Implement support for GSSAPI extension RFC 4178

RFC 4178 provides two support API calls that enable the caller to
manipulate the set of acceptable security mechanisms used in SPNEGO
protocol; for the given credentials, the gss_get_neg_mechs call is
used to indicate the current set of security mechanisms available
for negotiation, and the gss_set_neg_mechs call is used to specify
the set of security mechanisms avaiable for negotiation. Since
gss_get_neg_mechs is not implemented by MIT krb5, we are only
implementing the raw interface for the latter call. Note that
although RFC 4178 did not specify that the mech_set argument cannot
be an empty set, we are forcing it to be non empty in the low-level
API here since passing an empty set will always trigger an error in
MIT krb5 implementation.

gssapi/raw/__init__.py

@@ -69,6 +69,12 @@
 except ImportError:
     pass
 
+# optional RFC 4178 support
+try:
+    from gssapi.raw.ext_rfc4178 import *  # noqa
+except ImportError:
+    pass
+
 # optional RFC 5587 support
 try:
     from gssapi.raw.ext_rfc5587 import *  # noqa

gssapi/raw/ext_rfc4178.pyx

@@ -0,0 +1,48 @@
+GSSAPI="BASE"  # This ensures that a full module is generated by Cython
+
+from gssapi.raw.cython_types cimport *
+from gssapi.raw.cython_converters cimport c_get_mech_oid_set
+from gssapi.raw.creds cimport Creds
+
+from gssapi.raw.misc import GSSError
+
+cdef extern from "python_gssapi_ext.h":
+    OM_uint32 gss_set_neg_mechs(
+        OM_uint32 *minor_status,
+        gss_cred_id_t cred_handle,
+        const gss_OID_set mech_set) nogil
+
+
+def set_neg_mechs(Creds cred_handle not None, mech_set not None):
+    """
+    set_neg_mechs(cred_handle not None, mech_set not None)
+
+    Specify the set of security mechanisms that may be negotiated with
+    the credential identified by cred_handle.
+    If more than one mechanism is specified in mech_set, the order in
+    which those mechanisms are specified implies a relative preference.
+
+    Args:
+        cred_handle (Creds): credentials to set negotiable mechanisms for
+        mech_set ([MechType]): negotiable mechanisms to be set
+    Returns:
+        None
+    Raises:
+        GSSError
+    """
+
+    cdef gss_OID_set negotiable_mechs = c_get_mech_oid_set(mech_set)
+
+    cdef OM_uint32 maj_stat, min_stat
+
+    with nogil:
+        maj_stat = gss_set_neg_mechs(&min_stat, cred_handle.raw_creds,
+                                     negotiable_mechs)
+
+    cdef OM_uint32 tmp_min_stat
+    gss_release_oid_set(&tmp_min_stat, &negotiable_mechs)
+
+    if maj_stat == GSS_S_COMPLETE:
+        return None
+    else:
+        raise GSSError(maj_stat, min_stat)

gssapi/tests/test_raw.py

@@ -772,6 +772,67 @@ def test_sasl_names(self):
             cmp_mech.shouldnt_be_none()
             cmp_mech.should_be(mech)
 
+    @ktu.gssapi_extension_test('rfc4178', 'Negotiation Mechanism')
+    def test_set_neg_mechs(self):
+        all_mechs = gb.indicate_mechs()
+        spnego_mech = gb.OID.from_int_seq("1.3.6.1.5.5.2")
+        krb5_mech = gb.OID.from_int_seq("1.2.840.113554.1.2.2")
+        ntlm_mech = gb.OID.from_int_seq("1.3.6.1.4.1.311.2.2.10")
+
+        server_name = gb.import_name(TARGET_SERVICE_NAME,
+                                     gb.NameType.hostbased_service)
+
+        username = gb.import_name(name=b"user",
+                                  name_type=gb.NameType.user)
+        krb5_client_creds = gb.acquire_cred(None, usage='initiate',
+                                            mechs=all_mechs).creds
+        ntlm_client_creds = gb.acquire_cred_with_password(
+                                            name=username,
+                                            password=b'password',
+                                            mechs=all_mechs).creds
+        server_creds = gb.acquire_cred(server_name, usage='accept',
+                                       mechs=all_mechs).creds
+
+        neg_resp = gb.set_neg_mechs(server_creds, [ntlm_mech])
+        neg_resp.should_be_none()
+
+        client_ctx_resp = gb.init_sec_context(server_name,
+                                              creds=ntlm_client_creds,
+                                              mech=spnego_mech)
+        client_token = client_ctx_resp.token
+
+        server_ctx_resp = gb.accept_sec_context(client_token,
+                                                acceptor_creds=server_creds)
+        server_ctx_resp.shouldnt_be_none()
+
+        client_ctx_resp = gb.init_sec_context(server_name,
+                                              creds=krb5_client_creds,
+                                              mech=spnego_mech)
+        client_token = client_ctx_resp.token
+
+        gb.accept_sec_context.should_raise(gb.GSSError, client_token,
+                                           acceptor_creds=server_creds)
+
+        neg_resp = gb.set_neg_mechs(server_creds, [krb5_mech])
+        neg_resp.should_be_none()
+
+        client_ctx_resp = gb.init_sec_context(server_name,
+                                              creds=krb5_client_creds,
+                                              mech=spnego_mech)
+        client_token = client_ctx_resp.token
+
+        server_ctx_resp = gb.accept_sec_context(client_token,
+                                                acceptor_creds=server_creds)
+        server_ctx_resp.shouldnt_be_none()
+
+        client_ctx_resp = gb.init_sec_context(server_name,
+                                              creds=ntlm_client_creds,
+                                              mech=spnego_mech)
+        client_token = client_ctx_resp.token
+
+        gb.accept_sec_context.should_raise(gb.GSSError, client_token,
+                                           acceptor_creds=server_creds)
+
     @ktu.gssapi_extension_test('ggf', 'Global Grid Forum')
     @ktu.gssapi_extension_test('s4u', 'S4U')
     @ktu.krb_minversion_test('1.16',

setup.py

@@ -269,6 +269,7 @@ def gssapi_modules(lst):
         main_file('chan_bindings'),
         extension_file('s4u', 'gss_acquire_cred_impersonate_name'),
         extension_file('cred_store', 'gss_store_cred_into'),
+        extension_file('rfc4178', 'gss_set_neg_mechs'),
         extension_file('rfc5587', 'gss_indicate_mechs_by_attrs'),
         extension_file('rfc5588', 'gss_store_cred'),
         extension_file('rfc5801', 'gss_inquire_saslname_for_mech'),