make hash_to_field output an array

Author: carloskiki

hash2curve/src/group_digest.rs

@@ -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};
 
@@ -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))
@@ -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))
     }
@@ -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])
     }
 }

hash2curve/src/hash2field.rs

@@ -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,
 {
     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)
 }