Group trait overhaul part 2 (#53)

* Rely on elliptic-curve for hash-to-curve and P-256 implementations

* Update MSRV

* Remove unnecessary `#[macro_use]`

* Re-introduce `CipherSuite`

* Provide types for length shortcuts

* Remove `SUITE_ID` from `Group`

* Blanket implementation for RustCrypto `Curve`s

* Remove the p256 crate feature

* Rename `ristretto_*` crate features to `ristretto-*` for consistency

* Remove unnecessary allowed Clippy lints

* Remove some unnecessary constraints

src/ciphersuite.rs

@@ -0,0 +1,48 @@
+// 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.
+
+//! Defines the CipherSuite trait to specify the underlying primitives for VOPRF
+
+use digest::core_api::BlockSizeUser;
+use digest::{Digest, OutputSizeUser};
+use elliptic_curve::VoprfParameters;
+use generic_array::typenum::{IsLess, IsLessOrEqual, U256};
+
+use crate::Group;
+
+/// Configures the underlying primitives used in VOPRF
+pub trait CipherSuite
+where
+    <Self::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<Self::Hash as BlockSizeUser>::BlockSize>,
+{
+    /// The ciphersuite identifier as dictated by
+    /// <https://www.ietf.org/archive/id/draft-irtf-cfrg-voprf-08.html>
+    const ID: u16;
+
+    /// A finite cyclic group along with a point representation that allows some
+    /// customization on how to hash an input to a curve point. See [`Group`].
+    type Group: Group;
+
+    /// The main hash function to use (for HKDF computations and hashing
+    /// transcripts).
+    type Hash: BlockSizeUser + Digest;
+}
+
+impl<T: VoprfParameters> CipherSuite for T
+where
+    T: Group,
+    T::Hash: BlockSizeUser + Digest,
+    <T::Hash as OutputSizeUser>::OutputSize:
+        IsLess<U256> + IsLessOrEqual<<T::Hash as BlockSizeUser>::BlockSize>,
+{
+    const ID: u16 = T::ID;
+
+    type Group = T;
+
+    type Hash = T::Hash;
+}