Implement hash_to_scalar

Author: daxpedda

elliptic-curve/src/hash2curve/group_digest.rs

@@ -1,17 +1,20 @@
 use super::MapToCurve;
 use crate::{
     hash2field::{hash_to_field, ExpandMsg, FromOkm},
-    Result,
+    ProjectiveArithmetic, Result,
 };
 use group::cofactor::CofactorGroup;
 
 /// Adds hashing arbitrary byte sequences to a valid group element
-pub trait GroupDigest {
+pub trait GroupDigest: ProjectiveArithmetic<ProjectivePoint = Self::Output> {
     /// The field element representation for a group value with multiple elements
-    type FieldElement: FromOkm + Default + Copy;
+    type FieldElement: FromOkm
+        + MapToCurve<Output = Self::Output>
+        + Default
+        + Copy
+        + Into<Self::Scalar>;
     /// The resulting group element
-    type Output: CofactorGroup<Subgroup = Self::Output>
-        + MapToCurve<FieldElement = Self::FieldElement, Output = Self::Output>;
+    type Output: CofactorGroup<Subgroup = Self::Output>;
 
     /// Computes the hash to curve routine according to
     /// <https://www.ietf.org/archive/id/draft-irtf-cfrg-hash-to-curve-13.html>
@@ -69,4 +72,13 @@ pub trait GroupDigest {
         let q0 = Self::Output::map_to_curve(u[0]);
         Ok(q0.clear_cofactor())
     }
+
+    /// Computes the hash to field routine according to
+    /// <https://www.ietf.org/archive/id/draft-irtf-cfrg-hash-to-curve-13.html#section-5>
+    /// and returns a scalar.
+    fn hash_to_scalar<X: ExpandMsg>(msg: &[u8], dst: &'static [u8]) -> Result<Self::Scalar> {
+        let mut u = [Self::FieldElement::default()];
+        hash_to_field::<X, _>(msg, dst, &mut u)?;
+        Ok(u[0].into())
+    }
 }

elliptic-curve/src/hash2curve/map2curve.rs

@@ -2,11 +2,9 @@
 /// via a mapping method like Simplified Shallue-van de Woestijne-Ulas
 /// or Elligator
 pub trait MapToCurve {
-    /// The input values representing x and y
-    type FieldElement;
     /// The output point
     type Output;
 
     /// Map a field element into a point
-    fn map_to_curve(u: Self::FieldElement) -> Self::Output;
+    fn map_to_curve(u: Self) -> Self::Output;
 }