fix(derive-bits): support zero-length results

lib/src/impl_ffi/impl_ffi.ecdh.dart

@@ -111,8 +111,8 @@ final class _EcdhPrivateKeyImpl implements EcdhPrivateKeyImpl {
         'custom implementations of EcdhPublicKey is not supported',
       );
     }
-    if (length <= 0) {
-      throw ArgumentError.value(length, 'length', 'must be positive');
+    if (length < 0) {
+      throw ArgumentError.value(length, 'length', 'must be non-negative');
     }
 
     return _Scope.async((scope) async {

lib/src/impl_ffi/impl_ffi.pbkdf2.dart

@@ -60,16 +60,14 @@ final class _Pbkdf2SecretKeyImpl implements Pbkdf2SecretKeyImpl {
         'The length for PBKDF2 must be a multiple of 8 bits',
       );
     }
-    if (length == 0) {
-      throw operationError(
-        'A length of zero is not allowed Pbkdf2SecretKey.deriveBits',
-      );
-    }
     if (iterations <= 0) {
       throw operationError(
         'Iterations <= 0 is not allowed for Pbkdf2SecretKey.deriveBits',
       );
     }
+    if (length == 0) {
+      return Uint8List(0);
+    }
 
     final lengthInBytes = length ~/ 8;
 

lib/src/testing/regression/derive_bits_zero_length.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 'dart:typed_data';
+
+import 'package:webcrypto/webcrypto.dart';
+import '../utils/utils.dart';
+
+List<({String name, Future<void> Function() test})> tests() => [
+  (
+    name: 'ECDH derives an empty secret when length is zero',
+    test: () async {
+      final alice = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+      final bob = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+
+      final secret = await alice.privateKey.deriveBits(0, bob.publicKey);
+
+      check(secret.isEmpty, 'Expected an empty ECDH secret');
+    },
+  ),
+  (
+    name: 'PBKDF2 derives an empty secret when length is zero',
+    test: () async {
+      final key = await Pbkdf2SecretKey.importRawKey(Uint8List(16));
+
+      final secret = await key.deriveBits(0, Hash.sha256, Uint8List(16), 1);
+
+      check(secret.isEmpty, 'Expected an empty PBKDF2 secret');
+    },
+  ),
+];

lib/src/testing/testing.dart

@@ -30,6 +30,7 @@ import 'webcrypto/rsassapkcs1v15.dart' as rsassapkcs1v15;
 // Other test files, that don't use TestRunner
 import 'webcrypto/random.dart' as random;
 import 'webcrypto/digest.dart' as digest;
+import 'regression/derive_bits_zero_length.dart' as derive_bits_zero_length;
 import 'regression/issue_60_trailing_bytes.dart' as issue_60_trailing_bytes;
 
 /// Test runners from all test files except `digest.dart` and
@@ -60,6 +61,7 @@ void runAllTests(
     ...random.tests(),
     ...digest.tests(),
     ...issue_60_trailing_bytes.tests(),
+    ...derive_bits_zero_length.tests(),
   ];
 
   for (final (:name, :test) in allTests) {