Fix TLS-PSK for TLS 1.3

SSL_CTX_set_session_id_context() is a server-side only operation.
Using this on the client-side is causing authentication errors

Author: grantramsay

Doc/library/ssl.rst

@@ -2003,6 +2003,12 @@ to speed up repeated connections from the same clients.
 
    Setting ``callback`` to :const:`None` removes any existing callback.
 
+   .. note::
+      When using TLS 1.3:
+
+      - the ``hint`` parameter is always :const:`None`.
+      - client-identity must be a non-empty string.
+
    Example usage::
 
       context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
@@ -2041,6 +2047,9 @@ to speed up repeated connections from the same clients.
 
    The parameter ``identity_hint`` is an optional identity hint sent to the client.
 
+   .. note::
+      When using TLS 1.3 the ``identity_hint`` parameter is not sent to the client.
+
    Example usage::
 
       context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)

Lib/test/test_ssl.py

@@ -4259,6 +4259,38 @@ def server_callback(identity):
         with self.assertRaisesRegex(ssl.SSLError, 'Cannot add PSK client callback'):
             server_context.set_psk_client_callback(client_callback)
 
+    @requires_tls_version('TLSv1_3')
+    def test_psk_tls1_3(self):
+        psk = bytes.fromhex('deadbeef')
+        identity_hint = 'identity-hint'
+        client_identity = 'client-identity'
+
+        def client_callback(hint):
+            # identity_hint is not sent to the client in TLS 1.3
+            self.assertIsNone(hint)
+            return client_identity, psk
+
+        def server_callback(identity):
+            self.assertEqual(identity, client_identity)
+            return psk
+
+        client_context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+        client_context.check_hostname = False
+        client_context.verify_mode = ssl.CERT_NONE
+        client_context.minimum_version = ssl.TLSVersion.TLSv1_3
+        client_context.set_ciphers('PSK')
+        client_context.set_psk_client_callback(client_callback)
+
+        server_context = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
+        server_context.minimum_version = ssl.TLSVersion.TLSv1_3
+        server_context.set_ciphers('PSK')
+        server_context.set_psk_server_callback(server_callback, identity_hint)
+
+        server = ThreadedEchoServer(context=server_context)
+        with server:
+            with client_context.wrap_socket(socket.socket()) as s:
+                s.connect((HOST, server.port))
+
 
 @unittest.skipUnless(has_tls_version('TLSv1_3'), "Test needs TLS 1.3")
 class TestPostHandshakeAuth(unittest.TestCase):

Modules/_ssl.c

@@ -3171,10 +3171,14 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
        usage for no cost at all. */
     SSL_CTX_set_mode(self->ctx, SSL_MODE_RELEASE_BUFFERS);
 
+    /* Setting the session id context is a server-side only operation.
+     * It can cause unexpected behaviour on client-side connections. */
+    if (proto_version == PY_SSL_VERSION_TLS_SERVER) {
 #define SID_CTX "Python"
-    SSL_CTX_set_session_id_context(self->ctx, (const unsigned char *) SID_CTX,
-                                   sizeof(SID_CTX));
+        SSL_CTX_set_session_id_context(self->ctx, (const unsigned char *) SID_CTX,
+                                       sizeof(SID_CTX));
 #undef SID_CTX
+    }
 
     params = SSL_CTX_get0_param(self->ctx);
     /* Improve trust chain building when cross-signed intermediate