fix(ffi): validate JWK base64url encoding

lib/src/impl_ffi/impl_ffi.utils.dart

@@ -486,7 +486,11 @@ Uint8List _jwkDecodeBase64UrlNoPadding(String unpadded, String prop) {
       unpadded.length + ((4 - (unpadded.length % 4)) % 4),
       '=',
     );
-    return base64Url.decode(padded);
+    final decoded = base64Url.decode(padded);
+    if (_jwkEncodeBase64UrlNoPadding(decoded) != unpadded) {
+      throw const FormatException();
+    }
+    return decoded;
   } on FormatException {
     throw FormatException(
       'JWK property "$prop" is not url-safe base64 without padding',

lib/src/testing/regression/jwk_base64url.dart

@@ -0,0 +1,42 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:webcrypto/webcrypto.dart';
+
+import '../utils/utils.dart';
+
+List<({String name, Future<void> Function() test})> tests() => [
+  (
+    name: 'JWK: rejects padded base64url key data',
+    test: () => _checkInvalidKeyData('AQ=='),
+  ),
+  (
+    name: 'JWK: rejects the standard base64 alphabet',
+    test: () => _checkInvalidKeyData('+w'),
+  ),
+];
+
+Future<void> _checkInvalidKeyData(String keyData) async {
+  var threw = false;
+  try {
+    await HmacSecretKey.importJsonWebKey({
+      'kty': 'oct',
+      'alg': 'HS256',
+      'k': keyData,
+    }, Hash.sha256);
+  } on FormatException {
+    threw = true;
+  }
+  check(threw, 'Should throw FormatException for invalid JWK base64url data');
+}

lib/src/testing/testing.dart

@@ -31,6 +31,7 @@ import 'webcrypto/rsassapkcs1v15.dart' as rsassapkcs1v15;
 import 'webcrypto/random.dart' as random;
 import 'webcrypto/digest.dart' as digest;
 import 'regression/issue_60_trailing_bytes.dart' as issue_60_trailing_bytes;
+import 'regression/jwk_base64url.dart' as jwk_base64url;
 
 /// Test runners from all test files except `digest.dart` and
 /// `random.dart`, which do not use [TestRunner].
@@ -60,6 +61,7 @@ void runAllTests(
     ...random.tests(),
     ...digest.tests(),
     ...issue_60_trailing_bytes.tests(),
+    ...jwk_base64url.tests(),
   ];
 
   for (final (:name, :test) in allTests) {