Ensure immutability of MuSigPublicKeys.pubKeys set

Author: MatthewLM

coinlib/lib/src/musig/keys.dart

@@ -10,7 +10,8 @@ class MuSigPublicKeys {
 
   late final OpaqueMuSigCache _aggCache;
 
-  MuSigPublicKeys._(this.pubKeys, this.aggregate, this._aggCache);
+  MuSigPublicKeys._(Set<ECPublicKey> pubKeys, this.aggregate, this._aggCache)
+    : pubKeys = Set.unmodifiable(pubKeys);
 
   factory MuSigPublicKeys(Set<ECPublicKey> pubKeys) {
 

coinlib/test/musig/keys_test.dart

@@ -21,6 +21,11 @@ void main() {
       () => expect(() => MuSigPublicKeys({}), throwsArgumentError),
     );
 
+    test("pubKeys is immutable", () {
+      final keySet = getMuSigKeys().pubKeys;
+      expect(() => keySet.remove(keySet.first), throwsA(anything));
+    });
+
     test("aggregation works regardless of format", () {
 
       final compressed = getMuSigKeys(true).aggregate;