Group revamp (#261)

* Revamp `KeGroup` trait

* Update dependencies

* Fix `hash_to_scalar` using `OprfGroup` instead of `KeGroup`

* Relax constraints on associated types of `KeGroup`

* Improve `KeGroup` implementation on `Curve`

* Improve `KeyExchange` trait

* Fix new Clippy 1.59 warnings

src/key_exchange/group/elliptic_curve.rs

@@ -0,0 +1,85 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under both the MIT license found in the
+// LICENSE-MIT file in the root directory of this source tree and the Apache
+// License, Version 2.0 found in the LICENSE-APACHE file in the root directory
+// of this source tree.
+
+use digest::core_api::BlockSizeUser;
+use digest::Digest;
+use elliptic_curve::group::cofactor::CofactorGroup;
+use elliptic_curve::hash2curve::{ExpandMsgXmd, FromOkm, GroupDigest};
+use elliptic_curve::sec1::{FromEncodedPoint, ModulusSize, ToEncodedPoint};
+use elliptic_curve::{
+    AffinePoint, Curve, FieldSize, NonZeroScalar, ProjectiveArithmetic, ProjectivePoint, PublicKey,
+    Scalar, SecretKey,
+};
+use generic_array::typenum::{IsLess, IsLessOrEqual, U256};
+use generic_array::GenericArray;
+use rand::{CryptoRng, RngCore};
+
+use super::KeGroup;
+use crate::errors::InternalError;
+
+impl<G: Curve + GroupDigest + ProjectiveArithmetic> KeGroup for G
+where
+    FieldSize<Self>: ModulusSize,
+    AffinePoint<Self>: FromEncodedPoint<Self> + ToEncodedPoint<Self>,
+    ProjectivePoint<Self>: CofactorGroup + ToEncodedPoint<Self>,
+    Scalar<Self>: FromOkm,
+{
+    type Pk = PublicKey<Self>;
+
+    type PkLen = <FieldSize<Self> as ModulusSize>::CompressedPointSize;
+
+    type Sk = SecretKey<Self>;
+
+    type SkLen = FieldSize<Self>;
+
+    fn serialize_pk(pk: &Self::Pk) -> GenericArray<u8, Self::PkLen> {
+        GenericArray::clone_from_slice(pk.to_encoded_point(true).as_bytes())
+    }
+
+    fn deserialize_pk(bytes: &GenericArray<u8, Self::PkLen>) -> Result<Self::Pk, InternalError> {
+        PublicKey::from_sec1_bytes(bytes).map_err(|_| InternalError::PointError)
+    }
+
+    fn random_sk<R: RngCore + CryptoRng>(rng: &mut R) -> Self::Sk {
+        SecretKey::random(rng)
+    }
+
+    // Implements the `HashToScalar()` function
+    fn hash_to_scalar<H>(input: &[&[u8]], dst: &[u8]) -> Result<Self::Sk, InternalError>
+    where
+        H: Digest + BlockSizeUser,
+        H::OutputSize: IsLess<U256> + IsLessOrEqual<H::BlockSize>,
+    {
+        Self::hash_to_scalar::<ExpandMsgXmd<H>>(input, dst)
+            .ok()
+            .and_then(|scalar| Option::<NonZeroScalar<Self>>::from(NonZeroScalar::new(scalar)))
+            .map(SecretKey::from)
+            .ok_or(InternalError::HashToScalar)
+    }
+
+    fn public_key(sk: &Self::Sk) -> Self::Pk {
+        sk.public_key()
+    }
+
+    fn diffie_hellman(pk: &Self::Pk, sk: &Self::Sk) -> GenericArray<u8, Self::PkLen> {
+        GenericArray::clone_from_slice(
+            (pk.to_projective() * sk.to_nonzero_scalar().as_ref())
+                .to_encoded_point(true)
+                .as_bytes(),
+        )
+    }
+
+    fn zeroize_sk_on_drop(_sk: &mut Self::Sk) {}
+
+    fn serialize_sk(sk: &Self::Sk) -> GenericArray<u8, Self::SkLen> {
+        sk.to_be_bytes()
+    }
+
+    fn deserialize_sk(bytes: &GenericArray<u8, Self::SkLen>) -> Result<Self::Sk, InternalError> {
+        SecretKey::from_be_bytes(bytes).map_err(|_| InternalError::PointError)
+    }
+}