[WIP] Adding allowed algorithms (#252)

* Added allowed policy fields

Added allowed fields for host keys kex ciphers and macs

* Adapted policy.py to newest dev version

* Added allow_algorithm_subset_and_reordering flag

* Removed allowed policy entries as they are redundant now

* Fixed call to append_error

src/ssh_audit/policy.py

@@ -54,6 +54,7 @@ class Policy:
         self._hostkey_sizes: Optional[Dict[str, Dict[str, Union[int, str, bytes]]]] = None
         self._dh_modulus_sizes: Optional[Dict[str, int]] = None
         self._server_policy = True
+        self._allow_algorithm_subset_and_reordering = False
 
         self._name_and_version: str = ''
 
@@ -112,7 +113,7 @@ class Policy:
             key = key.strip()
             val = val.strip()
 
-            if key not in ['name', 'version', 'banner', 'compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs', 'client policy', 'host_key_sizes', 'dh_modulus_sizes'] and not key.startswith('hostkey_size_') and not key.startswith('cakey_size_') and not key.startswith('dh_modulus_size_'):
+            if key not in ['name', 'version', 'banner', 'compressions', 'host keys', 'optional host keys', 'key exchanges', 'ciphers', 'macs', 'client policy', 'host_key_sizes', 'dh_modulus_sizes', 'allow_algorithm_subset_and_reordering'] and not key.startswith('hostkey_size_') and not key.startswith('cakey_size_') and not key.startswith('dh_modulus_size_'):
                 raise ValueError("invalid field found in policy: %s" % line)
 
             if key in ['name', 'banner']:
@@ -205,7 +206,8 @@ class Policy:
 
             elif key.startswith('client policy') and val.lower() == 'true':
                 self._server_policy = False
-
+            elif key == 'allow_algorithm_subset_and_reordering' and val.lower() == 'true':
+                self._allow_algorithm_subset_and_reordering = True
 
         if self._name is None:
             raise ValueError('The policy does not have a name field.')
@@ -332,7 +334,7 @@ macs = %s
 
         # All subsequent tests require a valid kex, so end here if we don't have one.
         if kex is None:
-            return ret, errors, self._get_error_str(errors)
+            return ret, errors, self._get_error_str(errors, self._allow_algorithm_subset_and_reordering)
 
         if (self._compressions is not None) and (kex.server.compression != self._compressions):
             ret = False
@@ -343,10 +345,19 @@ macs = %s
         if self._optional_host_keys is not None:
             pruned_host_keys = [x for x in kex.key_algorithms if x not in self._optional_host_keys]
         
-        if (self._host_keys is not None) and (pruned_host_keys != self._host_keys):
+        # Checking Hostkeys
-            ret = False
+        if self._host_keys is not None:
-            self._append_error(errors, 'Host keys', self._host_keys, self._optional_host_keys, kex.key_algorithms)
+            if self._allow_algorithm_subset_and_reordering:
+                for hostkey_t in kex.key_algorithms:
+                    if hostkey_t not in self._host_keys:
+                        ret = False
+                        self._append_error(errors, 'Host keys', self._host_keys, None, kex.key_algorithms)
+                        break
+            elif pruned_host_keys != self._host_keys:
+                ret = False
+                self._append_error(errors, 'Host keys', self._host_keys, None, kex.key_algorithms)
 
+        # Checking Host Key Sizes
         if self._hostkey_sizes is not None:
             hostkey_types = list(self._hostkey_sizes.keys())
             hostkey_types.sort()  # Sorted to make testing output repeatable.
@@ -375,17 +386,41 @@ macs = %s
                             ret = False
                             self._append_error(errors, 'CA signature size (%s)' % actual_ca_key_type, [str(expected_ca_key_size)], None, [str(actual_ca_key_size)])
         
