Add OpenSSL.crypto.verify_chain method.

This change adds support for verifying a certificate or a certificate
chain. This implementation uses OpenSSL's underlying X509_STORE_CTX_*
class of functions to accomplish this.

This change also adds an intermediate signing certificate/key and a
service certificate/key signed with the intermediate signing
certificate, to make testing the OpenSSL.crypto.verify_chain method
easier to test. I figured I would add it to the top level module so
other people can use an intermediate signing certificate in their own
tests.

Issue: https://github.com/pyca/pyopenssl/issues/154
diff --git a/OpenSSL/test/test_crypto.py b/OpenSSL/test/test_crypto.py
index f704ac0..8c04938 100644
--- a/OpenSSL/test/test_crypto.py
+++ b/OpenSSL/test/test_crypto.py
@@ -10,6 +10,7 @@
 import base64
 import os
 import re
+import sys
 from subprocess import PIPE, Popen
 from datetime import datetime, timedelta
 
@@ -17,7 +18,8 @@
 
 from OpenSSL.crypto import TYPE_RSA, TYPE_DSA, Error, PKey, PKeyType
 from OpenSSL.crypto import X509, X509Type, X509Name, X509NameType
-from OpenSSL.crypto import X509Store, X509StoreType, X509Req, X509ReqType
+from OpenSSL.crypto import X509Store, X509StoreType, X509StoreContext
+from OpenSSL.crypto import X509Req, X509ReqType
 from OpenSSL.crypto import X509Extension, X509ExtensionType
 from OpenSSL.crypto import load_certificate, load_privatekey
 from OpenSSL.crypto import FILETYPE_PEM, FILETYPE_ASN1, FILETYPE_TEXT
@@ -28,7 +30,7 @@
 from OpenSSL.crypto import CRL, Revoked, load_crl
 from OpenSSL.crypto import NetscapeSPKI, NetscapeSPKIType
 from OpenSSL.crypto import (
-    sign, verify, get_elliptic_curve, get_elliptic_curves)
+    sign, verify, verify_cert, get_elliptic_curve, get_elliptic_curves)
 from OpenSSL.test.util import EqualityTestsMixin, TestCase
 from OpenSSL._util import native, lib
 
@@ -83,6 +85,40 @@
 -----END RSA PRIVATE KEY-----
 """)
 
+intermediate_cert_pem = b("""-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+""")
+
+intermediate_key_pem = b("""-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----
+""")
+
 server_cert_pem = b("""-----BEGIN CERTIFICATE-----
 MIICKDCCAZGgAwIBAgIJAJn/HpR21r/8MA0GCSqGSIb3DQEBBQUAMFgxCzAJBgNV
 BAYTAlVTMQswCQYDVQQIEwJJTDEQMA4GA1UEBxMHQ2hpY2FnbzEQMA4GA1UEChMH
@@ -116,6 +152,40 @@
 -----END RSA PRIVATE KEY-----
 """))
 
+intermediate_server_cert_pem = b("""-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
+""")
+
+intermediate_server_key_pem = b("""-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----
+""")
+
 client_cert_pem = b("""-----BEGIN CERTIFICATE-----
 MIICJjCCAY+gAwIBAgIJAKxpFI5lODkjMA0GCSqGSIb3DQEBBQUAMFgxCzAJBgNV
 BAYTAlVTMQswCQYDVQQIEwJJTDEQMA4GA1UEBxMHQ2hpY2FnbzEQMA4GA1UEChMH
@@ -3105,6 +3175,88 @@
         self.assertRaises(Error, load_crl, FILETYPE_PEM, b"hello, world")
 
 
+class VerifyCertTests(TestCase):
+    """
+    Tests for :py:obj:`OpenSSL.crypto.verify_cert`.
+    """
+    root_cert = load_certificate(FILETYPE_PEM, root_cert_pem)
+    intermediate_cert = load_certificate(FILETYPE_PEM, intermediate_cert_pem)
+    intermediate_server_cert = load_certificate(FILETYPE_PEM, intermediate_server_cert_pem)
+
+    def test_valid(self):
+        """
+        :py:obj:`verify_cert` does nothing when called with a certificate and
+        valid chain.
+        """
+        store = X509Store()
+        store.add_cert(self.root_cert)
+        store.add_cert(self.intermediate_cert)
+        store_ctx = X509StoreContext(store, self.intermediate_server_cert)
+        self.assertEqual(verify_cert(store_ctx), None)
+
+    def test_reuse(self):
+        """
+        :py:obj:`verify_cert` can be called multiple times.
+        """
+        store = X509Store()
+        store.add_cert(self.root_cert)
+        store.add_cert(self.intermediate_cert)
+        store_ctx = X509StoreContext(store, self.intermediate_server_cert)
+        self.assertEqual(verify_cert(store_ctx), None)
+        self.assertEqual(verify_cert(store_ctx), None)
+
+    def test_trusted_self_signed(self):
+        """
+        :py:obj:`verify_cert` does nothign when called with a self-signed
+        certificate and itself in the chain.
+        """
+        store = X509Store()
+        store.add_cert(self.root_cert)
+        store_ctx = X509StoreContext(store, self.root_cert)
+        self.assertEqual(verify_cert(store_ctx), None)
+
+    def test_untrusted_self_signed(self):
+        """
+        :py:obj:`verify_cert` raises error when a self-signed certificate is
+        verified without itself in the chain.
+        """
+        store = X509Store()
+        store_ctx = X509StoreContext(store, self.root_cert)
+        try:
+          verify_cert(store_ctx)
+          self.assertTrue(False)
+        except Error as e:
+          self.assertTrue('self signed certificate' in str(e))
+          self.assertEqual(e.certificate.get_subject().CN, 'Testing Root CA')
+
+    def test_invalid_chain_no_root(self):
+        """
+        :py:obj:`verify_cert` raises error when a root certificate is missing
+        from the chain.
+        """
+        store = X509Store()
+        store.add_cert(self.intermediate_cert)
+        store_ctx = X509StoreContext(store, self.intermediate_server_cert)
+        try:
+          verify_cert(store_ctx)
+        except Error as e:
+          self.assertTrue('unable to get issuer certificate' in str(e))
+          self.assertEqual(e.certificate.get_subject().CN, 'intermediate')
+
+    def test_invalid_chain_no_intermediate(self):
+        """
+        :py:obj:`verify_cert` raises error when an intermediate certificate is
+        missing from the chain.
+        """
+        store = X509Store()
+        store.add_cert(self.root_cert)
+        store_ctx = X509StoreContext(store, self.intermediate_server_cert)
+        try:
+          verify_cert(store_ctx)
+        except Error as e:
+          self.assertTrue('unable to get local issuer certificate' in str(e))
+          self.assertEqual(e.certificate.get_subject().CN, 'intermediate-service')
+
 
 class SignVerifyTests(TestCase):
     """