From 58424cfa4e7d2c5fd8b387ae9f424bfb12f30c13 Mon Sep 17 00:00:00 2001
From: Tony Arcieri <bascule@gmail.com>
Date: Thu, 5 Feb 2026 08:48:36 -0700
Subject: [PATCH] Remove generics from `*Decapsulate`/`Encapsulate`

Companion PR to RustCrypto/traits#2282
---
 Cargo.lock                      |  7 +++----
 Cargo.toml                      |  2 ++
 dhkem/src/ecdh_kem.rs           | 27 +++++++++++++++++++++------
 dhkem/src/lib.rs                | 16 ++++++++--------
 dhkem/src/x25519_kem.rs         | 18 ++++++++++++++----
 dhkem/tests/tests.rs            | 20 +++++++++++---------
 ml-kem/src/decapsulation_key.rs | 18 +++++++++++-------
 ml-kem/src/encapsulation_key.rs |  4 +++-
 ml-kem/src/lib.rs               |  3 +--
 ml-kem/tests/encap-decap.rs     |  7 +++----
 x-wing/src/lib.rs               | 24 ++++++++++++++----------
 11 files changed, 91 insertions(+), 55 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index ef88697..9f3e5bc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -314,9 +314,9 @@ dependencies = [
 
 [[package]]
 name = "crypto-common"
-version = "0.2.0-rc.15"
+version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6f8441110cea75afde0b89a8d796e2bc67b23432f5a9566cb15d9d365d91a2b0"
+checksum = "211f05e03c7d03754740fd9e585de910a095d6b99f8bcfffdef8319fa02a8331"
 dependencies = [
  "getrandom",
  "hybrid-array",
@@ -732,8 +732,7 @@ dependencies = [
 [[package]]
 name = "kem"
 version = "0.3.0-rc.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1c154411ffe1b199ef0b7e209e0cd2f05460aa5b9483873979042bfb02d0addd"
+source = "git+https://github.com/RustCrypto/traits#62a42bccc4d0d3934a0d7e9e737127fab8585e83"
 dependencies = [
  "crypto-common",
  "rand_core",
diff --git a/Cargo.toml b/Cargo.toml
index a5dedb5..2a2fd5f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -58,3 +58,5 @@ debug = true
 [patch.crates-io]
 ml-kem = { path = "./ml-kem" }
 module-lattice = { path = "./module-lattice" }
+
+kem = { git = "https://github.com/RustCrypto/traits" }
diff --git a/dhkem/src/ecdh_kem.rs b/dhkem/src/ecdh_kem.rs
index a220c74..89100a3 100644
--- a/dhkem/src/ecdh_kem.rs
+++ b/dhkem/src/ecdh_kem.rs
@@ -8,8 +8,8 @@ use elliptic_curve::{
     sec1::{FromSec1Point, ModulusSize, ToSec1Point, UncompressedPoint, UncompressedPointSize},
 };
 use kem::{
-    Ciphertext, Encapsulate, Generate, InvalidKey, Kem, KeyExport, KeySizeUser, SharedKey,
-    TryDecapsulate, TryKeyInit,
+    Ciphertext, Decapsulator, Encapsulate, Generate, InvalidKey, Kem, KeyExport, KeySizeUser,
+    SharedKey, TryDecapsulate, TryKeyInit,
 };
 use rand_core::{CryptoRng, TryCryptoRng};
 
@@ -27,8 +27,8 @@ impl<C> Kem for EcdhKem<C>
 where
     C: CurveArithmetic,
     FieldBytesSize<C>: ModulusSize,
-    EcdhDecapsulationKey<C>: TryDecapsulate<Self> + Generate,
-    EcdhEncapsulationKey<C>: Encapsulate<Self> + Clone,
+    EcdhDecapsulationKey<C>: TryDecapsulate<Kem = Self> + Generate,
+    EcdhEncapsulationKey<C>: Encapsulate<Kem = Self> + Clone,
 {
     type DecapsulationKey = EcdhDecapsulationKey<C>;
     type EncapsulationKey = EcdhEncapsulationKey<C>;
@@ -41,6 +41,19 @@ where
 /// Generic around an elliptic curve `C`.
 pub type EcdhDecapsulationKey<C> = DecapsulationKey<SecretKey<C>, PublicKey<C>>;
 
