gh-96931: Fix incorrect results in ssl.SSLSocket.shared_ciphers (#96932)

Doc/library/ssl.rst

@@ -1218,7 +1218,7 @@ SSL sockets also have the following additional methods and attributes:
 
 .. method:: SSLSocket.shared_ciphers()
 
-   Return the list of ciphers shared by the client during the handshake.  Each
+   Return the list of ciphers available in both the client and server.  Each
    entry of the returned list is a three-value tuple containing the name of the
    cipher, the version of the SSL protocol that defines its use, and the number
    of secret bits the cipher uses.  :meth:`~SSLSocket.shared_ciphers` returns

Lib/test/test_ssl.py

@@ -2082,13 +2082,13 @@ class SimpleBackgroundTests(unittest.TestCase):
         self.assertIs(sslobj._sslobj.owner, sslobj)
         self.assertIsNone(sslobj.cipher())
         self.assertIsNone(sslobj.version())
-        self.assertIsNotNone(sslobj.shared_ciphers())
+        self.assertIsNone(sslobj.shared_ciphers())
         self.assertRaises(ValueError, sslobj.getpeercert)
         if 'tls-unique' in ssl.CHANNEL_BINDING_TYPES:
             self.assertIsNone(sslobj.get_channel_binding('tls-unique'))
         self.ssl_io_loop(sock, incoming, outgoing, sslobj.do_handshake)
         self.assertTrue(sslobj.cipher())
-        self.assertIsNotNone(sslobj.shared_ciphers())
+        self.assertIsNone(sslobj.shared_ciphers())
         self.assertIsNotNone(sslobj.version())
         self.assertTrue(sslobj.getpeercert())
         if 'tls-unique' in ssl.CHANNEL_BINDING_TYPES:
@@ -4051,7 +4051,7 @@ class ThreadedTests(unittest.TestCase):
     def test_shared_ciphers(self):
         client_context, server_context, hostname = testing_context()
         client_context.set_ciphers("AES128:AES256")
-        server_context.set_ciphers("AES256")
+        server_context.set_ciphers("AES256:eNULL")
         expected_algs = [
             "AES256", "AES-256",
             # TLS 1.3 ciphers are always enabled

Misc/NEWS.d/next/Library/2022-09-19-08-12-58.gh-issue-96931.x0WQhh.rst

@@ -0,0 +1 @@
+Fix incorrect results from :meth:`ssl.SSLSocket.shared_ciphers`

Modules/_ssl.c

@@ -1998,24 +1998,44 @@ static PyObject *
 _ssl__SSLSocket_shared_ciphers_impl(PySSLSocket *self)
 /*[clinic end generated code: output=3d174ead2e42c4fd input=0bfe149da8fe6306]*/
 {
-    STACK_OF(SSL_CIPHER) *ciphers;
-    int i;
+    STACK_OF(SSL_CIPHER) *server_ciphers;
+    STACK_OF(SSL_CIPHER) *client_ciphers;
+    int i, len;
     PyObject *res;
+    const SSL_CIPHER* cipher;
 
-    ciphers = SSL_get_ciphers(self->ssl);
-    if (!ciphers)
+    /* Rather than use SSL_get_shared_ciphers, we use an equivalent algorithm because:
+
+       1) It returns a colon seperated list of strings, in an undefined
+          order, that we would have to post process back into tuples.
+       2) It will return a truncated string with no indication that it has
+          done so, if the buffer is too small.
+     */
+
+    server_ciphers = SSL_get_ciphers(self->ssl);
+    if (!server_ciphers)
         Py_RETURN_NONE;
-    res = PyList_New(sk_SSL_CIPHER_num(ciphers));
+    client_ciphers = SSL_get_client_ciphers(self->ssl);
+    if (!client_ciphers)
+        Py_RETURN_NONE;
+
+    res = PyList_New(sk_SSL_CIPHER_num(server_ciphers));
     if (!res)
         return NULL;
-    for (i = 0; i < sk_SSL_CIPHER_num(ciphers); i++) {
-        PyObject *tup = cipher_to_tuple(sk_SSL_CIPHER_value(ciphers, i));
+    len = 0;
+    for (i = 0; i < sk_SSL_CIPHER_num(server_ciphers); i++) {
+        cipher = sk_SSL_CIPHER_value(server_ciphers, i);
+        if (sk_SSL_CIPHER_find(client_ciphers, cipher) < 0)
+            continue;
+
+        PyObject *tup = cipher_to_tuple(cipher);
         if (!tup) {
             Py_DECREF(res);
             return NULL;
         }
-        PyList_SET_ITEM(res, i, tup);
+        PyList_SET_ITEM(res, len++, tup);
     }
+    Py_SET_SIZE(res, len);
     return res;
 }