { LIMBS_less_than(a.as_ptr(), b.as_ptr(), b.len()) }
	}

	#[inline]
	pub fn limbs_less_than_limbs_vartime(a: &[Limb], b: &[Limb]) -> bool {
	limbs_less_than_limbs_consttime(a, b) == LimbMask::True
	}

	#[inline]
	#[cfg(feature = "alloc")]
	pub fn limbs_less_than_limb_constant_time(a: &[Limb], b: Limb) -> LimbMask {
	unsafe { LIMBS_less_than_limb(a.as_ptr(), b, a.len()) }
	}

	#[inline]
	pub fn limbs_are_zero_constant_time(limbs: &[Limb]) -> LimbMask {
	unsafe { LIMBS_are_zero(limbs.as_ptr(), limbs.len()) }
	}

	#[cfg(feature = "alloc")]
	#[inline]
	pub fn limbs_are_even_constant_time(limbs: &[Limb]) -> LimbMask {
	unsafe { LIMBS_are_even(limbs.as_ptr(), limbs.len()) }
	}

	#[cfg(feature = "alloc")]
	#[inline]
	pub fn limbs_equal_limb_constant_time(a: &[Limb], b: Limb) -> LimbMask {
	unsafe { LIMBS_equal_limb(a.as_ptr(), b, a.len()) }
	}

	/// Returns the number of bits in `a`.
	//
	// This strives to be constant-time with respect to the values of all bits
	// except the most significant bit. This does not attempt to be constant-time
	// with respect to `a.len()` or the value of the result or the value of the
	// most significant bit (It's 1, unless the input is zero, in which case it's
	// zero.)
	#[cfg(feature = "alloc")]
	pub fn limbs_minimal_bits(a: &[Limb]) -> bits::BitLength {
	for num_limbs in (1..=a.len()).rev() {
	let high_limb = a[num_limbs - 1];

	// Find the number of set bits in \|high_limb\| by a linear scan from the
	// most significant bit to the least significant bit. This works great
	// for the most common inputs because usually the most significant bit
	// it set.
	for high_limb_num_bits in (1..=LIMB_BITS).rev() {
	let shifted = unsafe { LIMB_shr(high_limb, high_limb_num_bits - 1) };
	if shifted != 0 {
	return bits::BitLength::from_usize_bits(
	((num_limbs - 1) * LIMB_BITS) + high_limb_num_bits,
	);
	}
	}
	}

	// No bits were set.
	bits::BitLength::from_usize_bits(0)
	}

	/// Equivalent to `if (r >= m) { r -= m; }`
	#[inline]
	pub fn limbs_reduce_once_constant_time(r: &mut [Limb], m: &[Limb]) {
	assert_eq!(r.len(), m.len());
	unsafe { LIMBS_reduce_once(r.as_mut_ptr(), m.as_ptr(), m.len()) };
	}

	#[derive(Clone, Copy, PartialEq)]
	pub enum AllowZero {
	No,
	Yes,
	}

	/// Parses `input` into `result`, reducing it via conditional subtraction
	/// (mod `m`). Assuming 2*((self.num_limbs LIMB_BITS) - 1) < m and
	/// m < 2*(self.num_limbs LIMB_BITS), the value will be reduced mod `m` in
	/// constant time so that the result is in the range [0, m) if `allow_zero` is
	/// `AllowZero::Yes`, or [1, m) if `allow_zero` is `AllowZero::No`. `result` is
	/// padded with zeros to its length.
	pub fn parse_big_endian_in_range_partially_reduced_and_pad_consttime(
	input: untrusted::Input,
	allow_zero: AllowZero,
	m: &[Limb],
	result: &mut [Limb],
	) -> Result<(), error::Unspecified> {
	parse_big_endian_and_pad_consttime(input, result)?;
	limbs_reduce_once_constant_time(result, m);
	if allow_zero != AllowZero::Yes {
	if limbs_are_zero_constant_time(&result) != LimbMask::False {
	return Err(error::Unspecified);
	}
	}
	Ok(())
	}

