P256 implementation (#213)

* Initial P256 implementation

* Add P256 VOPRF test vectors

* Fix implementation

* Forgot `Cargo.toml` and CI

* Fix MSRV

* Fix MSRV tests

* Remove accidental leftovers

* Document constants

* Use included constant time checks

* Add P-256 VOPRF test vector checks

* Remove unnecessary bound on `CipherSuite::Group`

* P-256 improvement and fixes

* Add OPAQUE test vectors for P-256

* Remove fixed key size for 3DH

* Hide P-256 test vectors behind feature flag

* Split `Group` into modules

* Fix `hash_to_scalar`

* Point to source

* Move constants to `once_cell`

Co-authored-by: Kevin Lewi <[email protected]>

src/ciphersuite.rs

@@ -9,7 +9,6 @@ use crate::{
     hash::Hash, key_exchange::traits::KeyExchange, map_to_curve::GroupWithMapToCurve,
     slow_hash::SlowHash,
 };
-use digest::Digest;
 
 /// Configures the underlying primitives used in OPAQUE
 /// * `Group`: a finite cyclic group along with a point representation, along
@@ -24,7 +23,7 @@ pub trait CipherSuite {
     /// 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`.
-    type Group: GroupWithMapToCurve<UniformBytesLen = <Self::Hash as Digest>::OutputSize>;
+    type Group: GroupWithMapToCurve;
     /// A key exchange protocol
     type KeyExchange: KeyExchange<Self::Hash, Self::Group>;
     /// The main hash function use (for HKDF computations and hashing transcripts)