Handle WebAuthn assertions without user handles

Sources/Panels/BrowserWebAuthnSupport.swift

@@ -1179,6 +1179,39 @@ extension BrowserWebAuthnCoordinator: ASAuthorizationControllerDelegate, ASAutho
     }
 }
 
+@MainActor
+extension BrowserWebAuthnCoordinator {
+    func assertionReply(
+        credentialID: Data,
+        clientDataJSON: Data,
+        authenticatorData: Data,
+        signature: Data,
+        userHandle: Data?,
+        attachment: String,
+        clientExtensionResults: [String: Any]
+    ) -> [String: Any] {
+        var response: [String: Any] = [
+            "clientDataJSON": clientDataJSON.base64URLEncodedString(),
+            "authenticatorData": authenticatorData.base64URLEncodedString(),
+            "signature": signature.base64URLEncodedString(),
+        ]
+
+        if let userHandle, !userHandle.isEmpty {
+            response["userHandle"] = userHandle.base64URLEncodedString()
+        }
+
+        return [
+            "type": "public-key",
+            "id": credentialID.base64URLEncodedString(),
+            "rawId": credentialID.base64URLEncodedString(),
+            "authenticatorAttachment": attachment,
+            "responseKind": "assertion",
+            "response": response,
+            "clientExtensionResults": clientExtensionResults,
+        ]
+    }
+}
+
 @MainActor
 private extension BrowserWebAuthnCoordinator {
     enum BrowserWebAuthnAuthorizationErrorCode {
@@ -1695,36 +1728,6 @@ private extension BrowserWebAuthnCoordinator {
         return credential
     }
 
-    func assertionReply(
-        credentialID: Data,
-        clientDataJSON: Data,
-        authenticatorData: Data,
-        signature: Data,
-        userHandle: Data,
-        attachment: String,
-        clientExtensionResults: [String: Any]
-    ) -> [String: Any] {
-        var response: [String: Any] = [
-            "clientDataJSON": clientDataJSON.base64URLEncodedString(),
-            "authenticatorData": authenticatorData.base64URLEncodedString(),
-            "signature": signature.base64URLEncodedString(),
-        ]
-
-        if !userHandle.isEmpty {
-            response["userHandle"] = userHandle.base64URLEncodedString()
-        }
-
-        return [
-            "type": "public-key",
-            "id": credentialID.base64URLEncodedString(),
-            "rawId": credentialID.base64URLEncodedString(),
-            "authenticatorAttachment": attachment,
-            "responseKind": "assertion",
-            "response": response,
-            "clientExtensionResults": clientExtensionResults,
-        ]
-    }
-
     func securityKeyTransportValues(
         from registration: ASAuthorizationSecurityKeyPublicKeyCredentialRegistration
     ) -> [String] {

cmuxTests/BrowserWebContentProcessTests.swift

@@ -1032,6 +1032,25 @@ struct BrowserWebContentProcessTests {
         #expect(probe.receivedKinds == ["getCredential"])
     }
 
+    @Test(arguments: [nil, Data()] as [Data?])
+    func webAuthnAssertionReplyOmitsAbsentUserHandle(userHandle: Data?) throws {
+        let coordinator = BrowserWebAuthnCoordinator()
+        let credential = coordinator.assertionReply(
+            credentialID: Data([1, 2, 3]),
+            clientDataJSON: Data([4, 5]),
+            authenticatorData: Data([6, 7]),
+            signature: Data([8, 9]),
+            userHandle: userHandle,
+            attachment: "cross-platform",
+            clientExtensionResults: [:]
+        )
+        let response = try #require(credential["response"] as? [String: Any])
+
+        #expect(credential["id"] as? String == "AQID")
+        #expect(response["signature"] as? String == "CAk")
+        #expect(response["userHandle"] == nil)
+    }
+
     @Test
     func webAuthnNativeBridgeScopesParentDomainRelyingPartyIDs() throws {
         let googleOrigin = try #require(