	/// Parses `input` into `result`, verifies that the value is less than
	/// `max_exclusive`, and pads `result` with zeros to its length. If `allow_zero`
	/// is not `AllowZero::Yes`, zero values are rejected.
	///
	/// This attempts to be constant-time with respect to the actual value only if
	/// the value is actually in range. In other words, this won't leak anything
	/// about a valid value, but it might leak small amounts of information about an
	/// invalid value (which constraint it failed).
	pub fn parse_big_endian_in_range_and_pad_consttime(
	input: untrusted::Input,
	allow_zero: AllowZero,
	max_exclusive: &[Limb],
	result: &mut [Limb],
	) -> Result<(), error::Unspecified> {
	parse_big_endian_and_pad_consttime(input, result)?;
	if limbs_less_than_limbs_consttime(&result, max_exclusive) != LimbMask::True {
	return Err(error::Unspecified);
	}
	if allow_zero != AllowZero::Yes {
	if limbs_are_zero_constant_time(&result) != LimbMask::False {
	return Err(error::Unspecified);
	}
	}
	Ok(())
	}

	/// Parses `input` into `result`, padding `result` with zeros to its length.
	/// This attempts to be constant-time with respect to the value but not with
	/// respect to the length; it is assumed that the length is public knowledge.
	pub fn parse_big_endian_and_pad_consttime(
	input: untrusted::Input,
	result: &mut [Limb],
	) -> Result<(), error::Unspecified> {
	if input.is_empty() {
	return Err(error::Unspecified);
	}

	// `bytes_in_current_limb` is the number of bytes in the current limb.
	// It will be `LIMB_BYTES` for all limbs except maybe the highest-order
	// limb.
	let mut bytes_in_current_limb = input.len() % LIMB_BYTES;
	if bytes_in_current_limb == 0 {
	bytes_in_current_limb = LIMB_BYTES;
	}

	let num_encoded_limbs = (input.len() / LIMB_BYTES)
	+ (if bytes_in_current_limb == LIMB_BYTES {
	0
	} else {
	1
	});
	if num_encoded_limbs > result.len() {
	return Err(error::Unspecified);
	}

	for r in &mut result[..] {
	*r = 0;
	}

	// XXX: Questionable as far as constant-timedness is concerned.
	// TODO: Improve this.
	input.read_all(error::Unspecified, \|input\| {
	for i in 0..num_encoded_limbs {
	let mut limb: Limb = 0;
	for _ in 0..bytes_in_current_limb {
	let b: Limb = input.read_byte()?.into();
	limb = (limb << 8) \| b;
	}
	result[num_encoded_limbs - i - 1] = limb;
	bytes_in_current_limb = LIMB_BYTES;
	}
	Ok(())
	})
	}

	pub fn big_endian_from_limbs(limbs: &[Limb], out: &mut [u8]) {
	let num_limbs = limbs.len();
	let out_len = out.len();
	assert_eq!(out_len, num_limbs * LIMB_BYTES);
	for i in 0..num_limbs {
	let mut limb = limbs[i];
	for j in 0..LIMB_BYTES {
	out[((num_limbs - i - 1) * LIMB_BYTES) + (LIMB_BYTES - j - 1)] = (limb & 0xff) as u8;
	limb >>= 8;
	}
	}
	}

	#[cfg(feature = "alloc")]
	pub type Window = Limb;

	/// Processes `limbs` as a sequence of 5-bit windows, folding the windows from
	/// most significant to least significant and returning the accumulated result.
	/// The first window will be mapped by `init` to produce the initial value for
	/// the accumulator. Then `f` will be called to fold the accumulator and the
	/// next window until all windows are processed. When the input's bit length
	/// isn't divisible by 5, the window passed to `init` will be partial; all
	/// windows passed to `fold` will be full.
	///
	/// This is designed to avoid leaking the contents of `limbs` through side
	/// channels as long as `init` and `fold` are side-channel free.
	///
	/// Panics if `limbs` is empty.
	#[cfg(feature = "alloc")]
	pub fn fold_5_bit_windows<R, I: FnOnce(Window) -> R, F: Fn(R, Window) -> R>(
	limbs: &[Limb],
	init: I,
	fold: F,
	) -> R {
	#[derive(Clone, Copy)]
	#[repr(transparent)]
	struct BitIndex(Wrapping<c::size_t>);

