fix(hmac): reject empty imported keys

lib/src/impl_ffi/impl_ffi.hmac.dart

@@ -37,6 +37,7 @@ Future<HmacSecretKeyImpl> hmacSecretKey_importRawKey(
   HashImpl hash, {
   int? length,
 }) async {
+  _checkData(keyData.isNotEmpty, message: 'HMAC key data must not be empty');
   return _HmacSecretKeyImpl(
     _asUint8ListZeroedToBitLength(keyData, length),
     _HashImpl.fromHash(hash),

lib/src/testing/regression/hmac_empty_key.dart

@@ -0,0 +1,49 @@
+// 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: 'HMAC rejects empty raw keys',
+    test: () => _expectEmptyKeyRejected(
+      HmacSecretKey.importRawKey(const [], Hash.sha256),
+    ),
+  ),
+  (
+    name: 'HMAC rejects empty JWK keys',
+    test: () => _expectEmptyKeyRejected(
+      HmacSecretKey.importJsonWebKey(const {
+        'kty': 'oct',
+        'alg': 'HS256',
+        'k': '',
+      }, Hash.sha256),
+    ),
+  ),
+];
+
+Future<void> _expectEmptyKeyRejected(Future<HmacSecretKey> import) async {
+  try {
+    await import;
+  } on FormatException catch (error) {
+    check(
+      error.message == 'HMAC key data must not be empty',
+      'Expected an empty HMAC key error',
+    );
+    return;
+  }
+  check(false, 'Expected an empty HMAC key to be rejected');
+}

lib/src/testing/testing.dart

@@ -34,6 +34,7 @@ import 'regression/aes_gcm_invalid_tag_length.dart'
     as aes_gcm_invalid_tag_length;
 import 'regression/derive_bits_zero_length.dart' as derive_bits_zero_length;
 import 'regression/ecdh_invalid_length.dart' as ecdh_invalid_length;
+import 'regression/hmac_empty_key.dart' as hmac_empty_key;
 import 'regression/issue_60_trailing_bytes.dart' as issue_60_trailing_bytes;
 import 'regression/jwk_base64url.dart' as jwk_base64url;
 import 'regression/rsa_oaep_sha1_jwk_alg.dart' as rsa_oaep_sha1_jwk_alg;
@@ -71,6 +72,7 @@ void runAllTests(
     ...jwk_base64url.tests(),
     ...derive_bits_zero_length.tests(),
     ...ecdh_invalid_length.tests(),
+    ...hmac_empty_key.tests(),
     ...rsa_oaep_sha1_jwk_alg.tests(),
     ...rsa_modulus_length.tests(),
   ];