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Increase constant-time properties of from_repr/from_bytes

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It's still not perfect, as it's Option -> CtOption which requires an 
unwrap_or, but...
This commit is contained in:
Luke Parker 2022-07-08 15:30:56 -04:00
parent a4c2f71610
commit 41eaa1b124
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GPG key ID: F9F1386DB1E119B6
1 changed files with 16 additions and 12 deletions
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28
crypto/dalek-ff-group/src/lib.rs
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@ -32,6 +32,12 @@ use dalek::{
use ff::{Field, PrimeField, FieldBits, PrimeFieldBits};
use group::{Group, GroupEncoding, prime::PrimeGroup};
fn choice(value: bool) -> Choice {
let bit = value as u8;
debug_assert_eq!(bit | 1, 1);
Choice::from(bit)
}
macro_rules! deref_borrow {
($Source: ident, $Target: ident) => {
impl Deref for $Source {
@ -160,7 +166,7 @@ impl Field for Scalar {
fn square(&self) -> Self { *self * self }
fn double(&self) -> Self { *self + self }
fn invert(&self) -> CtOption<Self> {
CtOption::new(Self(self.0.invert()), Choice::from(1 as u8))
CtOption::new(Self(self.0.invert()), self.is_zero())
}
fn sqrt(&self) -> CtOption<Self> { unimplemented!() }
fn is_zero(&self) -> Choice { self.0.ct_eq(&DScalar::zero()) }
@ -177,11 +183,9 @@ impl PrimeField for Scalar {
const NUM_BITS: u32 = 253;
const CAPACITY: u32 = 252;
fn from_repr(bytes: [u8; 32]) -> CtOption<Self> {
let scalar = DScalar::from_canonical_bytes(bytes).map(|x| Scalar(x));
CtOption::new(
scalar.unwrap_or(Scalar::zero()),
Choice::from(if scalar.is_some() { 1 } else { 0 })
)
let scalar = DScalar::from_canonical_bytes(bytes);
// TODO: This unwrap_or isn't constant time, yet do we have an alternative?
CtOption::new(Scalar(scalar.unwrap_or(DScalar::zero())), choice(scalar.is_some()))
}
fn to_repr(&self) -> [u8; 32] { self.0.to_bytes() }
@ -237,6 +241,8 @@ macro_rules! dalek_group {
impl Group for $Point {
type Scalar = Scalar;
// Ideally, this would be cryptographically secure, yet that's not a bound on the trait
// k256 also does this
fn random(rng: impl RngCore) -> Self { &$BASEPOINT_TABLE * Scalar::random(rng) }
fn identity() -> Self { Self($DPoint::identity()) }
fn generator() -> Self { $BASEPOINT_POINT }
@ -248,12 +254,10 @@ macro_rules! dalek_group {
type Repr = [u8; 32];
fn from_bytes(bytes: &Self::Repr) -> CtOption<Self> {
if let Some(point) = $DCompressed(*bytes).decompress() {
if $torsion_free(point) {
return CtOption::new($Point(point), Choice::from(1));
}
}
CtOption::new($Point::identity(), Choice::from(0))
let decompressed = $DCompressed(*bytes).decompress();
// TODO: Same note on unwrap_or as above
let point = decompressed.unwrap_or($DPoint::identity());
CtOption::new($Point(point), choice(decompressed.is_some()) & choice($torsion_free(point)))
}
fn from_bytes_unchecked(bytes: &Self::Repr) -> CtOption<Self> {