	const WINDOW_BITS: Wrapping<c::size_t> = Wrapping(5);

	extern "C" {
	fn LIMBS_window5_split_window(
	lower_limb: Limb,
	higher_limb: Limb,
	index_within_word: BitIndex,
	) -> Window;
	fn LIMBS_window5_unsplit_window(limb: Limb, index_within_word: BitIndex) -> Window;
	}

	let num_limbs = limbs.len();
	let mut window_low_bit = {
	let num_whole_windows = (num_limbs * LIMB_BITS) / 5;
	let mut leading_bits = (num_limbs * LIMB_BITS) - (num_whole_windows * 5);
	if leading_bits == 0 {
	leading_bits = WINDOW_BITS.0;
	}
	BitIndex(Wrapping(LIMB_BITS - leading_bits))
	};

	let initial_value = {
	let leading_partial_window =
	unsafe { LIMBS_window5_split_window(*limbs.last().unwrap(), 0, window_low_bit) };
	window_low_bit.0 -= WINDOW_BITS;
	init(leading_partial_window)
	};

	let mut low_limb = 0;
	limbs
	.iter()
	.rev()
	.fold(initial_value, \|mut acc, current_limb\| {
	let higher_limb = low_limb;
	low_limb = *current_limb;

	if window_low_bit.0 > Wrapping(LIMB_BITS) - WINDOW_BITS {
	let window =
	unsafe { LIMBS_window5_split_window(low_limb, higher_limb, window_low_bit) };
	window_low_bit.0 -= WINDOW_BITS;
	acc = fold(acc, window);
	};
	while window_low_bit.0 < Wrapping(LIMB_BITS) {
	let window = unsafe { LIMBS_window5_unsplit_window(low_limb, window_low_bit) };
	// The loop exits when this subtraction underflows, causing `window_low_bit` to
	// wrap around to a very large value.
	window_low_bit.0 -= WINDOW_BITS;
	acc = fold(acc, window);
	}
	window_low_bit.0 += Wrapping(LIMB_BITS); // "Fix" the underflow.

	acc
	})
	}

	extern "C" {
	#[cfg(feature = "alloc")]
	fn LIMB_shr(a: Limb, shift: c::size_t) -> Limb;

	#[cfg(feature = "alloc")]
	fn LIMBS_are_even(a: *const Limb, num_limbs: c::size_t) -> LimbMask;
	fn LIMBS_are_zero(a: *const Limb, num_limbs: c::size_t) -> LimbMask;
	#[cfg(feature = "alloc")]
	fn LIMBS_equal_limb(a: *const Limb, b: Limb, num_limbs: c::size_t) -> LimbMask;
	fn LIMBS_less_than(a: const Limb, b: const Limb, num_limbs: c::size_t) -> LimbMask;
	#[cfg(feature = "alloc")]
	fn LIMBS_less_than_limb(a: *const Limb, b: Limb, num_limbs: c::size_t) -> LimbMask;
	fn LIMBS_reduce_once(r: mut Limb, m: const Limb, num_limbs: c::size_t);
	}

	#[cfg(test)]
	mod tests {
	use super::*;

	const MAX: Limb = LimbMask::True as Limb;

	#[test]
	fn test_limbs_are_even() {
	static EVENS: &[&[Limb]] = &[
	&[],
	&[0],
	&[2],
	&[0, 0],
	&[2, 0],
	&[0, 1],
	&[0, 2],
	&[0, 3],
	&[0, 0, 0, 0, MAX],
	];
	for even in EVENS {
	assert_eq!(limbs_are_even_constant_time(even), LimbMask::True);
	}
	static ODDS: &[&[Limb]] = &[
	&[1],
	&[3],
	&[1, 0],
	&[3, 0],
	&[1, 1],
	&[1, 2],
	&[1, 3],
	&[1, 0, 0, 0, MAX],
	];
	for odd in ODDS {
	assert_eq!(limbs_are_even_constant_time(odd), LimbMask::False);
	}
	}

