Skip to content

Commit

Permalink
p256: name some magic numbers
Browse files Browse the repository at this point in the history
  • Loading branch information
@ctz
ctz committed Nov 22, 2024
1 parent 1528f11 commit 7cf31df
Showing 1 changed file with 41 additions and 25 deletions.
66 changes: 41 additions & 25 deletions graviola/src/mid/p256.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ use crate::mid::rng::{RandomSource, SystemRandom};
use crate::Error;

use core::fmt;
use core::ops::Range;

mod precomp;

Expand Down Expand Up @@ -102,7 +103,7 @@ impl PrivateKey {
}

/// Return a fixed-length encoding of this private key's value.
pub fn as_bytes(&self) -> [u8; 32] {
pub fn as_bytes(&self) -> [u8; Scalar::BYTES] {
let _ = low::Entry::new_secret();
self.scalar.as_bytes()
}
Expand Down Expand Up @@ -148,7 +149,7 @@ impl PrivateKey {
pub(crate) fn generate(rng: &mut dyn RandomSource) -> Result<Self, Error> {
let _ = low::Entry::new_secret();
for _ in 0..64 {
let mut r = [0u8; 32];
let mut r = [0u8; Scalar::BYTES];
rng.fill(&mut r)?;
if let Ok(p) = Self::from_bytes(&r) {
return Ok(p);
Expand Down Expand Up @@ -178,7 +179,7 @@ impl fmt::Debug for PrivateKey {
}

/// A shared secret output from a P-256 Diffie-Hellman operation.
pub struct SharedSecret(pub [u8; 32]);
pub struct SharedSecret(pub [u8; FieldElement::BYTES]);

impl Drop for SharedSecret {
fn drop(&mut self) {
Expand All @@ -192,19 +193,22 @@ struct AffineMontPoint {
}

impl AffineMontPoint {
const X: Range<usize> = 0..4;
const Y: Range<usize> = 4..8;

fn from_x962_uncompressed(bytes: &[u8]) -> Result<Self, Error> {
match bytes.first() {
Some(&0x04) => (),
Some(_) => return Err(Error::NotUncompressed),
None => return Err(Error::WrongLength),
}

if bytes.len() != 1 + 64 {
if bytes.len() != 1 + FieldElement::BYTES + FieldElement::BYTES {
return Err(Error::WrongLength);
}

let x = &bytes[1..33];
let y = &bytes[33..65];
let (_, xy) = bytes.split_at(1);
let (x, y) = xy.split_at(FieldElement::BYTES);

let point = Self::from_xy(
FieldElement(util::big_endian_slice_to_u64x4(x).unwrap()).as_mont(),
Expand All @@ -220,22 +224,22 @@ impl AffineMontPoint {

fn x_scalar(&self) -> Scalar {
let bytes = self.as_bytes_uncompressed();
Scalar::from_bytes_reduced(&bytes[1..33]).unwrap()
Scalar::from_bytes_reduced(&bytes[1..1 + Scalar::BYTES]).unwrap()
}

fn from_xy(x: FieldElement, y: FieldElement) -> Self {
let mut r = Self::default();
r.xy[0..4].copy_from_slice(&x.0[..]);
r.xy[4..8].copy_from_slice(&y.0[..]);
r.xy[Self::X].copy_from_slice(&x.0[..]);
r.xy[Self::Y].copy_from_slice(&y.0[..]);
r
}

fn x(&self) -> FieldElement {
FieldElement(self.xy[0..4].try_into().unwrap())
FieldElement(self.xy[Self::X].try_into().unwrap())
}

fn y(&self) -> FieldElement {
FieldElement(self.xy[4..8].try_into().unwrap())
FieldElement(self.xy[Self::Y].try_into().unwrap())
}

fn on_curve(&self) -> bool {
Expand All @@ -259,9 +263,12 @@ impl AffineMontPoint {

fn as_bytes_uncompressed(&self) -> [u8; 65] {
let mut r = [0u8; 65];
r[0] = 0x04;
r[1..33].copy_from_slice(&util::u64x4_to_big_endian(&self.x().demont().0));
r[33..65].copy_from_slice(&util::u64x4_to_big_endian(&self.y().demont().0));
let (indicator, xy) = r.split_at_mut(1);
let (x, y) = xy.split_at_mut(FieldElement::BYTES);

indicator[0] = 0x04;
x.copy_from_slice(&util::u64x4_to_big_endian(&self.x().demont().0));
y.copy_from_slice(&util::u64x4_to_big_endian(&self.y().demont().0));
r
}

