Tighten authentication logic

Use enumerations for specifying authentication support. This will
quickly throw a value error should we hit an unknown authentication
scheme instead of falling back on default behaviours that might not be
correct.

Always throw a key error if part of the authentication header is
missing, instead of defaulting to None or empty strings. This should
also help quickly catch errors if we send out invalid authentication
headers.

aiosip/auth.py

@@ -1,14 +1,25 @@
-import logging
-
-from hashlib import md5
 from collections import MutableMapping
+from enum import Enum
+from hashlib import md5
+import logging
 
 from . import utils
 
 
 LOG = logging.getLogger(__name__)
 
 
+class Algorithm(Enum):
+    MD5 = 'md5'
+    MD5Sess = 'md5-sess'
+
+
+class Directive(Enum):
+    Unspecified = ''
+    Auth = 'auth'
+    AuthInt = 'auth-int'
+
+
 def md5digest(*args):
     return md5(':'.join(args).encode()).hexdigest()
 
@@ -78,35 +89,38 @@ def __parse_digest(cls, header):
         return params
 
     def _calculate_response(self, password, payload, username=None, uri=None, cnonce=None, nonce_count=None):
+        if self.mode != 'Digest':
+            raise ValueError('Authentication method not supported')
+
+        algorithm = Algorithm(self.get('algorithm', 'md5').lower())
+        qop = Directive(self.get('qop', '').lower())
+
         if username is None:
             username = self['username']
         if uri is None:
             uri = self['uri']
-        if cnonce is None:
-            cnonce = self.get('cnonce')
-        if nonce_count is None:
-            nonce_count = self.get('nc')
 
-        if self.mode == 'Digest':
-            algorithm = self.get('algorithm', 'md5')
-            if algorithm == 'md5-sess':
-                ha1 = md5digest(md5digest(username, self['realm'], password), self['nonce'], cnonce)
-            else:
-                ha1 = md5digest(username, self['realm'], password)
-
-            qop = self.get('qop', '').lower()
-            if qop == 'auth-int':
-                ha2 = md5digest(self['method'], self['uri'], md5digest(payload))
-            else:
-                ha2 = md5digest(self['method'], uri)
-
-            if qop in ('auth', 'auth-int'):
-                response = md5digest(ha1, self['nonce'], nonce_count, cnonce, self['qop'], ha2)
-            else:
-                response = md5digest(ha1, self['nonce'], ha2)
-            return response
+        ha1 = md5digest(username, self['realm'], password)
+        if algorithm is Algorithm.MD5Sess:
+            ha1 = md5digest(ha1, self['nonce'], cnonce or self['cnonce'])
+
+        if qop is Directive.AuthInt:
+            ha2 = md5digest(self['method'], uri, md5digest(payload))
         else:
-            raise ValueError('Authentication method not supported')
+            ha2 = md5digest(self['method'], uri)
+
+        # If there's no quality of prootection specified, we can return early,
+        # our computation is much simpler
+        if qop is Directive.Unspecified:
+            return md5digest(ha1, self['nonce'], ha2)
+
+        return md5digest(
+            ha1,
+            self['nonce'],
+            nonce_count or self['nc'],
+            cnonce or self['cnonce'],
+            self['qop'],
+            ha2)
 
     # MutableMapping API
     def __eq__(self, other):