Introducing a trait for key exchange (#20)

src/ciphersuite.rs

@@ -7,6 +7,7 @@
 
 use crate::{
     errors::InternalPakeError,
+    key_exchange::traits::KeyExchange,
     keypair::{Key, KeyPair},
     map_to_curve::GroupWithMapToCurve,
     slow_hash::SlowHash,
@@ -15,6 +16,13 @@ use crate::{
 use rand_core::{CryptoRng, RngCore};
 
 /// Configures the underlying primitives used in OPAQUE
+/// * `Group`: a finite cyclic group along with a point representation, along
+///   with an extension trait PasswordToCurve that allows some customization on
+///   how to hash a password to a curve point. See `group::Group` and
+///   `map_to_curve::GroupWithMapToCurve`.
+/// * `KeyFormat`: a keypair type composed of public and private components
+/// * `KeyExchange`: The key exchange protocol to use in the login step
+/// * `SlowHash`: a slow hashing function, typically used for password hashing
 pub trait CipherSuite {
     /// A finite cyclic group along with a point representation along with
     /// an extension trait PasswordToCurve that allows some customization on
@@ -23,6 +31,8 @@ pub trait CipherSuite {
     type Group: GroupWithMapToCurve;
     /// A keypair type composed of public and private components
     type KeyFormat: KeyPair<Repr = Key> + PartialEq;
+    /// A key exchange protocol
+    type KeyExchange: KeyExchange;
     /// A slow hashing function, typically used for password hashing
     type SlowHash: SlowHash;