Add a new constant-time is_zero() check for RsaIdentity

There are some places in the protocol where we have an all-zero RSA
identity that does not truly represent a key, but rather represents
an absent or unknown key.  For these, it's better to use
`RsaIdentity::is_zero` instead of manually checking for a set of
zero bytes: it expresses the intent better, and ensures that the
operation is constant-time.

I am deliberately not introducing a more general IsZero trait here,
or implementing is_zero for anything else: This is the only one we
seem to need right now.  We can generalize it later if we have to.

crates/tor-llcrypto/semver.md

@@ -0,0 +1,2 @@
+MODIFIED: New is_zero API on RsaIdentity.
+

crates/tor-llcrypto/src/pk/rsa.rs

@@ -159,6 +159,17 @@ impl RsaIdentity {
             Ok(()) => Some(RsaIdentity::from(array)),
         }
     }
+
+    /// Return true if this `RsaIdentity` is composed entirely of zero-valued
+    /// bytes.
+    ///
+    /// Such all-zero values should not be used internally, since they are not
+    /// the ID of any valid key.  Instead, they are used in some places in the
+    /// Tor protocols.
+    pub fn is_zero(&self) -> bool {
+        // We do a constant-time comparison to avoid side-channels.
+        self.id.ct_eq(&[0; RSA_ID_LEN]).into()
+    }
 }
 
 impl From<[u8; 20]> for RsaIdentity {

crates/tor-llcrypto/tests/misc.rs

@@ -63,6 +63,26 @@ fn test_wrong_hex_rsa_ids() {
     assert!(RsaIdentity::from_hex("listen carefully, spider of destiny  -FZ").is_none());
 }
 
+#[test]
+fn test_rsa_is_zero() {
+    use ll::pk::rsa::RsaIdentity;
+    assert!(
+        RsaIdentity::from_hex("0000000000000000000000000000000000000000")
+            .unwrap()
+            .is_zero()
+    );
+    assert!(
+        !RsaIdentity::from_hex("000000000000000000000000000000000000000F")
+            .unwrap()
+            .is_zero()
+    );
+    assert!(
+        !RsaIdentity::from_hex("F000000000000000000000000000000000000000")
+            .unwrap()
+            .is_zero()
+    );
+}
+
 // TODO: Proper tests for RSA keys
 
 #[test]