ml-kem+xwing: add KeyInit and KeySizeUser impls (#156)

...to the decapsulator types, which initialize from a seed

Author: tarcieri

Cargo.lock

@@ -12,3 +12,5 @@ debug = true
 
 [patch.crates-io]
 ml-kem = { path = "./ml-kem" }
+
+kem = { git = "https://github.com/RustCrypto/traits" }

Cargo.toml

@@ -1,6 +1,6 @@
 use core::convert::Infallible;
 use core::marker::PhantomData;
-use hybrid_array::typenum::U32;
+use hybrid_array::typenum::{U32, U64};
 use rand_core::{CryptoRng, TryCryptoRng};
 use subtle::{ConditionallySelectable, ConstantTimeEq};
 
@@ -98,6 +98,23 @@ where
     }
 }
 
+impl<P> ::kem::KeySizeUser for DecapsulationKey<P>
+where
+    P: KemParams,
+{
+    type KeySize = U64;
+}
+
+impl<P> ::kem::KeyInit for DecapsulationKey<P>
+where
+    P: KemParams,
+{
+    #[inline]
+    fn new(seed: &Seed) -> Self {
+        Self::from_seed(*seed)
+    }
+}
+
 impl<P> ::kem::Decapsulate<EncodedCiphertext<P>, SharedKey> for DecapsulationKey<P>
 where
     P: KemParams,

ml-kem/src/kem.rs

@@ -29,7 +29,7 @@
 pub use kem::{self, Decapsulate, Encapsulate};
 
 use core::convert::Infallible;
-use ml_kem::array::ArrayN;
+use ml_kem::array::{ArrayN, typenum::consts::U32};
 use ml_kem::{B32, EncodedSizeUser, KemCore, MlKem768, MlKem768Params};
 use rand_core::{CryptoRng, TryCryptoRng};
 #[cfg(feature = "os_rng")]
@@ -145,6 +145,16 @@ impl Decapsulate<Ciphertext, SharedSecret> for DecapsulationKey {
     }
 }
 
+impl ::kem::KeySizeUser for DecapsulationKey {
+    type KeySize = U32;
+}
+
+impl ::kem::KeyInit for DecapsulationKey {
+    fn new(key: &ArrayN<u8, 32>) -> Self {
+        Self { sk: key.0 }
+    }
+}
+
 impl DecapsulationKey {
     /// Generate a new `DecapsulationKey` using `OsRng`.
     #[cfg(feature = "os_rng")]