SIGMA-I Key Exchange (#378)

* Move `KeGroup` to `KeyExchange::Group`

- Introduce `KeyExchange::Hash`, which separates the OPRF hash from the one used in `KeyExchange`.
- Remove `De/Serialize` requirement on key exchange messages and states, which forced a lot of where bounds on downstream users.
- Rename `KeGroup` to `Group`.
- Replace `D` generic for hash with `H`.

* Use `voprf::derive_key()` directly

* Implement SIGMA-I key exchange

* Improve `KeyExchange` for SIGMA-I and Ed25519

* Implement EdDSA

* Un-qualify some method calls

* SIGMA-I: only include client identity in client mac

* SIGMA-I: include server mac in client signature

* Expose key exchange types in `crate` & move modules

* Implement Ed25519ph

* Document `ed25519` crate feature

* Remove `ristretto255-voprf` crate feature

* Adjust CI crate feature testing

* Fix Rustdoc

* Remove unnecessary generic parameters from SIGMA-I

* Properly mark to-do's with TODO

* Assorted fixes

* SIGMA-I: include context in signature

* SIGMA-I: include identifiers in signature

* Merge `ServerLoginStart/FinishParameters`

* Re-export more necessary types

* More carefully expose types

* Add ECDSA test

* SIGMA-I: share context hashing

* De-duplicate client static public key storage

* Hide `KeyExchange` better

* Use the correct hash in the root documentation

* Bump `derive-where`

* Format documentation examples a bit further

* Add remote OPRF seed documentation

* Rename `deserialize_key_pair` to `deserialize_take_key_pair`

* Add more key tests

* Remove `SharedSecret` trait

* SIGMA-I refactor message API

* Share more implementation between 3DH and SIGMA-I

* Remove unnecessary zero scalar check for Curve25519

* Use correct hash in test

* Add some more TODOs

* Exclude `tests` folder from Cargo publishing

* Enable missing dependencies

* Use right crate for testing Ed25519

* Remove unnecessary `Sized` constraints

* Remove unnecessary `ecdsa` crate features

* Move signature de/serialization to trait methods

* Nit: move import to appropriate location

* Add warning to SIGMA-I

src/errors.rs

@@ -96,9 +96,6 @@ pub enum ProtocolError<T = Infallible> {
     /** This error occurs when the client detects that the server has
     reflected the OPRF value (beta == alpha) */
     ReflectedValueError,
-    /** Identity group element was encountered during deserialization, which is
-    invalid */
-    IdentityGroupElementError,
     /// Custom [`SecretKey`](crate::keypair::PrivateKeySerialization) error type
     Custom(T),
 }
@@ -122,7 +119,6 @@ impl<T: Debug> Debug for ProtocolError<T> {
                 .field("actual_len", actual_len)
                 .finish(),
             Self::ReflectedValueError => f.debug_tuple("ReflectedValueError").finish(),
-            Self::IdentityGroupElementError => f.debug_tuple("IdentityGroupElementError").finish(),
             Self::Custom(custom) => f.debug_tuple("Custom").field(custom).finish(),
         }
     }
@@ -164,41 +160,6 @@ impl ProtocolError {
                 actual_len,
             },
             Self::ReflectedValueError => ProtocolError::ReflectedValueError,
-            Self::IdentityGroupElementError => ProtocolError::IdentityGroupElementError,
         }
     }
 }
-
-pub(crate) mod utils {
-    use super::*;
-
-    pub fn check_slice_size<'a>(
-        slice: &'a [u8],
-        expected_len: usize,
-        arg_name: &'static str,
-    ) -> Result<&'a [u8], ProtocolError> {
-        if slice.len() != expected_len {
-            return Err(ProtocolError::SizeError {
-                name: arg_name,
-                len: expected_len,
-                actual_len: slice.len(),
-            });
-        }
-        Ok(slice)
-    }
-
-    pub fn check_slice_size_atleast<'a>(
-        slice: &'a [u8],
-        expected_len: usize,
-        arg_name: &'static str,
-    ) -> Result<&'a [u8], ProtocolError> {
-        if slice.len() < expected_len {
-            return Err(ProtocolError::SizeError {
-                name: arg_name,
-                len: expected_len,
-                actual_len: slice.len(),
-            });
-        }
-        Ok(slice)
-    }
-}