bpo-29136: Add TLS 1.3 cipher suites and OP_NO_TLSv1_3 (#1363)

* bpo-29136: Add TLS 1.3 support

TLS 1.3 introduces a new, distinct set of cipher suites. The TLS 1.3
cipher suites don't overlap with cipher suites from TLS 1.2 and earlier.
Since Python sets its own set of permitted ciphers, TLS 1.3 handshake
will fail as soon as OpenSSL 1.1.1 is released. Let's enable the common
AES-GCM and ChaCha20 suites.

Additionally the flag OP_NO_TLSv1_3 is added. It defaults to 0 (no op) with
OpenSSL prior to 1.1.1. This allows applications to opt-out from TLS 1.3
now.

Signed-off-by: Christian Heimes <christian@python.org>

5 files changed, 79 insertions, 3 deletions

diff --git a/Doc/library/ssl.rst b/Doc/library/ssl.rst
index 69bea1e..031c361 100644
--- a/Doc/library/ssl.rst
+++ b/Doc/library/ssl.rst
@@ -193,11 +193,11 @@ instead.
      .. table::
 
        ========================  ============  ============  =============  =========  ===========  ===========
-        *client* / **server**    **SSLv2**     **SSLv3**     **TLS**        **TLSv1**  **TLSv1.1**  **TLSv1.2**
+        *client* / **server**    **SSLv2**     **SSLv3**     **TLS** [3]_   **TLSv1**  **TLSv1.1**  **TLSv1.2**
        ------------------------  ------------  ------------  -------------  ---------  -----------  -----------
         *SSLv2*                    yes           no            no [1]_        no         no         no
         *SSLv3*                    no            yes           no [2]_        no         no         no
-        *TLS* (*SSLv23*)           no [1]_       no [2]_       yes            yes        yes        yes
+        *TLS* (*SSLv23*) [3]_      no [1]_       no [2]_       yes            yes        yes        yes
         *TLSv1*                    no            no            yes            yes        no         no
         *TLSv1.1*                  no            no            yes            no         yes        no
         *TLSv1.2*                  no            no            yes            no         no         yes
@@ -206,6 +206,9 @@ instead.
    .. rubric:: Footnotes
    .. [1] :class:`SSLContext` disables SSLv2 with :data:`OP_NO_SSLv2` by default.
    .. [2] :class:`SSLContext` disables SSLv3 with :data:`OP_NO_SSLv3` by default.
+   .. [3] TLS 1.3 protocol will be available with :data:`PROTOCOL_TLS` in
+      OpenSSL >= 1.1.1. There is no dedicated PROTOCOL constant for just
+      TLS 1.3.
 
    .. note::
 
@@ -294,6 +297,11 @@ purposes.
 
      3DES was dropped from the default cipher string.
 
+   .. versionchanged:: 3.7
+
+     TLS 1.3 cipher suites TLS_AES_128_GCM_SHA256, TLS_AES_256_GCM_SHA384,
+     and TLS_CHACHA20_POLY1305_SHA256 were added to the default cipher string.
+
 
 Random generation
 ^^^^^^^^^^^^^^^^^
@@ -764,6 +772,16 @@ Constants
 
    .. versionadded:: 3.4
 
+.. data:: OP_NO_TLSv1_3
+
+   Prevents a TLSv1.3 connection. This option is only applicable in conjunction
+   with :const:`PROTOCOL_TLS`. It prevents the peers from choosing TLSv1.3 as
+   the protocol version. TLS 1.3 is available with OpenSSL 1.1.1 or later.
+   When Python has been compiled against an older version of OpenSSL, the
+   flag defaults to *0*.
+
+   .. versionadded:: 3.7
+
 .. data:: OP_CIPHER_SERVER_PREFERENCE
 
    Use the server's cipher ordering preference, rather than the client's.
@@ -838,6 +856,12 @@ Constants
 
    .. versionadded:: 3.3
 
+.. data:: HAS_TLSv1_3
+
+   Whether the OpenSSL library has built-in support for the TLS 1.3 protocol.
+
+   .. versionadded:: 3.7
+
 .. data:: CHANNEL_BINDING_TYPES
 
