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hash2curve: make hash_to_field output an array instead of taking an out parameter. #1296

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Merged
merged 3 commits into from
Jul 18, 2025
Merged

hash2curve: make hash_to_field output an array instead of taking an out parameter. #1296

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6f022b9
make hash_to_field output an array
carloskiki Jul 13, 2025
4a0553a
use primitive array instead of `hybrid_array::Array`
carloskiki Jul 18, 2025
c4b4c04
fix: all uses of hash_to_field in other crates
carloskiki Jul 18, 2025
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10 changes: 4 additions & 6 deletions hash2curve/src/group_digest.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
//! Traits for handling hash to curve.

use super::{ExpandMsg, MapToCurve, hash_to_field};
use digest::consts::{U1, U2};
use elliptic_curve::array::typenum::Unsigned;
use elliptic_curve::{ProjectivePoint, Result};

Expand Down Expand Up @@ -36,8 +37,7 @@ pub trait GroupDigest: MapToCurve {
where
X: ExpandMsg<Self::K>,
{
let mut u = [Self::FieldElement::default(), Self::FieldElement::default()];
hash_to_field::<X, _, _>(msg, dst, &mut u)?;
let u = hash_to_field::<X, _, Self::FieldElement, U2>(msg, dst)?;
let q0 = Self::map_to_curve(u[0]);
let q1 = Self::map_to_curve(u[1]);
Ok(Self::add_and_map_to_subgroup(q0, q1))
Expand Down Expand Up @@ -67,8 +67,7 @@ pub trait GroupDigest: MapToCurve {
where
X: ExpandMsg<Self::K>,
{
let mut u = [Self::FieldElement::default()];
hash_to_field::<X, _, _>(msg, dst, &mut u)?;
let u = hash_to_field::<X, _, Self::FieldElement, U1>(msg, dst)?;
let q0 = Self::map_to_curve(u[0]);
Ok(Self::map_to_subgroup(q0))
}
Expand All @@ -91,8 +90,7 @@ pub trait GroupDigest: MapToCurve {
where
X: ExpandMsg<Self::K>,
{
let mut u = [Self::Scalar::default()];
hash_to_field::<X, _, _>(msg, dst, &mut u)?;
let u = hash_to_field::<X, _, Self::Scalar, U1>(msg, dst)?;
Ok(u[0])
}
}
8 changes: 5 additions & 3 deletions hash2curve/src/hash2field.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -38,21 +38,23 @@ pub trait FromOkm {
/// [`ExpandMsgXmd`]: crate::hash2field::ExpandMsgXmd
/// [`ExpandMsgXof`]: crate::hash2field::ExpandMsgXof
#[doc(hidden)]
pub fn hash_to_field<E, K, T>(data: &[&[u8]], domain: &[&[u8]], out: &mut [T]) -> Result<()>
pub fn hash_to_field<E, K, T, C>(data: &[&[u8]], domain: &[&[u8]]) -> Result<Array<T, C>>
where
E: ExpandMsg<K>,
T: FromOkm + Default,
C: ArraySize,
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@daxpedda daxpedda Jul 15, 2025 •
edited
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Actually, if we leave this to ArraySize instead of const C, we could do the following:

Suggested change
T: FromOkm + Default,
C: ArraySize,
T: FromOkm + Default,
C: ArraySize,
T::Length: Mul<C, Output: IsLess<U65536, Output = True>>,

Eliminating one more run-time error.

However propagating this requirement up for users might be quite ugly. So I'm in favor of actually going ahead with the const generic, but thought I'd drop the thought at least.

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Yes this is a good idea, but I don't like the complex trait bounds either. With inline consts we could do something like:

const { if N * T::Length > u16::MAX {
    compile_error!("...");
} };

Although this causes non-local errors and so I did not add it for now.

{
let len_in_bytes = T::Length::USIZE
.checked_mul(out.len())
.checked_mul(C::USIZE)
.and_then(|len| len.try_into().ok())
.and_then(NonZeroU16::new)
.ok_or(Error)?;
let mut tmp = Array::<u8, <T as FromOkm>::Length>::default();
let mut expander = E::expand_message(data, domain, len_in_bytes)?;
let mut out = Array::<T, C>::default();
for o in out.iter_mut() {
expander.fill_bytes(&mut tmp);
*o = T::from_okm(&tmp);
}
Ok(())
Ok(out)
}