+impl<C> Decapsulator for EcdhDecapsulationKey<C>
+where
+    C: CurveArithmetic,
+    FieldBytesSize<C>: ModulusSize,
+    AffinePoint<C>: FromSec1Point<C> + ToSec1Point<C>,
+{
+    type Kem = EcdhKem<C>;
+
+    fn encapsulation_key(&self) -> &EcdhEncapsulationKey<C> {
+        &self.ek
+    }
+}
+
 impl<C> KeySizeUser for EcdhDecapsulationKey<C>
 where
     C: CurveArithmetic,
@@ -103,7 +116,7 @@ where
 /// To produce something suitable for e.g. symmetric key(s), use the [`Expander`] type to derive
 /// output keys.
 /// </div>
-impl<C> TryDecapsulate<EcdhKem<C>> for EcdhDecapsulationKey<C>
+impl<C> TryDecapsulate for EcdhDecapsulationKey<C>
 where
     C: CurveArithmetic,
     FieldBytesSize<C>: ModulusSize,
@@ -189,12 +202,14 @@ where
 /// To produce something suitable for e.g. symmetric key(s), use the [`Expander`] type to derive
 /// output keys.
 /// </div>
-impl<C> Encapsulate<EcdhKem<C>> for EcdhEncapsulationKey<C>
+impl<C> Encapsulate for EcdhEncapsulationKey<C>
 where
     C: CurveArithmetic,
     FieldBytesSize<C>: ModulusSize,
     AffinePoint<C>: FromSec1Point<C> + ToSec1Point<C>,
 {
+    type Kem = EcdhKem<C>;
+
     fn encapsulate_with_rng<R>(
         &self,
         rng: &mut R,
diff --git a/dhkem/src/lib.rs b/dhkem/src/lib.rs
index b8ef932..4882b76 100644
--- a/dhkem/src/lib.rs
+++ b/dhkem/src/lib.rs
@@ -32,7 +32,7 @@
 mod expander;
 
 pub use expander::{Expander, InvalidLength};
-pub use kem::{self, Encapsulate, Generate, Kem, TryDecapsulate};
+pub use kem::{self, Decapsulator, Encapsulate, Generate, Kem, TryDecapsulate};
 
 #[cfg(feature = "ecdh")]
 mod ecdh_kem;
@@ -62,6 +62,13 @@ pub struct DecapsulationKey<DK, EK> {
     ek: EncapsulationKey<EK>,
 }
 
+impl<DK, EK> DecapsulationKey<DK, EK> {
+    /// Consumes `self` and returns the wrapped value
+    pub fn into_inner(self) -> DK {
+        self.dk
+    }
+}
+
 impl<DK, EK> AsRef<EncapsulationKey<EK>> for DecapsulationKey<DK, EK> {
     fn as_ref(&self) -> &EncapsulationKey<EK> {
         &self.ek
@@ -78,13 +85,6 @@ where
     }
 }
 
-impl<DK, EK> DecapsulationKey<DK, EK> {
-    /// Consumes `self` and returns the wrapped value
-    pub fn into_inner(self) -> DK {
-        self.dk
-    }
-}
-
 /// Newtype for a piece of data that may be encapsulated
 #[derive(Clone, Copy, Debug, Ord, PartialOrd, Eq, PartialEq, Hash, Default)]
 pub struct EncapsulationKey<EK>(EK);
diff --git a/dhkem/src/x25519_kem.rs b/dhkem/src/x25519_kem.rs
index 5c7a684..020c217 100644
--- a/dhkem/src/x25519_kem.rs
+++ b/dhkem/src/x25519_kem.rs
@@ -1,7 +1,7 @@
 use crate::{DecapsulationKey, EncapsulationKey};
 use kem::{
-    Decapsulate, Encapsulate, Generate, InvalidKey, Kem, Key, KeyExport, KeyInit, KeySizeUser,
-    TryKeyInit,
+    Decapsulate, Decapsulator, Encapsulate, Generate, InvalidKey, Kem, Key, KeyExport, KeyInit,
+    KeySizeUser, TryKeyInit,
     common::array::{Array, sizes::U32},
 };
 use rand_core::{CryptoRng, TryCryptoRng};
@@ -34,6 +34,14 @@ impl Kem for X25519Kem {
 /// Generic around an elliptic curve `C`.
 pub type X25519DecapsulationKey = DecapsulationKey<StaticSecret, PublicKey>;
 