    List of supported TLS channel binding types.  Strings in this list
diff --git a/Lib/ssl.py b/Lib/ssl.py
index 7a574dc..1f3a31a 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -115,7 +115,7 @@ except ImportError:
     pass
 
 
-from _ssl import HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN
+from _ssl import HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN, HAS_TLSv1_3
 from _ssl import _OPENSSL_API_VERSION
 
 
@@ -178,6 +178,7 @@ else:
 # (OpenSSL's default setting is 'DEFAULT:!aNULL:!eNULL')
 # Enable a better set of ciphers by default
 # This list has been explicitly chosen to:
+#   * TLS 1.3 ChaCha20 and AES-GCM cipher suites
 #   * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
 #   * Prefer ECDHE over DHE for better performance
 #   * Prefer AEAD over CBC for better performance and security
@@ -189,6 +190,8 @@ else:
 #   * Disable NULL authentication, NULL encryption, 3DES and MD5 MACs
 #     for security reasons
 _DEFAULT_CIPHERS = (
+    'TLS13-AES-256-GCM-SHA384:TLS13-CHACHA20-POLY1305-SHA256:'
+    'TLS13-AES-128-GCM-SHA256:'
     'ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:'
     'ECDH+AES128:DH+AES:ECDH+HIGH:DH+HIGH:RSA+AESGCM:RSA+AES:RSA+HIGH:'
     '!aNULL:!eNULL:!MD5:!3DES'
@@ -196,6 +199,7 @@ _DEFAULT_CIPHERS = (
 
 # Restricted and more secure ciphers for the server side
 # This list has been explicitly chosen to:
+#   * TLS 1.3 ChaCha20 and AES-GCM cipher suites
 #   * Prefer cipher suites that offer perfect forward secrecy (DHE/ECDHE)
 #   * Prefer ECDHE over DHE for better performance
 #   * Prefer AEAD over CBC for better performance and security
@@ -206,6 +210,8 @@ _DEFAULT_CIPHERS = (
 #   * Disable NULL authentication, NULL encryption, MD5 MACs, DSS, RC4, and
 #     3DES for security reasons
 _RESTRICTED_SERVER_CIPHERS = (
+    'TLS13-AES-256-GCM-SHA384:TLS13-CHACHA20-POLY1305-SHA256:'
+    'TLS13-AES-128-GCM-SHA256:'
     'ECDH+AESGCM:ECDH+CHACHA20:DH+AESGCM:DH+CHACHA20:ECDH+AES256:DH+AES256:'
     'ECDH+AES128:DH+AES:ECDH+HIGH:DH+HIGH:RSA+AESGCM:RSA+AES:RSA+HIGH:'
     '!aNULL:!eNULL:!MD5:!DSS:!RC4:!3DES'
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 747661b..fe9f693 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -170,6 +170,13 @@ class BasicSocketTests(unittest.TestCase):
             ssl.OP_NO_COMPRESSION
         self.assertIn(ssl.HAS_SNI, {True, False})
         self.assertIn(ssl.HAS_ECDH, {True, False})
+        ssl.OP_NO_SSLv2
+        ssl.OP_NO_SSLv3
+        ssl.OP_NO_TLSv1
+        ssl.OP_NO_TLSv1_3
+        if ssl.OPENSSL_VERSION_INFO >= (1, 0, 1):
+            ssl.OP_NO_TLSv1_1
+            ssl.OP_NO_TLSv1_2
 
     def test_str_for_enums(self):
         # Make sure that the PROTOCOL_* constants have enum-like string
@@ -3098,12 +3105,33 @@ class ThreadedTests(unittest.TestCase):
                 self.assertEqual(s.version(), 'TLSv1')
             self.assertIs(s.version(), None)
 
