Using voprf as a dependency (#248)

* Using voprf as a dependency

* Adding back x25519 and KeGroup

* Addressing comments

src/key_exchange/group/mod.rs

@@ -0,0 +1,39 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+
+//! Includes the KeGroup trait and definitions for the
+//! key exchange groups
+
+use crate::errors::InternalError;
+use generic_array::{ArrayLength, GenericArray};
+use rand::{CryptoRng, RngCore};
+
+/// A group representation for use in the key exchange
+pub trait KeGroup: Sized + Clone {
+    /// Length of the public key
+    type PkLen: ArrayLength<u8> + 'static;
+    /// Length of the secret key
+    type SkLen: ArrayLength<u8> + 'static;
+
+    /// Return a public key from its fixed-length bytes representation
+    fn from_pk_slice(element_bits: &GenericArray<u8, Self::PkLen>) -> Result<Self, InternalError>;
+
+    /// Generate a random secret key
+    fn random_sk<R: RngCore + CryptoRng>(rng: &mut R) -> GenericArray<u8, Self::SkLen>;
+
+    /// Return a public key from its secret key
+    fn public_key(sk: &GenericArray<u8, Self::SkLen>) -> Self;
+
+    /// Serializes `self`
+    fn to_arr(&self) -> GenericArray<u8, Self::PkLen>;
+
+    /// Diffie-Hellman key exchange
+    fn diffie_hellman(&self, sk: &GenericArray<u8, Self::SkLen>) -> GenericArray<u8, Self::PkLen>;
+}
+
+#[cfg(feature = "p256")]
+pub mod p256;
+pub mod ristretto255;
+pub mod x25519;