fix(hmac): use block size for default key length

CHANGELOG.md

@@ -1,6 +1,7 @@
 # 0.6.2-wip
 * Fixed JS interop to enable WebAssembly.
 * Fixed native RSA public-key JWK imports to reject private key material.
+* Fixed native HMAC key generation to use the hash block size by default.
 
 # 0.6.1
 * Added Dart native build hooks and native asset lookup for the bundled

lib/src/impl_ffi/impl_ffi.digest.dart

@@ -72,6 +72,11 @@ abstract class _HashImpl implements HashImpl {
   /// https://www.iana.org/assignments/jose/jose.xhtml
   String get hmacJwkAlg;
 
+  /// Recommended HMAC key length in bits.
+  ///
+  /// Web Crypto defines this as the block size of the hash function.
+  int get hmacBlockSizeInBits;
+
   /// Algorithm (`alg` for JWK) when this hash algorithm is used in RSA-OAEP.
   ///
   /// For SHA-1, it returns 'RSA-OAEP'.
@@ -112,6 +117,9 @@ final class _Sha1 extends _HashImpl {
   @override
   String get hmacJwkAlg => 'HS1';
 
+  @override
+  int get hmacBlockSizeInBits => 512;
+
   @override
   String get rsaOaepJwkAlg => 'RSA-OAEP';
 
@@ -131,6 +139,9 @@ final class _Sha256 extends _HashImpl {
   @override
   String get hmacJwkAlg => 'HS256';
 
+  @override
+  int get hmacBlockSizeInBits => 512;
+
   @override
   String get rsaOaepJwkAlg => 'RSA-OAEP-256';
 
@@ -150,6 +161,9 @@ final class _Sha384 extends _HashImpl {
   @override
   String get hmacJwkAlg => 'HS384';
 
+  @override
+  int get hmacBlockSizeInBits => 1024;
+
   @override
   String get rsaOaepJwkAlg => 'RSA-OAEP-384';
 
@@ -169,6 +183,9 @@ final class _Sha512 extends _HashImpl {
   @override
   String get hmacJwkAlg => 'HS512';
 
+  @override
+  int get hmacBlockSizeInBits => 1024;
+
   @override
   String get rsaOaepJwkAlg => 'RSA-OAEP-512';
 

lib/src/impl_ffi/impl_ffi.hmac.dart

@@ -74,7 +74,7 @@ Future<HmacSecretKeyImpl> hmacSecretKey_generateKey(
   int? length,
 }) async {
   final h = _HashImpl.fromHash(hash);
-  length ??= ssl.EVP_MD_size(h._md) * 8;
+  length ??= h.hmacBlockSizeInBits;
   final keyData = Uint8List((length / 8).ceil());
   fillRandomBytes(keyData);
 
@@ -103,7 +103,7 @@ final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
   }
 
   @override
-  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length = 32}) {
+  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length}) {
     return hmacSecretKey_generateKey(hash, length: length);
   }
 }

lib/src/impl_js/impl_js.hmac.dart

@@ -102,7 +102,7 @@ final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
   }
 
   @override
-  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length = 32}) {
+  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length}) {
     return hmacSecretKey_generateKey(hash, length: length);
   }
 }

lib/src/impl_stub/impl_stub.hmac.dart

@@ -37,7 +37,7 @@ final class _StaticHmacSecretKeyImpl implements StaticHmacSecretKeyImpl {
   }
 
   @override
-  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length = 32}) {
+  Future<HmacSecretKeyImpl> generateKey(HashImpl hash, {int? length}) {
     throw UnimplementedError('Not implemented');
   }
 }

lib/src/webcrypto/webcrypto.hmac.dart

@@ -181,8 +181,9 @@ final class HmacSecretKey {
   /// Generate random [HmacSecretKey].
   ///
   /// The [length] specifies the length of the secret key in bits. If omitted
-  /// the random key will use the same number of bits as the underlying hash
-  /// algorithm given in [hash].
+  /// the random key will use the block size of the underlying hash algorithm
+  /// given in [hash] (512 bits for SHA-1 and SHA-256, and 1024 bits for
+  /// SHA-384 and SHA-512).
   ///
   /// **Example**
   /// ```dart

test/hmac_default_key_length_test.dart

@@ -0,0 +1,39 @@
+// 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:test/test.dart';
+import 'package:webcrypto/webcrypto.dart';
+
+void main() {
+  group('HMAC default key length', () {
+    for (final testCase in [
+      (name: 'SHA-1', hash: Hash.sha1, expectedBits: 512),
+      (name: 'SHA-256', hash: Hash.sha256, expectedBits: 512),
+      (name: 'SHA-384', hash: Hash.sha384, expectedBits: 1024),
+      (name: 'SHA-512', hash: Hash.sha512, expectedBits: 1024),
+    ]) {
+      test(testCase.name, () async {
+        final key = await HmacSecretKey.generateKey(testCase.hash);
+
+        expect(await key.exportRawKey(), hasLength(testCase.expectedBits ~/ 8));
+      });
+    }
+
+    test('explicit length takes precedence', () async {
+      final key = await HmacSecretKey.generateKey(Hash.sha512, length: 256);
+
+      expect(await key.exportRawKey(), hasLength(32));
+    });
+  });
+}