+impl Decapsulator for X25519DecapsulationKey {
+    type Kem = X25519Kem;
+
+    fn encapsulation_key(&self) -> &X25519EncapsulationKey {
+        &self.ek
+    }
+}
+
 impl KeySizeUser for X25519DecapsulationKey {
     type KeySize = U32;
 }
@@ -79,7 +87,7 @@ impl Generate for X25519DecapsulationKey {
 /// To produce something suitable for e.g. symmetric key(s), use the [`Expander`] type to derive
 /// output keys.
 /// </div>
-impl Decapsulate<X25519Kem> for X25519DecapsulationKey {
+impl Decapsulate for X25519DecapsulationKey {
     fn decapsulate(&self, encapsulated_key: &Ciphertext) -> SharedKey {
         let public_key = PublicKey::from(encapsulated_key.0);
         self.dk.diffie_hellman(&public_key).to_bytes().into()
@@ -137,7 +145,9 @@ impl KeyExport for X25519EncapsulationKey {
 /// To produce something suitable for e.g. symmetric key(s), use the [`Expander`] type to derive
 /// output keys.
 /// </div>
-impl Encapsulate<X25519Kem> for X25519EncapsulationKey {
+impl Encapsulate for X25519EncapsulationKey {
+    type Kem = X25519Kem;
+
     fn encapsulate_with_rng<R>(&self, rng: &mut R) -> (Ciphertext, SharedKey)
     where
         R: CryptoRng + ?Sized,
diff --git a/dhkem/tests/tests.rs b/dhkem/tests/tests.rs
index cf4f2f2..3ea97b6 100644
--- a/dhkem/tests/tests.rs
+++ b/dhkem/tests/tests.rs
@@ -1,19 +1,21 @@
 //! DHKEM tests.
 
-#![cfg(any(
-    feature = "k256",
-    feature = "p256",
-    feature = "p384",
-    feature = "p521",
-    feature = "x25519"
+#![cfg(all(
+    feature = "getrandom",
+    any(
+        feature = "k256",
+        feature = "p256",
+        feature = "p384",
+        feature = "p521",
+        feature = "x25519"
+    )
 ))]
 #![allow(clippy::unwrap_used, reason = "tests")]
 
-use kem::{Encapsulate, Generate, Kem, TryDecapsulate};
+use kem::{Encapsulate, Kem, TryDecapsulate};
 
 fn test_kem<K: Kem>() {
-    let dk = K::DecapsulationKey::generate();
-    let ek = dk.as_ref().clone();
+    let (dk, ek) = K::generate_keypair();
     let (ek, ss1) = ek.encapsulate();
     let ss2 = dk.try_decapsulate(&ek).unwrap();
     assert_eq!(ss1.as_slice(), ss2.as_slice());
diff --git a/ml-kem/src/decapsulation_key.rs b/ml-kem/src/decapsulation_key.rs
index 30571a3..e3bcf13 100644
--- a/ml-kem/src/decapsulation_key.rs
+++ b/ml-kem/src/decapsulation_key.rs
@@ -1,7 +1,6 @@
 use crate::{
     B32, EncapsulationKey, Seed, SharedKey,
     crypto::{G, J},
-    kem::{Generate, InvalidKey, Kem, KeyExport, KeyInit, KeySizeUser},
     param::{DecapsulationKeySize, ExpandedDecapsulationKey, KemParams},
     pke::{DecryptionKey, EncryptionKey},
 };
@@ -9,7 +8,10 @@ use array::{
     Array, ArraySize,
     sizes::{U32, U64},
 };
-use kem::{Ciphertext, Decapsulate};
+use kem::{
+    Ciphertext, Decapsulate, Decapsulator, Generate, InvalidKey, Kem, KeyExport, KeyInit,
+    KeySizeUser,
+};
 use rand_core::{TryCryptoRng, TryRng};
 use subtle::{ConditionallySelectable, ConstantTimeEq};
 
@@ -145,7 +147,7 @@ where
     }
 }
 
-impl<P> Decapsulate<P> for DecapsulationKey<P>
+impl<P> Decapsulate for DecapsulationKey<P>
 where
     P: Kem<EncapsulationKey = EncapsulationKey<P>, SharedKeySize = U32> + KemParams,
 {
@@ -158,11 +160,13 @@ where
     }
 }
 
-impl<P> AsRef<EncapsulationKey<P>> for DecapsulationKey<P>
+impl<P> Decapsulator for DecapsulationKey<P>
 where
-    P: KemParams,
+    P: Kem<EncapsulationKey = EncapsulationKey<P>, SharedKeySize = U32> + KemParams,
 {
-    fn as_ref(&self) -> &EncapsulationKey<P> {
+    type Kem = P;
+
+    fn encapsulation_key(&self) -> &EncapsulationKey<P> {
         &self.ek
     }
 }
@@ -271,7 +275,7 @@ where
 {
     fn from_seed(seed: &Seed) -> (K::DecapsulationKey, K::EncapsulationKey) {
         let dk = K::DecapsulationKey::new(seed);
-        let ek = dk.as_ref().clone();
+        let ek = dk.encapsulation_key().clone();
         (dk, ek)
     }
 }
diff --git a/ml-kem/src/encapsulation_key.rs b/ml-kem/src/encapsulation_key.rs
index 58ee4e9..f09e246 100644
--- a/ml-kem/src/encapsulation_key.rs
+++ b/ml-kem/src/encapsulation_key.rs
@@ -63,10 +63,12 @@ where
     }
 }
 
