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p256: add 32-bit arithmetic and optimizations (#1033)
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Co-authored-by: davideciacciolo <[email protected]>
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davideciacciolo and davideciacciolo authored Mar 13, 2024
1 parent 7652c58 commit 6ff3bb7
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1 change: 0 additions & 1 deletion p256/src/arithmetic.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,6 @@ pub(crate) mod field;
#[cfg(feature = "hash2curve")]
mod hash2curve;
pub(crate) mod scalar;
pub(crate) mod util;

use self::{field::FieldElement, scalar::Scalar};
use crate::NistP256;
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203 changes: 22 additions & 181 deletions p256/src/arithmetic/field.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,17 +2,18 @@
#![allow(clippy::assign_op_pattern, clippy::op_ref)]

use crate::{
arithmetic::util::{adc, mac, sbb, u256_to_u64x4, u64x4_to_u256},
FieldBytes,
};
#[cfg_attr(target_pointer_width = "32", path = "field/field32.rs")]
#[cfg_attr(target_pointer_width = "64", path = "field/field64.rs")]
mod field_impl;

use crate::FieldBytes;
use core::{
iter::{Product, Sum},
ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign},
};
use elliptic_curve::ops::Invert;
use elliptic_curve::{
bigint::{ArrayEncoding, U256},
bigint::{ArrayEncoding, U256, U512},
ff::{Field, PrimeField},
rand_core::RngCore,
subtle::{Choice, ConditionallySelectable, ConstantTimeEq, ConstantTimeLess, CtOption},
Expand Down Expand Up @@ -116,171 +117,28 @@ impl FieldElement {

/// Returns self + rhs mod p
pub const fn add(&self, rhs: &Self) -> Self {
let a = u256_to_u64x4(self.0);
let b = u256_to_u64x4(rhs.0);

// Bit 256 of p is set, so addition can result in five words.
let (w0, carry) = adc(a[0], b[0], 0);
let (w1, carry) = adc(a[1], b[1], carry);
let (w2, carry) = adc(a[2], b[2], carry);
let (w3, w4) = adc(a[3], b[3], carry);

// Attempt to subtract the modulus, to ensure the result is in the field.
let modulus = u256_to_u64x4(MODULUS.0);
let (result, _) = Self::sub_inner(
w0, w1, w2, w3, w4, modulus[0], modulus[1], modulus[2], modulus[3], 0,
);
result
Self(field_impl::add(self.0, rhs.0))
}

/// Returns 2*self.
/// Returns 2 * self.
pub const fn double(&self) -> Self {
self.add(self)
}

/// Returns self - rhs mod p
pub const fn sub(&self, rhs: &Self) -> Self {
let a = u256_to_u64x4(self.0);
let b = u256_to_u64x4(rhs.0);
Self::sub_inner(a[0], a[1], a[2], a[3], 0, b[0], b[1], b[2], b[3], 0).0
Self(field_impl::sub(self.0, rhs.0))
}

/// Negate element.
pub const fn neg(&self) -> Self {
Self::sub(&Self::ZERO, self)
}

fn from_bytes_wide(bytes: [u8; 64]) -> Self {
#[allow(clippy::unwrap_used)]
FieldElement::montgomery_reduce(
u64::from_be_bytes(bytes[0..8].try_into().unwrap()),
u64::from_be_bytes(bytes[8..16].try_into().unwrap()),
u64::from_be_bytes(bytes[16..24].try_into().unwrap()),
u64::from_be_bytes(bytes[24..32].try_into().unwrap()),
u64::from_be_bytes(bytes[32..40].try_into().unwrap()),
u64::from_be_bytes(bytes[40..48].try_into().unwrap()),
u64::from_be_bytes(bytes[48..56].try_into().unwrap()),
u64::from_be_bytes(bytes[56..64].try_into().unwrap()),
)
}

