ed: Make it possible to convert between VerifyingKey and EdwardsPoint (#624)

Adds VerifyingKey::to_edwards and a From conversion

See #623

ed25519-dalek/src/verifying.rs

@@ -505,6 +505,11 @@ impl VerifyingKey {
     pub fn to_montgomery(&self) -> MontgomeryPoint {
         self.point.to_montgomery()
     }
+
+    /// Return this verifying key in Edwards form.
+    pub fn to_edwards(&self) -> EdwardsPoint {
+        self.point
+    }
 }
 
 impl Verifier<ed25519::Signature> for VerifyingKey {
@@ -563,6 +568,12 @@ impl TryFrom<&[u8]> for VerifyingKey {
     }
 }
 
+impl From<VerifyingKey> for EdwardsPoint {
+    fn from(vk: VerifyingKey) -> EdwardsPoint {
+        vk.point
+    }
+}
+
 #[cfg(all(feature = "alloc", feature = "pkcs8"))]
 impl pkcs8::EncodePublicKey for VerifyingKey {
     fn to_public_key_der(&self) -> pkcs8::spki::Result<pkcs8::Document> {

ed25519-dalek/tests/ed25519.rs

@@ -459,6 +459,29 @@ mod integrations {
         assert_eq!(v, "Second public key");
         assert_eq!(m.len(), 2usize);
     }
+
+    #[test]
+    fn montgomery_and_edwards_conversion() {
+        let mut rng = rand::rngs::OsRng;
+        let signing_key = SigningKey::generate(&mut rng);
+        let verifying_key = signing_key.verifying_key();
+
+        let ed = verifying_key.to_edwards();
+
+        // Check that to_edwards and From return same result:
+        assert_eq!(ed, curve25519_dalek::EdwardsPoint::from(verifying_key));
+
+        // The verifying key serialization is simply the compressed Edwards point
+        assert_eq!(verifying_key.to_bytes(), ed.compress().0);
+
+        // Check that modulo sign, to_montgomery().to_edwards() returns the original point
+        let monty = verifying_key.to_montgomery();
+        let via_monty0 = monty.to_edwards(0).unwrap();
+        let via_monty1 = monty.to_edwards(1).unwrap();
+
+        assert!(via_monty0 != via_monty1);
+        assert!(ed == via_monty0 || ed == via_monty1);
+    }
 }
 
 #[cfg(all(test, feature = "serde"))]