Explicitly ban the identity point as an Ethereum Schnorr public key (002)

This doesn't have a well-defined affine representation. k256's behavior,
mapping it to (0, 0), means this would've been rejected anyways (so this isn't
a change of any current behavior), but it's best not to rely on such an
implementation detail.

networks/ethereum/schnorr/src/public_key.rs

@@ -1,5 +1,5 @@
 use subtle::Choice;
-use group::ff::PrimeField;
+use group::{ff::PrimeField, Group};
 use k256::{
   elliptic_curve::{
     ops::Reduce,
@@ -22,6 +22,10 @@ impl PublicKey {
   /// bounds such as parity).
   #[must_use]
   pub fn new(A: ProjectivePoint) -> Option<PublicKey> {
+    if bool::from(A.is_identity()) {
+      None?;
+    }
+
     let affine = A.to_affine();
 
     // Only allow even keys to save a word within Ethereum

networks/ethereum/schnorr/src/tests/public_key.rs

@@ -27,6 +27,11 @@ pub(crate) fn test_key() -> (Scalar, PublicKey) {
   }
 }
 
+#[test]
+fn test_identity_key() {
+  assert!(PublicKey::new(ProjectivePoint::IDENTITY).is_none());
+}
+
 #[test]
 fn test_odd_key() {
   // We generate a valid key to ensure there's not some distinct reason this key is invalid