#[inline]
#[allow(clippy::too_many_arguments)]
const fn sub_inner(
l0: u64,
l1: u64,
l2: u64,
l3: u64,
l4: u64,
r0: u64,
r1: u64,
r2: u64,
r3: u64,
r4: u64,
) -> (Self, u64) {
let (w0, borrow) = sbb(l0, r0, 0);
let (w1, borrow) = sbb(l1, r1, borrow);
let (w2, borrow) = sbb(l2, r2, borrow);
let (w3, borrow) = sbb(l3, r3, borrow);
let (_, borrow) = sbb(l4, r4, borrow);

// If underflow occurred on the final limb, borrow = 0xfff...fff, otherwise
// borrow = 0x000...000. Thus, we use it as a mask to conditionally add the
// modulus.
let modulus = u256_to_u64x4(MODULUS.0);
let (w0, carry) = adc(w0, modulus[0] & borrow, 0);
let (w1, carry) = adc(w1, modulus[1] & borrow, carry);
let (w2, carry) = adc(w2, modulus[2] & borrow, carry);
let (w3, _) = adc(w3, modulus[3] & borrow, carry);

(Self(u64x4_to_u256([w0, w1, w2, w3])), borrow)
}

/// Montgomery Reduction
///
/// The general algorithm is:
/// ```text
/// A <- input (2n b-limbs)
/// for i in 0..n {
/// k <- A[i] p' mod b
/// A <- A + k p b^i
/// }
/// A <- A / b^n
/// if A >= p {
/// A <- A - p
/// }
/// ```
///
/// For secp256r1, we have the following simplifications:
///
/// - `p'` is 1, so our multiplicand is simply the first limb of the intermediate A.
///
/// - The first limb of p is 2^64 - 1; multiplications by this limb can be simplified
/// to a shift and subtraction:
/// ```text
/// a_i * (2^64 - 1) = a_i * 2^64 - a_i = (a_i << 64) - a_i
/// ```
/// However, because `p' = 1`, the first limb of p is multiplied by limb i of the
/// intermediate A and then immediately added to that same limb, so we simply
/// initialize the carry to limb i of the intermediate.
///
/// - The third limb of p is zero, so we can ignore any multiplications by it and just
/// add the carry.
///
/// References:
/// - Handbook of Applied Cryptography, Chapter 14
/// Algorithm 14.32
/// http://cacr.uwaterloo.ca/hac/about/chap14.pdf
///
/// - Efficient and Secure Elliptic Curve Cryptography Implementation of Curve P-256
/// Algorithm 7) Montgomery Word-by-Word Reduction
/// https://csrc.nist.gov/csrc/media/events/workshop-on-elliptic-curve-cryptography-standards/documents/papers/session6-adalier-mehmet.pdf
#[inline]
#[allow(clippy::too_many_arguments)]
const fn montgomery_reduce(
r0: u64,
r1: u64,
r2: u64,
r3: u64,
r4: u64,
r5: u64,
r6: u64,
r7: u64,
) -> Self {
let modulus = u256_to_u64x4(MODULUS.0);

let (r1, carry) = mac(r1, r0, modulus[1], r0);
let (r2, carry) = adc(r2, 0, carry);
let (r3, carry) = mac(r3, r0, modulus[3], carry);
let (r4, carry2) = adc(r4, 0, carry);

let (r2, carry) = mac(r2, r1, modulus[1], r1);
let (r3, carry) = adc(r3, 0, carry);
let (r4, carry) = mac(r4, r1, modulus[3], carry);
let (r5, carry2) = adc(r5, carry2, carry);

let (r3, carry) = mac(r3, r2, modulus[1], r2);
let (r4, carry) = adc(r4, 0, carry);
let (r5, carry) = mac(r5, r2, modulus[3], carry);
let (r6, carry2) = adc(r6, carry2, carry);

let (r4, carry) = mac(r4, r3, modulus[1], r3);
let (r5, carry) = adc(r5, 0, carry);
let (r6, carry) = mac(r6, r3, modulus[3], carry);
let (r7, r8) = adc(r7, carry2, carry);

