fix(bls): fix is_infinity when aggregating onto empty AggregateSignature (#8496)

Co-Authored-By: figtracer <1gusredo@gmail.com>

Co-Authored-By: Michael Sproul <michaelsproul@users.noreply.github.com>

crypto/bls/src/generic_aggregate_signature.rs

@@ -124,13 +124,15 @@ where
     /// Aggregates a signature onto `self`.
     pub fn add_assign(&mut self, other: &GenericSignature<Pub, Sig>) {
         if let Some(other_point) = other.point() {
-            self.is_infinity = self.is_infinity && other.is_infinity;
             if let Some(self_point) = &mut self.point {
-                self_point.add_assign(other_point)
+                self_point.add_assign(other_point);
+                self.is_infinity = self.is_infinity && other.is_infinity;
             } else {
                 let mut self_point = AggSig::infinity();
                 self_point.add_assign(other_point);
-                self.point = Some(self_point)
+                self.point = Some(self_point);
+                // the result is infinity, if `other` is
+                self.is_infinity = other.is_infinity;
             }
         }
     }
@@ -138,13 +140,15 @@ where
     /// Aggregates an aggregate signature onto `self`.
     pub fn add_assign_aggregate(&mut self, other: &Self) {
         if let Some(other_point) = other.point() {
-            self.is_infinity = self.is_infinity && other.is_infinity;
             if let Some(self_point) = &mut self.point {
-                self_point.add_assign_aggregate(other_point)
+                self_point.add_assign_aggregate(other_point);
+                self.is_infinity = self.is_infinity && other.is_infinity;
             } else {
                 let mut self_point = AggSig::infinity();
                 self_point.add_assign_aggregate(other_point);
-                self.point = Some(self_point)
+                self.point = Some(self_point);
+                // the result is infinity, if `other` is
+                self.is_infinity = other.is_infinity;
             }
         }
     }

crypto/bls/tests/tests.rs

@@ -356,6 +356,17 @@ macro_rules! test_suite {
                 .assert_single_message_verify(true)
         }
 
+        #[test]
+        fn empty_aggregate_plus_infinity_should_be_infinity() {
+            let mut agg = AggregateSignature::empty();
+            let infinity_sig = Signature::deserialize(&INFINITY_SIGNATURE).unwrap();
+            agg.add_assign(&infinity_sig);
+            assert!(
+                agg.is_infinity(),
+                "is_infinity flag should be true after adding infinity to empty"
+            );
+        }
+
         #[test]
         fn deserialize_infinity_public_key() {
             PublicKey::deserialize(&bls::INFINITY_PUBLIC_KEY).unwrap_err();