+    @unittest.skipUnless(ssl.HAS_TLSv1_3,
+                         "test requires TLSv1.3 enabled OpenSSL")
+    def test_tls1_3(self):
+        context = ssl.SSLContext(ssl.PROTOCOL_TLS)
+        context.load_cert_chain(CERTFILE)
+        # disable all but TLS 1.3
+        context.options |= (
+            ssl.OP_NO_TLSv1 | ssl.OP_NO_TLSv1_1 | ssl.OP_NO_TLSv1_2
+        )
+        with ThreadedEchoServer(context=context) as server:
+            with context.wrap_socket(socket.socket()) as s:
+                s.connect((HOST, server.port))
+                self.assertIn(s.cipher()[0], [
+                    'TLS13-AES-256-GCM-SHA384',
+                    'TLS13-CHACHA20-POLY1305-SHA256',
+                    'TLS13-AES-128-GCM-SHA256',
+                ])
+
     @unittest.skipUnless(ssl.HAS_ECDH, "test requires ECDH-enabled OpenSSL")
     def test_default_ecdh_curve(self):
         # Issue #21015: elliptic curve-based Diffie Hellman key exchange
         # should be enabled by default on SSL contexts.
         context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
         context.load_cert_chain(CERTFILE)
+        # TLSv1.3 defaults to PFS key agreement and no longer has KEA in
+        # cipher name.
+        context.options |= ssl.OP_NO_TLSv1_3
         # Prior to OpenSSL 1.0.0, ECDH ciphers have to be enabled
         # explicitly using the 'ECCdraft' cipher alias.  Otherwise,
         # our default cipher list should prefer ECDH-based ciphers
@@ -3532,6 +3560,10 @@ class ThreadedTests(unittest.TestCase):
         context2.load_verify_locations(CERTFILE)
         context2.load_cert_chain(CERTFILE)
 
+        # TODO: session reuse does not work with TLS 1.3
+        context.options |= ssl.OP_NO_TLSv1_3
+        context2.options |= ssl.OP_NO_TLSv1_3
+
         server = ThreadedEchoServer(context=context, chatty=False)
         with server:
             with context.wrap_socket(socket.socket()) as s:
diff --git a/Misc/NEWS.d/next/Library/2017-09-04-16-39-49.bpo-29136.vSn1oR.rst b/Misc/NEWS.d/next/Library/2017-09-04-16-39-49.bpo-29136.vSn1oR.rst
new file mode 100644
index 0000000..e76997e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-09-04-16-39-49.bpo-29136.vSn1oR.rst
@@ -0,0 +1 @@
+Add TLS 1.3 cipher suites and OP_NO_TLSv1_3.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index 5ea354a..f737925 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -5350,6 +5350,11 @@ PyInit__ssl(void)
     PyModule_AddIntConstant(m, "OP_NO_TLSv1_1", SSL_OP_NO_TLSv1_1);
     PyModule_AddIntConstant(m, "OP_NO_TLSv1_2", SSL_OP_NO_TLSv1_2);
 #endif
+#ifdef SSL_OP_NO_TLSv1_3
+    PyModule_AddIntConstant(m, "OP_NO_TLSv1_3", SSL_OP_NO_TLSv1_3);
+#else
+    PyModule_AddIntConstant(m, "OP_NO_TLSv1_3", 0);
+#endif
     PyModule_AddIntConstant(m, "OP_CIPHER_SERVER_PREFERENCE",
                             SSL_OP_CIPHER_SERVER_PREFERENCE);
     PyModule_AddIntConstant(m, "OP_SINGLE_DH_USE", SSL_OP_SINGLE_DH_USE);
@@ -5398,6 +5403,14 @@ PyInit__ssl(void)
     Py_INCREF(r);
     PyModule_AddObject(m, "HAS_ALPN", r);
 
+#if defined(TLS1_3_VERSION) && !defined(OPENSSL_NO_TLS1_3)
+    r = Py_True;
+#else
+    r = Py_False;
+#endif
+    Py_INCREF(r);
+    PyModule_AddObject(m, "HAS_TLSv1_3", r);
+
     /* Mappings for error codes */
     err_codes_to_names = PyDict_New();
     err_names_to_codes = PyDict_New();