Issue #21013: Enhance ssl.create_default_context() for server side contexts

Closes #21013 by modfying ssl.create_default_context() to:

* Move the restricted ciphers to only apply when using
  ssl.Purpose.CLIENT_AUTH. The major difference between restricted and not
  is the lack of RC4 in the restricted. However there are servers that exist
  that only expose RC4 still.
* Switches the default protocol to ssl.PROTOCOL_SSLv23 so that the context
  will select TLS1.1 or TLS1.2 if it is available.
* Add ssl.OP_NO_SSLv3 by default to continue to block SSL3.0 sockets
* Add ssl.OP_SINGLE_DH_USE and ssl.OP_SINGLE_ECDG_USE to improve the security
  of the perfect forward secrecy
* Add ssl.OP_CIPHER_SERVER_PREFERENCE so that when used for a server side
  socket the context will prioritize our ciphers which have been carefully
  selected to maximize security and performance.
* Documents the failure conditions when a SSL3.0 connection is required so
  that end users can more easily determine if they need to unset
  ssl.OP_NO_SSLv3.

1 files changed, 23 insertions, 3 deletions

diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 891720e..331d6ba 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -1014,23 +1014,43 @@ class ContextTests(unittest.TestCase):
 
     def test_create_default_context(self):
         ctx = ssl.create_default_context()
-        self.assertEqual(ctx.protocol, ssl.PROTOCOL_TLSv1)
+        self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_REQUIRED)
         self.assertTrue(ctx.check_hostname)
         self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self.assertEqual(
+            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
+            getattr(ssl, "OP_NO_COMPRESSION", 0),
+        )
 
         with open(SIGNING_CA) as f:
             cadata = f.read()
         ctx = ssl.create_default_context(cafile=SIGNING_CA, capath=CAPATH,
                                          cadata=cadata)
-        self.assertEqual(ctx.protocol, ssl.PROTOCOL_TLSv1)
+        self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_REQUIRED)
         self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self.assertEqual(
+            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
+            getattr(ssl, "OP_NO_COMPRESSION", 0),
+        )
 
         ctx = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
-        self.assertEqual(ctx.protocol, ssl.PROTOCOL_TLSv1)
+        self.assertEqual(ctx.protocol, ssl.PROTOCOL_SSLv23)
         self.assertEqual(ctx.verify_mode, ssl.CERT_NONE)
         self.assertEqual(ctx.options & ssl.OP_NO_SSLv2, ssl.OP_NO_SSLv2)
+        self.assertEqual(
+            ctx.options & getattr(ssl, "OP_NO_COMPRESSION", 0),
+            getattr(ssl, "OP_NO_COMPRESSION", 0),
+        )
+        self.assertEqual(
+            ctx.options & getattr(ssl, "OP_SINGLE_DH_USE", 0),
+            getattr(ssl, "OP_SINGLE_DH_USE", 0),
+        )
+        self.assertEqual(
+            ctx.options & getattr(ssl, "OP_SINGLE_ECDH_USE", 0),
+            getattr(ssl, "OP_SINGLE_ECDH_USE", 0),
+        )
 
     def test__create_stdlib_context(self):
         ctx = ssl._create_stdlib_context()