-impl<P> Encapsulate<P> for EncapsulationKey<P>
+impl<P> Encapsulate for EncapsulationKey<P>
 where
     P: Kem + KemParams,
 {
+    type Kem = P;
+
     fn encapsulate_with_rng<R>(&self, rng: &mut R) -> (Ciphertext<P>, SharedKey)
     where
         R: CryptoRng + ?Sized,
diff --git a/ml-kem/src/lib.rs b/ml-kem/src/lib.rs
index 6a2dc10..d41bea5 100644
--- a/ml-kem/src/lib.rs
+++ b/ml-kem/src/lib.rs
@@ -278,8 +278,7 @@ mod test {
     where
         K: Kem,
     {
-        let dk = K::DecapsulationKey::generate();
-        let ek = dk.as_ref().clone();
+        let (dk, ek) = K::generate_keypair();
         let (ct, k_send) = ek.encapsulate();
         let k_recv = dk.try_decapsulate(&ct).unwrap();
         assert_eq!(k_send, k_recv);
diff --git a/ml-kem/tests/encap-decap.rs b/ml-kem/tests/encap-decap.rs
index 517d32c..4109f27 100644
--- a/ml-kem/tests/encap-decap.rs
+++ b/ml-kem/tests/encap-decap.rs
@@ -3,8 +3,8 @@
 #![allow(unreachable_pub, reason = "tests")]
 #![allow(clippy::unwrap_used, reason = "tests")]
 
-use ::kem::Decapsulate;
 use array::{Array, ArrayN};
+use kem::{Ciphertext, Decapsulate, Decapsulator};
 use ml_kem::*;
 use std::{fs::read_to_string, path::PathBuf};
 
@@ -92,11 +92,10 @@ fn verify_decap_group(tg: &acvp::DecapTestGroup) {
 fn verify_decap<K>(tc: &acvp::DecapTestCase, dk_slice: &[u8])
 where
     K: Kem,
-    K::DecapsulationKey: Decapsulate<K> + ExpandedKeyEncoding,
+    K::DecapsulationKey: Decapsulate + Decapsulator<Kem = K> + ExpandedKeyEncoding,
 {
     let dk = K::DecapsulationKey::from_expanded_bytes(dk_slice.try_into().unwrap()).unwrap();
-
-    let c = ::kem::Ciphertext::<K>::try_from(tc.c.as_slice()).unwrap();
+    let c = Ciphertext::<K>::try_from(tc.c.as_slice()).unwrap();
     let k = dk.decapsulate(&c);
     assert_eq!(k.as_slice(), tc.k.as_slice());
 }
diff --git a/x-wing/src/lib.rs b/x-wing/src/lib.rs
index 2e5dc38..db4d7c9 100644
--- a/x-wing/src/lib.rs
+++ b/x-wing/src/lib.rs
@@ -29,8 +29,8 @@
 //! ```
 
 pub use kem::{
-    self, Decapsulate, Encapsulate, Generate, InvalidKey, Kem, Key, KeyExport, KeyInit,
-    KeySizeUser, TryKeyInit,
+    self, Decapsulate, Decapsulator, Encapsulate, Generate, InvalidKey, Kem, Key, KeyExport,
+    KeyInit, KeySizeUser, TryKeyInit,
 };
 
 use core::fmt::{self, Debug};
@@ -132,7 +132,9 @@ impl EncapsulationKey {
     }
 }
 
-impl Encapsulate<XWingKem> for EncapsulationKey {
+impl Encapsulate for EncapsulationKey {
+    type Kem = XWingKem;
+
     fn encapsulate_with_rng<R>(&self, rng: &mut R) -> (Ciphertext, SharedKey)
     where
         R: CryptoRng + ?Sized,
@@ -196,12 +198,6 @@ impl DecapsulationKey {
     }
 }
 
-impl AsRef<EncapsulationKey> for DecapsulationKey {
-    fn as_ref(&self) -> &EncapsulationKey {
-        &self.ek
-    }
-}
-
 impl Debug for DecapsulationKey {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("DecapsulationKey")
@@ -210,7 +206,7 @@ impl Debug for DecapsulationKey {
     }
 }
 
-impl Decapsulate<XWingKem> for DecapsulationKey {
+impl Decapsulate for DecapsulationKey {
     #[allow(clippy::similar_names)] // So we can use the names as in the RFC
     fn decapsulate(&self, ct: &Ciphertext) -> SharedKey {
         let ct = CiphertextMessage::from(ct);
@@ -225,6 +221,14 @@ impl Decapsulate<XWingKem> for DecapsulationKey {
     }
 }
 
+impl Decapsulator for DecapsulationKey {
+    type Kem = XWingKem;
+
+    fn encapsulation_key(&self) -> &EncapsulationKey {
+        &self.ek
+    }
+}
+
 impl Drop for DecapsulationKey {
     fn drop(&mut self) {
         #[cfg(feature = "zeroize")]