	static ZEROES: &[&[Limb]] = &[
	&[],
	&[0],
	&[0, 0],
	&[0, 0, 0],
	&[0, 0, 0, 0],
	&[0, 0, 0, 0, 0],
	&[0, 0, 0, 0, 0, 0, 0],
	&[0, 0, 0, 0, 0, 0, 0, 0],
	&[0, 0, 0, 0, 0, 0, 0, 0, 0],
	];

	static NONZEROES: &[&[Limb]] = &[
	&[1],
	&[0, 1],
	&[1, 1],
	&[1, 0, 0, 0],
	&[0, 1, 0, 0],
	&[0, 0, 1, 0],
	&[0, 0, 0, 1],
	];

	#[test]
	fn test_limbs_are_zero() {
	for zero in ZEROES {
	assert_eq!(limbs_are_zero_constant_time(zero), LimbMask::True);
	}
	for nonzero in NONZEROES {
	assert_eq!(limbs_are_zero_constant_time(nonzero), LimbMask::False);
	}
	}

	#[test]
	fn test_limbs_equal_limb() {
	for zero in ZEROES {
	assert_eq!(limbs_equal_limb_constant_time(zero, 0), LimbMask::True);
	}
	for nonzero in NONZEROES {
	assert_eq!(limbs_equal_limb_constant_time(nonzero, 0), LimbMask::False);
	}
	static EQUAL: &[(&[Limb], Limb)] = &[
	(&[1], 1),
	(&[MAX], MAX),
	(&[1, 0], 1),
	(&[MAX, 0, 0], MAX),
	(&[0b100], 0b100),
	(&[0b100, 0], 0b100),
	];
	for &(a, b) in EQUAL {
	assert_eq!(limbs_equal_limb_constant_time(a, b), LimbMask::True);
	}
	static UNEQUAL: &[(&[Limb], Limb)] = &[
	(&[0], 1),
	(&[2], 1),
	(&[3], 1),
	(&[1, 1], 1),
	(&[0b100, 0b100], 0b100),
	(&[1, 0, 0b100, 0, 0, 0, 0, 0], 1),
	(&[1, 0, 0, 0, 0, 0, 0, 0b100], 1),
	(&[MAX, MAX], MAX),
	(&[MAX, 1], MAX),
	];
	for &(a, b) in UNEQUAL {
	assert_eq!(limbs_equal_limb_constant_time(a, b), LimbMask::False);
	}
	}

	#[test]
	#[cfg(feature = "alloc")]
	fn test_limbs_less_than_limb_constant_time() {
	static LESSER: &[(&[Limb], Limb)] = &[
	(&[0], 1),
	(&[0, 0], 1),
	(&[1, 0], 2),
	(&[2, 0], 3),
	(&[2, 0], 3),
	(&[MAX - 1], MAX),
	(&[MAX - 1, 0], MAX),
	];
	for &(a, b) in LESSER {
	assert_eq!(limbs_less_than_limb_constant_time(a, b), LimbMask::True);
	}
	static EQUAL: &[(&[Limb], Limb)] = &[
	(&[0], 0),
	(&[0, 0, 0, 0], 0),
	(&[1], 1),
	(&[1, 0, 0, 0, 0, 0, 0], 1),
	(&[MAX], MAX),
	];
	static GREATER: &[(&[Limb], Limb)] = &[
	(&[1], 0),
	(&[2, 0], 1),
	(&[3, 0, 0, 0], 1),
	(&[0, 1, 0, 0], 1),
	(&[0, 0, 1, 0], 1),
	(&[0, 0, 1, 1], 1),
	(&[MAX], MAX - 1),
	];
	for &(a, b) in EQUAL.iter().chain(GREATER.iter()) {
	assert_eq!(limbs_less_than_limb_constant_time(a, b), LimbMask::False);
	}
	}

	#[test]
	fn test_parse_big_endian_and_pad_consttime() {
	const LIMBS: usize = 4;

	{
	// Empty input.
	let inp = untrusted::Input::from(&[]);
	let mut result = [0; LIMBS];
	assert!(parse_big_endian_and_pad_consttime(inp, &mut result).is_err());
	}