Expand All @@ -282,14 +289,14 @@ impl AffineMontPoint {

fn negate_y(&mut self) {
let neg_y = self.y().negate_mod_p();
self.xy[4..8].copy_from_slice(&neg_y.0);
self.xy[Self::Y].copy_from_slice(&neg_y.0);
}

fn maybe_negate_y(&mut self, sign: u8) {
let y = self.y();
let neg_y = y.negate_mod_p();
let result = FieldElement::select(&y, &neg_y, sign);
self.xy[4..8].copy_from_slice(&result.0);
self.xy[Self::Y].copy_from_slice(&result.0);
}

/// Precomputes wNAF form (with 𝑤=6) for the point `self`
Expand Down Expand Up @@ -396,6 +403,11 @@ struct JacobianMontPoint {
}

impl JacobianMontPoint {
const X: Range<usize> = 0..4;
const Y: Range<usize> = 4..8;
const XY: Range<usize> = 0..8;
const Z: Range<usize> = 8..12;

fn infinity() -> Self {
Self {
xyz: [1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0],
Expand All @@ -411,19 +423,19 @@ impl JacobianMontPoint {
}

fn x(&self) -> FieldElement {
FieldElement(self.xyz[0..4].try_into().unwrap())
FieldElement(self.xyz[Self::X].try_into().unwrap())
}

fn y(&self) -> FieldElement {
FieldElement(self.xyz[4..8].try_into().unwrap())
FieldElement(self.xyz[Self::Y].try_into().unwrap())
}

fn z(&self) -> FieldElement {
FieldElement(self.xyz[8..12].try_into().unwrap())
FieldElement(self.xyz[Self::Z].try_into().unwrap())
}

fn set_z(&mut self, fe: &FieldElement) {
self.xyz[8..12].copy_from_slice(&fe.0);
self.xyz[Self::Z].copy_from_slice(&fe.0);
}

fn base_multiply(scalar: &Scalar) -> Self {
Expand Down Expand Up @@ -527,8 +539,8 @@ impl JacobianMontPoint {

fn from_affine(p: &AffineMontPoint) -> Self {
let mut xyz: [u64; 12] = Default::default();
xyz[..8].copy_from_slice(&p.xy);
xyz[8..12].copy_from_slice(&CURVE_ONE_MONT.0);
xyz[Self::XY].copy_from_slice(&p.xy);
xyz[Self::Z].copy_from_slice(&CURVE_ONE_MONT.0);
Self { xyz }
}

Expand Down Expand Up @@ -635,21 +647,23 @@ impl JacobianMontPoint {

fn negate_y(&mut self) {
let neg_y = self.y().negate_mod_p();
self.xyz[4..8].copy_from_slice(&neg_y.0);
self.xyz[Self::Y].copy_from_slice(&neg_y.0);
}

fn maybe_negate_y(&mut self, sign: u8) {
let y = self.y();
let neg_y = y.negate_mod_p();
let result = FieldElement::select(&y, &neg_y, sign);
self.xyz[4..8].copy_from_slice(&result.0);
self.xyz[Self::Y].copy_from_slice(&result.0);
}
}

#[derive(Clone, Copy, Debug, Default)]
struct FieldElement([u64; 4]);

impl FieldElement {
const BYTES: usize = 32;

fn inv(&self) -> Self {
let mut r = Self::default();
low::bignum_inv_p256(&mut r.0, &self.0);
Expand Down Expand Up @@ -715,6 +729,8 @@ impl FieldElement {
pub struct Scalar([u64; 4]);

impl Scalar {
const BYTES: usize = 32;

/// Create a scalar from the given slice, which can be any size.
///
/// If it is larger than 32 bytes, the leading bytes must be
Expand Down Expand Up @@ -745,7 +761,7 @@ impl Scalar {
}
}

pub(crate) fn as_bytes(&self) -> [u8; 32] {
pub(crate) fn as_bytes(&self) -> [u8; Self::BYTES] {
util::u64x4_to_big_endian(&self.0)
}

Expand Down

0 comments on commit 7cf31df

Please sign in to comment.