From 2043b6f511216627a67c3263fe40ec92441bdff5 Mon Sep 17 00:00:00 2001
From: Harshita Yadav <seemayadavanuj123@gmail.com>
Date: Mon, 13 Jul 2026 20:10:24 +0530
Subject: [PATCH] fix(derive-bits): support zero-length results

---
 lib/src/impl_ffi/impl_ffi.ecdh.dart           |  4 +-
 lib/src/impl_ffi/impl_ffi.pbkdf2.dart         |  8 ++--
 .../regression/derive_bits_zero_length.dart   | 42 +++++++++++++++++++
 lib/src/testing/testing.dart                  |  2 +
 4 files changed, 49 insertions(+), 7 deletions(-)
 create mode 100644 lib/src/testing/regression/derive_bits_zero_length.dart

diff --git a/lib/src/impl_ffi/impl_ffi.ecdh.dart b/lib/src/impl_ffi/impl_ffi.ecdh.dart
index 06cbf53c1..f7a3c0eae 100644
--- a/lib/src/impl_ffi/impl_ffi.ecdh.dart
+++ b/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 {
diff --git a/lib/src/impl_ffi/impl_ffi.pbkdf2.dart b/lib/src/impl_ffi/impl_ffi.pbkdf2.dart
index 0046cfc0e..25dd53933 100644
--- a/lib/src/impl_ffi/impl_ffi.pbkdf2.dart
+++ b/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;
 
diff --git a/lib/src/testing/regression/derive_bits_zero_length.dart b/lib/src/testing/regression/derive_bits_zero_length.dart
new file mode 100644
index 000000000..2da6d3139
--- /dev/null
+++ b/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');
+    },
+  ),
+];
diff --git a/lib/src/testing/testing.dart b/lib/src/testing/testing.dart
index 3c7b4fd26..075a73520 100644
--- a/lib/src/testing/testing.dart
+++ b/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) {