Add a key purpose parameter to Certificate PathBuilder.

net/data/verify_certificate_chain_unittest/generate-intermediate-restricts-eku-fail.py

+#!/usr/bin/python
+# Copyright (c) 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Certificate chain with 1 intermediate and a trusted root. The intermediate
+restricts the EKU to clientAuth, and the target has serverAuth +
+clientAuth. Verification is expected to fail when requesting serverAuth."""
+
+import common
+
+# Self-signed root certificate (used as trust anchor).
+root = common.create_self_signed_root_certificate('Root')
+
+# Intermediate certificate.
+intermediate = common.create_intermediate_certificate('Intermediate', root)
+intermediate.get_extensions().set_property('extendedKeyUsage',
+                                           'clientAuth')
+
+# Target certificate.
+target = common.create_end_entity_certificate('Target', intermediate)
+target.get_extensions().set_property('extendedKeyUsage',
+                                     'serverAuth,clientAuth')
+
+chain = [target, intermediate]
+trusted = common.TrustAnchor(root, constrained=False)
+time = common.DEFAULT_TIME
+key_purpose = common.KEY_PURPOSE_SERVER_AUTH
+verify_result = False
+errors = """----- Certificate i=1 (CN=Intermediate) -----
+ERROR: The extended key usage does not include server auth
+
+"""
+
+common.write_test_file(__doc__, chain, trusted, time, key_purpose,
+                       verify_result, errors)