// Result may be within MODULUS of the correct value
let (result, _) = Self::sub_inner(
r4, r5, r6, r7, r8, modulus[0], modulus[1], modulus[2], modulus[3], 0,
);
result
}

/// Translate a field element out of the Montgomery domain.
#[inline]
pub(crate) const fn to_canonical(self) -> Self {
let w = u256_to_u64x4(self.0);
FieldElement::montgomery_reduce(w[0], w[1], w[2], w[3], 0, 0, 0, 0)
Self(field_impl::to_canonical(self.0))
}

/// Translate a field element into the Montgomery domain.
Expand All @@ -291,31 +149,8 @@ impl FieldElement {

/// Returns self * rhs mod p
pub const fn multiply(&self, rhs: &Self) -> Self {
// Schoolbook multiplication.
let a = u256_to_u64x4(self.0);
let b = u256_to_u64x4(rhs.0);

let (w0, carry) = mac(0, a[0], b[0], 0);
let (w1, carry) = mac(0, a[0], b[1], carry);
let (w2, carry) = mac(0, a[0], b[2], carry);
let (w3, w4) = mac(0, a[0], b[3], carry);

let (w1, carry) = mac(w1, a[1], b[0], 0);
let (w2, carry) = mac(w2, a[1], b[1], carry);
let (w3, carry) = mac(w3, a[1], b[2], carry);
let (w4, w5) = mac(w4, a[1], b[3], carry);

let (w2, carry) = mac(w2, a[2], b[0], 0);
let (w3, carry) = mac(w3, a[2], b[1], carry);
let (w4, carry) = mac(w4, a[2], b[2], carry);
let (w5, w6) = mac(w5, a[2], b[3], carry);

let (w3, carry) = mac(w3, a[3], b[0], 0);
let (w4, carry) = mac(w4, a[3], b[1], carry);
let (w5, carry) = mac(w5, a[3], b[2], carry);
let (w6, w7) = mac(w6, a[3], b[3], carry);

FieldElement::montgomery_reduce(w0, w1, w2, w3, w4, w5, w6, w7)
let (lo, hi): (U256, U256) = self.0.split_mul(&rhs.0);
Self(field_impl::montgomery_reduce(lo, hi))
}

/// Returns self * self mod p
Expand Down Expand Up @@ -420,7 +255,8 @@ impl Field for FieldElement {
// negligible bias from the uniform distribution.
let mut buf = [0; 64];
rng.fill_bytes(&mut buf);
FieldElement::from_bytes_wide(buf)
let buf = U512::from_be_slice(&buf);
Self(field_impl::from_bytes_wide(buf))
}

#[must_use]
Expand Down Expand Up @@ -666,9 +502,13 @@ impl<'a> Product<&'a FieldElement> for FieldElement {

#[cfg(test)]
mod tests {
use super::{u64x4_to_u256, FieldElement};
use super::FieldElement;
use crate::{test_vectors::field::DBL_TEST_VECTORS, FieldBytes};
use core::ops::Mul;

#[cfg(target_pointer_width = "64")]
use crate::U256;
#[cfg(target_pointer_width = "64")]
use proptest::{num::u64::ANY, prelude::*};

#[test]
Expand Down Expand Up @@ -783,6 +623,7 @@ mod tests {
assert_eq!(four.sqrt().unwrap(), two);
}

#[cfg(target_pointer_width = "64")]
proptest! {
/// This checks behaviour well within the field ranges, because it doesn't set the
/// highest limb.
Expand All @@ -795,8 +636,8 @@ mod tests {
b1 in ANY,
b2 in ANY,
) {
let a = FieldElement(u64x4_to_u256([a0, a1, a2, 0]));
let b = FieldElement(u64x4_to_u256([b0, b1, b2, 0]));
let a = FieldElement(U256::from_words([a0, a1, a2, 0]));
let b = FieldElement(U256::from_words([b0, b1, b2, 0]));
assert_eq!(a.add(&b).sub(&a), b);
}
}
Expand Down
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