Adding a hash type to CipherSuite (#24)

src/hash.rs

@@ -0,0 +1,22 @@
+// 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.
+
+use digest::{BlockInput, FixedOutput, Reset, Update};
+use generic_array::ArrayLength;
+
+/// Trait inheriting the requirements from digest::Digest for compatibility with HKDF and HMAC
+// Associated types could be simplified when they are made as defaults:
+// https://github.com/rust-lang/rust/issues/29661
+pub trait Hash: Update + BlockInput + FixedOutput + Reset + Default + Clone {
+    /// The block size for the hash function
+    type BlockSize: ArrayLength<u8>;
+    /// The output size of the hash function
+    type OutputSize: ArrayLength<u8>;
+}
+
+impl<T: Update + BlockInput + FixedOutput + Reset + Default + Clone> Hash for T {
+    type BlockSize = T::BlockSize;
+    type OutputSize = T::OutputSize;
+}