Replace Deref impl on RsaPrivateKey with AsRef (#317)

The `RsaPrivateKey` type previously had a `Deref` impl providing access
to the associated `RsaPublicKey`.

`Deref` is intended for "smart pointer types", i.e. container types
which manage a (typically generic) inner type in some way. This doesn't
seem like one of those cases.

`AsRef`, on the other hand, is for cheap reference conversions, which is
exactly what's happening here, so it's a better fit and provides the
same functionality (albeit explicitly rather than via deref coercion).

src/key.rs

@@ -1,8 +1,5 @@
 use alloc::vec::Vec;
-use core::{
-    hash::{Hash, Hasher},
-    ops::Deref,
-};
+use core::hash::{Hash, Hasher};
 use num_bigint::traits::ModInverse;
 use num_bigint::Sign::Plus;
 use num_bigint::{BigInt, BigUint};
@@ -57,6 +54,12 @@ impl PartialEq for RsaPrivateKey {
     }
 }
 
+impl AsRef<RsaPublicKey> for RsaPrivateKey {
+    fn as_ref(&self) -> &RsaPublicKey {
+        &self.pubkey_components
+    }
+}
+
 impl Hash for RsaPrivateKey {
     fn hash<H: Hasher>(&self, state: &mut H) {
         // Domain separator for RSA private keys
@@ -73,13 +76,6 @@ impl Drop for RsaPrivateKey {
     }
 }
 
-impl Deref for RsaPrivateKey {
-    type Target = RsaPublicKey;
-    fn deref(&self) -> &RsaPublicKey {
-        &self.pubkey_components
-    }
-}
-
 impl ZeroizeOnDrop for RsaPrivateKey {}
 
 #[derive(Debug, Clone)]
@@ -124,9 +120,8 @@ impl From<RsaPrivateKey> for RsaPublicKey {
 
 impl From<&RsaPrivateKey> for RsaPublicKey {
     fn from(private_key: &RsaPrivateKey) -> Self {
-        let n = private_key.n.clone();
-        let e = private_key.e.clone();
-
+        let n = private_key.n().clone();
+        let e = private_key.e().clone();
         RsaPublicKey { n, e }
     }
 }
@@ -201,11 +196,11 @@ impl RsaPublicKey {
 
 impl PublicKeyParts for RsaPrivateKey {
     fn n(&self) -> &BigUint {
-        &self.n
+        &self.pubkey_components.n
     }
 
     fn e(&self) -> &BigUint {
-        &self.e
+        &self.pubkey_components.e
     }
 }
 
@@ -336,7 +331,7 @@ impl RsaPrivateKey {
             }
             m *= prime;
         }
-        if m != self.n {
+        if m != self.pubkey_components.n {
             return Err(Error::InvalidModulus);
         }
 
@@ -345,7 +340,7 @@ impl RsaPrivateKey {
         // inverse. Therefore e is coprime to lcm(p-1,q-1,r-1,...) =
         // exponent(ℤ/nℤ). It also implies that a^de ≡ a mod p as a^(p-1) ≡ 1
         // mod p. Thus a^de ≡ a mod n for all a coprime to n, as required.
-        let mut de = self.e.clone();
+        let mut de = self.e().clone();
         de *= self.d.clone();
         for prime in &self.primes {
             let congruence: BigUint = &de % (prime - BigUint::one());

src/pss.rs

@@ -405,6 +405,7 @@ mod test {
                 .expect("failed to sign");
 
             priv_key
+                .to_public_key()
                 .verify(Pss::new::<Sha1>(), &digest, &sig)
                 .expect("failed to verify");
         }
@@ -424,6 +425,7 @@ mod test {
                 .expect("failed to sign");
 
             priv_key
+                .to_public_key()
                 .verify(Pss::new::<Sha1>(), &digest, &sig)
                 .expect("failed to verify");
         }
@@ -595,6 +597,7 @@ mod test {
             .expect("failed to sign");
 
         priv_key
+            .to_public_key()
             .verify(Pss::new::<Sha1>(), &digest, &sig)
             .expect("failed to verify");
     }