Reject JWKs with conflicting use and key_ops

lib/src/jsonwebkey.dart

@@ -61,6 +61,39 @@ final class JsonWebKey {
     this.k,
   });
 
+  static void _verifyUseAndKeyOps(
+    String? use,
+    List<String>? keyOps,
+    Map<String, dynamic> json,
+  ) {
+    if (use == null || keyOps == null) {
+      return; // nothing to validate
+    }
+
+    const encryptionOps = {'encrypt', 'decrypt', 'wrapKey', 'unwrapKey'};
+
+    const signingOps = {'sign', 'verify'};
+
+    Set<String>? allowedOps;
+    if (use == 'enc') {
+      allowedOps = encryptionOps;
+    } else if (use == 'sig') {
+      allowedOps = signingOps;
+    } else {
+      // Unknown "use" values are ignored (spec-compatible)
+      return;
+    }
+
+    for (final op in keyOps) {
+      if (!allowedOps.contains(op)) {
+        throw FormatException(
+          'JWK property "key_ops" conflicts with "use": "$use"',
+          json,
+        );
+      }
+    }
+  }
+
   static JsonWebKey fromJson(Map<String, dynamic> json) {
     const stringKeys = [
       'kty',
@@ -95,6 +128,7 @@ final class JsonWebKey {
       }
       key_ops = (json['key_ops'] as List).map((e) => e as String).toList();
     }
+    _verifyUseAndKeyOps(json['use'] as String?, key_ops, json);
 
     if (json.containsKey('ext') && json['ext'] is! bool) {
       throw FormatException('JWK entry "ext" must be boolean', json);

lib/src/testing/regression/jwk_use_key_ops_conflict.dart

@@ -0,0 +1,88 @@
+// 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/src/jsonwebkey.dart';
+import '../utils/utils.dart';
+
+void main() => tests().runTests();
+
+List<({String name, Future<void> Function() test})> tests() {
+  final tests = <({String name, Future<void> Function() test})>[];
+  void test(String name, Future<void> Function() fn) =>
+      tests.add((name: name, test: fn));
+
+  test('JsonWebKey: rejects enc use with sign/verify key_ops', () async {
+    bool threw = false;
+    try {
+      JsonWebKey.fromJson({
+        'kty': 'RSA',
+        'use': 'enc',
+        'key_ops': ['sign'],
+        'n': 'x',
+        'e': 'x',
+      });
+    } on FormatException {
+      threw = true;
+    }
+    check(threw, 'enc use conflicts with sign operation');
+  });
+
+  test('JsonWebKey: rejects sig use with encrypt/decrypt key_ops', () async {
+    bool threw = false;
+    try {
+      JsonWebKey.fromJson({
+        'kty': 'RSA',
+        'use': 'sig',
+        'key_ops': ['encrypt'],
+        'n': 'x',
+        'e': 'x',
+      });
+    } on FormatException {
+      threw = true;
+    }
+    check(threw, 'sig use conflicts with encrypt operation');
+  });
+
+  test('JsonWebKey: accepts valid use with matching key_ops', () async {
+    // enc use with encrypt/decrypt operations
+    JsonWebKey.fromJson({
+      'kty': 'RSA',
+      'use': 'enc',
+      'key_ops': ['encrypt', 'decrypt'],
+      'n': 'x',
+      'e': 'x',
+    });
+
+    // sig use with sign/verify operations
+    JsonWebKey.fromJson({
+      'kty': 'RSA',
+      'use': 'sig',
+      'key_ops': ['sign', 'verify'],
+      'n': 'x',
+      'e': 'x',
+    });
+  });
+
+  test('JsonWebKey: ignores unknown use values', () async {
+    JsonWebKey.fromJson({
+      'kty': 'RSA',
+      'use': 'unknown',
+      'key_ops': ['sign', 'encrypt'],
+      'n': 'x',
+      'e': 'x',
+    });
+  });
+
+  return tests;
+}

lib/src/testing/testing.dart

@@ -30,6 +30,8 @@ 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/jwk_use_key_ops_conflict.dart'
+    as jwk_use_key_ops_conflict;
 
 /// Test runners from all test files except `digest.dart` and
 /// `random.dart`, which do not use [TestRunner].
@@ -58,6 +60,7 @@ void runAllTests(
     for (final r in _testRunners) ...r.tests(),
     ...random.tests(),
     ...digest.tests(),
+    ...jwk_use_key_ops_conflict.tests(),
   ];
 
   for (final (:name, :test) in allTests) {