-        if kex.kex_algorithms != self._kex:
+        # Checking KEX
-            ret = False
+        if self._kex is not None:
-            self._append_error(errors, 'Key exchanges', self._kex, None, kex.kex_algorithms)
+            if self._allow_algorithm_subset_and_reordering:
+                for kex_t in kex.kex_algorithms:
+                    if kex_t not in self._kex:
+                        ret = False
+                        self._append_error(errors, 'Key exchanges', self._kex, None, kex.kex_algorithms)
+                        break
+            elif kex.kex_algorithms != self._kex: # Requires perfect match
+                ret = False
+                self._append_error(errors, 'Key exchanges', self._kex, None, kex.kex_algorithms)
         
-        if (self._ciphers is not None) and (kex.server.encryption != self._ciphers):
+        # Checking Ciphers
-            ret = False
+        if self._ciphers is not None:
-            self._append_error(errors, 'Ciphers', self._ciphers, None, kex.server.encryption)
+            if self._allow_algorithm_subset_and_reordering:
+                for cipher_t in kex.server.encryption:
+                    if cipher_t not in self._ciphers:
+                        ret = False
+                        self._append_error(errors, 'Ciphers', self._ciphers, None, kex.server.encryption)
+                        break
+            elif kex.server.encryption != self._ciphers: # Requires perfect match
+                ret = False
+                self._append_error(errors, 'Ciphers', self._ciphers, None, kex.server.encryption)
         
-        if (self._macs is not None) and (kex.server.mac != self._macs):
+        # Checking MACs
-            ret = False
+        if self._macs is not None:
-            self._append_error(errors, 'MACs', self._macs, None, kex.server.mac)
+            if self._allow_algorithm_subset_and_reordering:
+                for mac_t in kex.server.mac:
+                    if mac_t not in self._macs:
+                        ret = False
+                        self._append_error(errors, 'MACs', self._macs, None, kex.server.mac)
+                        break
+            elif kex.server.mac != self._macs: # Requires perfect match
+                ret = False
+                self._append_error(errors, 'MACs', self._macs, None, kex.server.mac)
 
         if self._dh_modulus_sizes is not None:
             dh_modulus_types = list(self._dh_modulus_sizes.keys())
@@ -398,22 +433,28 @@ macs = %s
                         ret = False
                         self._append_error(errors, 'Group exchange (%s) modulus sizes' % dh_modulus_type, [str(expected_dh_modulus_size)], None, [str(actual_dh_modulus_size)])
 
-        return ret, errors, self._get_error_str(errors)
+        return ret, errors, self._get_error_str(errors, self._allow_algorithm_subset_and_reordering)
 
 
     @staticmethod
-    def _get_error_str(errors: List[Any]) -> str:
+    def _get_error_str(errors: List[Any], allow_algorithm_subset_and_reordering: bool = False) -> str:
         '''Transforms an error struct to a flat string of error messages.'''
 
+        if allow_algorithm_subset_and_reordering:
+            expected_str = 'allowed'
+        else:
+            expected_str = 'required'
+
         error_list = []
         spacer = ''
         for e in errors:
             e_str = "  * %s did not match.\n" % e['mismatched_field']
+
             if ('expected_optional' in e) and (e['expected_optional'] != ['']):
-                e_str += "    - Expected (required): %s\n    - Expected (optional): %s\n" % (Policy._normalize_error_field(e['expected_required']), Policy._normalize_error_field(e['expected_optional']))
+                e_str += "    - Expected (" + expected_str + "): %s\n    - Expected (optional): %s\n" % (Policy._normalize_error_field(e['expected_required']), Policy._normalize_error_field(e['expected_optional']))
                 spacer = '              '
             else:
-                e_str += "    - Expected: %s\n" % Policy._normalize_error_field(e['expected_required'])
+                e_str += "    - Expected (" + expected_str + "): %s\n" % Policy._normalize_error_field(e['expected_required'])
                 spacer = '   '
             e_str += "    - Actual:%s%s\n" % (spacer, Policy._normalize_error_field(e['actual']))
             error_list.append(e_str)