Implement reused secret keys for Noise protocol.

src/x25519.rs

@@ -95,6 +95,46 @@ impl<'a> From<&'a EphemeralSecret> for PublicKey {
     }
 }
 
+/// A Diffie-Hellman secret key which may be used more than once, but is
+/// purposefully not serialiseable in order to discourage key-reuse.  This is
+/// implemented to facilitate protocols such as Noise (e.g. Noise IK key usage,
+/// etc.) and X3DH which require an "ephemeral" key to conduct the
+/// Diffie-Hellman operation multiple times throughout the protocol, while the
+/// protocol run at a higher level is only conducted once per key.
+///
+/// If you're uncertain about whether you should use this, then you likely
+/// should not be using this.  Our strongly recommended advice is to use
+/// [`EphemeralSecret`] at all times, as that type enforces at compile-time that
+/// secret keys are never reused, which can have very serious security
+/// implications for many protocols.
+#[derive(Zeroize)]
+#[zeroize(drop)]
+pub struct NonSerializeableSecret(pub(crate) Scalar);
+
+impl NonSerializeableSecret {
+    /// Perform a Diffie-Hellman key agreement between `self` and
+    /// `their_public` key to produce a [`SharedSecret`].
+    pub fn diffie_hellman(&self, their_public: &PublicKey) -> SharedSecret {
+        SharedSecret(&self.0 * their_public.0)
+    }
+
+    /// Generate a non-serializeable x25519 key.
+    pub fn new<T: RngCore + CryptoRng>(mut csprng: T) -> Self {
+        let mut bytes = [0u8; 32];
+
+        csprng.fill_bytes(&mut bytes);
+
+        NonSerializeableSecret(clamp_scalar(bytes))
+    }
+}
+
+impl<'a> From<&'a NonSerializeableSecret> for PublicKey {
+    /// Given an x25519 [`NonSerializeableSecret`] key, compute its corresponding [`PublicKey`].
+    fn from(secret: &'a NonSerializeableSecret) -> PublicKey {
+        PublicKey((&ED25519_BASEPOINT_TABLE * &secret.0).to_montgomery())
+    }
+}
+
 /// A Diffie-Hellman secret key that can be used to compute multiple [`SharedSecret`]s.
 ///
 /// This type is identical to the [`EphemeralSecret`] type, except that the