From e72aaadac2f64693d8fedfbf6a45cb2f392ac789 Mon Sep 17 00:00:00 2001
From: Harshita Yadav <seemayadavanuj123@gmail.com>
Date: Sat, 7 Feb 2026 15:45:15 +0530
Subject: [PATCH 1/2] Document and test maximum deriveBits length for ECDH
 curves

---
 lib/src/webcrypto/webcrypto.ecdh.dart |  4 ++
 test/crypto_subtle_test.dart          | 67 +++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/lib/src/webcrypto/webcrypto.ecdh.dart b/lib/src/webcrypto/webcrypto.ecdh.dart
index 6a7770c2..bf55f3d5 100644
--- a/lib/src/webcrypto/webcrypto.ecdh.dart
+++ b/lib/src/webcrypto/webcrypto.ecdh.dart
@@ -206,6 +206,10 @@ final class EcdhPrivateKey {
   /// two parties.
   ///
   /// [length] specifies the length of the derived secret in bits.
+  /// The maximum length that can be derived depends on the elliptic curve:
+  ///  * [EllipticCurve.p256] can derive up to 256 bits.
+  ///  * [EllipticCurve.p384] can derive up to 384 bits.
+  ///  * [EllipticCurve.p521] can derive up to 528 bits.
   /// [publicKey] is [EcdhPublicKey] from the other party's ECDH key pair.
   ///
   /// Returns a [Uint8List] containing the derived shared secret.
diff --git a/test/crypto_subtle_test.dart b/test/crypto_subtle_test.dart
index 3c114a08..f02dcee5 100644
--- a/test/crypto_subtle_test.dart
+++ b/test/crypto_subtle_test.dart
@@ -310,6 +310,73 @@ void main() {
       );
     });
   });
+  group('ECDH deriveBits', () {
+    test('P-256 allows maximum deriveBits length', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+
+      final secret = await aliceKeyPair.privateKey.deriveBits(
+        256,
+        bobKeyPair.publicKey,
+      );
+
+      expect(secret.length, equals(32));
+    });
+
+    test('P-256 rejects deriveBits larger than maximum', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p256);
+
+      expect(
+        aliceKeyPair.privateKey.deriveBits(257, bobKeyPair.publicKey),
+        throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
+      );
+    });
+
+    test('P-384 allows maximum deriveBits length', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
+
+      final secret = await aliceKeyPair.privateKey.deriveBits(
+        384,
+        bobKeyPair.publicKey,
+      );
+
+      expect(secret.length, equals(48));
+    });
+
+    test('P-384 rejects deriveBits larger than maximum', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p384);
+
+      expect(
+        aliceKeyPair.privateKey.deriveBits(385, bobKeyPair.publicKey),
+        throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
+      );
+    });
+
+    test('P-521 allows maximum deriveBits length', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
+
+      final secret = await aliceKeyPair.privateKey.deriveBits(
+        528,
+        bobKeyPair.publicKey,
+      );
+
+      expect(secret.length, equals(66));
+    });
+
+    test('P-521 rejects deriveBits larger than maximum', () async {
+      final aliceKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
+      final bobKeyPair = await EcdhPrivateKey.generateKey(EllipticCurve.p521);
+
+      expect(
+        aliceKeyPair.privateKey.deriveBits(529, bobKeyPair.publicKey),
+        throwsA(anyOf(isA<subtle.JSDomException>(), isA<Error>())),
+      );
+    });
+  });
 }
 
 extension on JSArray<JSString> {

From c3f82313b3b77f2a109fc2f3c911d492f7df202c Mon Sep 17 00:00:00 2001
From: Harshita Yadav <seemayadavanuj123@gmail.com>
Date: Wed, 4 Mar 2026 23:20:56 +0530
Subject: [PATCH 2/2] Update lib/src/webcrypto/webcrypto.ecdh.dart

Co-authored-by: HamdaanAliQuatil <96776914+HamdaanAliQuatil@users.noreply.github.com>
---
 lib/src/webcrypto/webcrypto.ecdh.dart | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/src/webcrypto/webcrypto.ecdh.dart b/lib/src/webcrypto/webcrypto.ecdh.dart
index bf55f3d5..7aba31e0 100644
--- a/lib/src/webcrypto/webcrypto.ecdh.dart
+++ b/lib/src/webcrypto/webcrypto.ecdh.dart
@@ -206,7 +206,7 @@ final class EcdhPrivateKey {
   /// two parties.
   ///
   /// [length] specifies the length of the derived secret in bits.
-  /// The maximum length that can be derived depends on the elliptic curve:
+  /// The maximum allowed [length] is determined by the elliptic curve:
   ///  * [EllipticCurve.p256] can derive up to 256 bits.
   ///  * [EllipticCurve.p384] can derive up to 384 bits.
   ///  * [EllipticCurve.p521] can derive up to 528 bits.