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Add PrimeFieldBits support to Scalar #579

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merged 11 commits into from
Sep 20, 2023
Merged

Add PrimeFieldBits support to Scalar #579

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8b63cb2
Add PrimeFieldBits support to Scalar
kayabaNerve Sep 12, 2023
22c0d80
Sane char_le_bits
kayabaNerve Sep 12, 2023
6fe7d8b
Add bits feature
kayabaNerve Sep 12, 2023
dc276b1
Correct bits feature flagging
kayabaNerve Sep 12, 2023
5b6d4de
FieldBits, fmt
kayabaNerve Sep 12, 2023
9f8aec1
fmt, again, due to my fault
kayabaNerve Sep 12, 2023
268da71
Merge branch 'main' into patch-2
kayabaNerve Sep 12, 2023
5abdf53
group-bits feature flag
kayabaNerve Sep 12, 2023
f5c321b
Update CHANGELOG.md
kayabaNerve Sep 19, 2023
4dbf578
Merge branch 'main' into patch-2
rozbb Sep 20, 2023
1d57d14
Use BASEPOINT_ORDER_PRIVATE instead of BASEPOINT_ORDER
rozbb Sep 20, 2023
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18 changes: 17 additions & 1 deletion curve25519-dalek/src/scalar.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,7 +126,7 @@ use cfg_if::cfg_if;

#[cfg(feature = "group")]
use {
group::ff::{Field, FromUniformBytes, PrimeField},
group::ff::{Field, FromUniformBytes, PrimeField, PrimeFieldBits},
rand_core::RngCore,
};

Expand Down Expand Up @@ -1321,6 +1321,22 @@ impl PrimeField for Scalar {
};
}

#[cfg(feature = "group")]
impl PrimeFieldBits for Scalar {
type ReprBits = [u8; 32];

fn to_le_bits(&self) -> FieldBits<Self::ReprBits> {
self.to_repr().into()
}

fn char_le_bits() -> FieldBits<Self::ReprBits> {
let mut bytes = (Scalar::ZERO - Scalar::ONE).to_repr();
bytes[0] += 1;
debug_assert_eq!(DScalar::from_bytes_mod_order(bytes), DScalar::ZERO);
bytes.into()
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I think this should work but haven't tested it

Suggested change
let mut bytes = (Scalar::ZERO - Scalar::ONE).to_repr();
bytes[0] += 1;
debug_assert_eq!(DScalar::from_bytes_mod_order(bytes), DScalar::ZERO);
bytes.into()
crate::constants::BASEPOINT_ORDER.into()

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I'll test it now, thanks.

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@kayabaNerve kayabaNerve Sep 12, 2023 •
edited
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That's a Scalar, not a byte array. Doesn't that break safety to have an unreduced Scalar in system? Especially now that Scalar addition assumes only unreduced Scalars exist? I'd guess it doesn't in practice, as it won't have any shouldn't never be set bits set, yet still sounds like something which should be a [u8; 32].

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to_bytes().into() works, will push that.

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Doesn't that break safety to have an unreduced Scalar in system?

Yeah, seems a bit odd to have it in the public API if it's unreduced /cc @rozbb

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@pinkforest pinkforest Sep 12, 2023 •
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BASEPOINT_ORDER will be marked deprecated from 4.1.1 - this will be BASEPOINT_ORDER_PRIVATE as pub(crate)

}
}

#[cfg(feature = "group")]
impl FromUniformBytes<64> for Scalar {
fn from_uniform_bytes(bytes: &[u8; 64]) -> Self {
Expand Down