As a way to roll back the genericity, implement an extension trait of group for password-to-curve hashing,

This supersedes #18.

Co-authored-by: KevinLewi <[email protected]>

src/map_to_curve.rs

@@ -0,0 +1,23 @@
+use crate::group::Group;
+use curve25519_dalek::{edwards::EdwardsPoint, ristretto::RistrettoPoint};
+
+use generic_array::GenericArray;
+use hkdf::Hkdf;
+
+pub trait GroupWithMapToCurve: Group {
+    fn map_to_curve(password: &[u8], pepper: Option<&[u8]>) -> Self;
+}
+
+impl GroupWithMapToCurve for RistrettoPoint {
+    fn map_to_curve(password: &[u8], pepper: Option<&[u8]>) -> Self {
+        let (hashed_input, _) = Hkdf::<sha2::Sha512>::extract(pepper, password);
+        <Self as Group>::hash_to_curve(GenericArray::from_slice(&hashed_input))
+    }
+}
+
+impl GroupWithMapToCurve for EdwardsPoint {
+    fn map_to_curve(password: &[u8], pepper: Option<&[u8]>) -> Self {
+        let (hashed_input, _) = Hkdf::<sha2::Sha256>::extract(pepper, password);
+        <Self as Group>::hash_to_curve(GenericArray::from_slice(&hashed_input))
+    }
+}