	// The input is longer than will fit in the given number of limbs.
	{
	let inp = [1, 2, 3, 4, 5, 6, 7, 8, 9];
	let inp = untrusted::Input::from(&inp);
	let mut result = [0; 8 / LIMB_BYTES];
	assert!(parse_big_endian_and_pad_consttime(inp, &mut result[..]).is_err());
	}

	// Less than a full limb.
	{
	let inp = [0xfe];
	let inp = untrusted::Input::from(&inp);
	let mut result = [0; LIMBS];
	assert_eq!(
	Ok(()),
	parse_big_endian_and_pad_consttime(inp, &mut result[..])
	);
	assert_eq!(&[0xfe, 0, 0, 0], &result);
	}

	// A whole limb for 32-bit, half a limb for 64-bit.
	{
	let inp = [0xbe, 0xef, 0xf0, 0x0d];
	let inp = untrusted::Input::from(&inp);
	let mut result = [0; LIMBS];
	assert_eq!(Ok(()), parse_big_endian_and_pad_consttime(inp, &mut result));
	assert_eq!(&[0xbeeff00d, 0, 0, 0], &result);
	}

	// XXX: This is a weak set of tests. TODO: expand it.
	}

	#[test]
	fn test_big_endian_from_limbs_same_length() {
	#[cfg(target_pointer_width = "32")]
	let limbs = [
	0xbccddeef, 0x89900aab, 0x45566778, 0x01122334, 0xddeeff00, 0x99aabbcc, 0x55667788,
	0x11223344,
	];

	#[cfg(target_pointer_width = "64")]
	let limbs = [
	0x8990_0aab_bccd_deef,
	0x0112_2334_4556_6778,
	0x99aa_bbcc_ddee_ff00,
	0x1122_3344_5566_7788,
	];

	let expected = [
	0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee,
	0xff, 0x00, 0x01, 0x12, 0x23, 0x34, 0x45, 0x56, 0x67, 0x78, 0x89, 0x90, 0x0a, 0xab,
	0xbc, 0xcd, 0xde, 0xef,
	];

	let mut out = [0xabu8; 32];
	big_endian_from_limbs(&limbs[..], &mut out);
	assert_eq!(&out[..], &expected[..]);
	}

	#[should_panic]
	#[test]
	fn test_big_endian_from_limbs_fewer_limbs() {
	#[cfg(target_pointer_width = "32")]
	// Two fewer limbs.
	let limbs = [
	0xbccddeef, 0x89900aab, 0x45566778, 0x01122334, 0xddeeff00, 0x99aabbcc,
	];

	// One fewer limb.
	#[cfg(target_pointer_width = "64")]
	let limbs = [
	0x8990_0aab_bccd_deef,
	0x0112_2334_4556_6778,
	0x99aa_bbcc_ddee_ff00,
	];

	let mut out = [0xabu8; 32];

	big_endian_from_limbs(&limbs[..], &mut out);
	}

	#[test]
	fn test_limbs_minimal_bits() {
	const ALL_ONES: Limb = LimbMask::True as Limb;
	static CASES: &[(&[Limb], usize)] = &[
	(&[], 0),
	(&[0], 0),
	(&[ALL_ONES], LIMB_BITS),
	(&[ALL_ONES, 0], LIMB_BITS),
	(&[ALL_ONES, 1], LIMB_BITS + 1),
	(&[0, 0], 0),
	(&[1, 0], 1),
	(&[0, 1], LIMB_BITS + 1),
	(&[0, ALL_ONES], 2 * LIMB_BITS),
	(&[ALL_ONES, ALL_ONES], 2 * LIMB_BITS),
	(&[ALL_ONES, ALL_ONES >> 1], 2 * LIMB_BITS - 1),
	(&[ALL_ONES, 0b100_0000], LIMB_BITS + 7),
	(&[ALL_ONES, 0b101_0000], LIMB_BITS + 7),
	(&[ALL_ONES, ALL_ONES >> 1], LIMB_BITS + (LIMB_BITS) - 1),
	];
	for (limbs, bits) in CASES {
	assert_eq!(limbs_minimal_bits(limbs).as_usize_bits(), *bits);
	}
	}
	}