fix(aes-cbc): normalize invalid IV errors

lib/src/impl_ffi/impl_ffi.aescbc.dart

@@ -42,7 +42,7 @@ Stream<Uint8List> _aesCbcEncryptOrDecrypt(
 
     final ivSize = ssl.EVP_CIPHER_iv_length(cipher);
     if (iv.length != ivSize) {
-      throw ArgumentError.value(iv, 'iv', 'must be $ivSize bytes');
+      throw operationError('iv must be $ivSize bytes');
     }
 
     final ctx = scope.createEVP_CIPHER_CTX();

lib/src/testing/regression/aes_cbc_invalid_iv.dart

@@ -0,0 +1,62 @@
+// 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() => [
+  for (final ivLength in [15, 17])
+    (
+      name: 'AES-CBC rejects IVs with $ivLength bytes',
+      test: () async {
+        final key = await AesCbcSecretKey.importRawKey(List.filled(16, 0));
+        const plaintext = [1, 2, 3];
+        final ciphertext = await key.encryptBytes(
+          plaintext,
+          List.filled(16, 0),
+        );
+        final iv = List.filled(ivLength, 0);
+
+        await _expectOperationError(
+          () => key.encryptBytes(plaintext, iv),
+          'Expected encryption with a $ivLength-byte IV to be rejected',
+        );
+        await _expectOperationError(
+          () => key.decryptBytes(ciphertext, iv),
+          'Expected decryption with a $ivLength-byte IV to be rejected',
+        );
+        await _expectOperationError(
+          () => key.encryptStream(Stream.value(plaintext), iv).drain(),
+          'Expected stream encryption with a $ivLength-byte IV to be rejected',
+        );
+        await _expectOperationError(
+          () => key.decryptStream(Stream.value(ciphertext), iv).drain(),
+          'Expected stream decryption with a $ivLength-byte IV to be rejected',
+        );
+      },
+    ),
+];
+
+Future<void> _expectOperationError(
+  Future<Object?> Function() callback,
+  String message,
+) async {
+  try {
+    await callback();
+  } on OperationError {
+    return;
+  }
+  check(false, message);
+}

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/aes_cbc_invalid_iv.dart' as aes_cbc_invalid_iv;
 import 'regression/issue_302_hmac_jwk_length.dart' as issue_302_hmac_jwk_length;
 import 'regression/aes_gcm_invalid_tag_length.dart'
     as aes_gcm_invalid_tag_length;
@@ -67,6 +68,7 @@ void runAllTests(
     for (final r in _testRunners) ...r.tests(),
     ...random.tests(),
     ...digest.tests(),
+    ...aes_cbc_invalid_iv.tests(),
     ...issue_302_hmac_jwk_length.tests(),
     ...aes_gcm_invalid_tag_length.tests(),
     ...issue_60_trailing_bytes.tests(),