diff --git a/aiosip/auth.py b/aiosip/auth.py
index 2ef1fda..10c1a48 100644
--- a/aiosip/auth.py
+++ b/aiosip/auth.py
@@ -1,7 +1,7 @@
-import logging
-
-from hashlib import md5
 from collections import MutableMapping
+from enum import Enum
+from hashlib import md5
+import logging
 
 from . import utils
 
@@ -9,6 +9,17 @@
 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):
diff --git a/tests/test_registration.py b/tests/test_registration.py
new file mode 100644
index 0000000..92f43d1
--- /dev/null
+++ b/tests/test_registration.py
@@ -0,0 +1,48 @@
+from aiosip import auth
+
+
+AUTH = {
+    'auth_with_qop': 'Digest  realm="asterisk",'
+                     'nonce="1535646722/5d9e709c8f2ccd74601946bfbd77b032",'
+                     'algorithm=md5,'
+                     'qop="auth",'
+                     'nc="00000001",'
+                     'response="7aafeb20b391dfb0af52c6d39bbef36e",'
+                     'cnonce="0a4f113b"',
+    'auth_without_qop': 'Digest  realm="asterisk",'
+                        'nonce="1535646722/5d9e709c8f2ccd74601946bfbd77b032",'
+                        'algorithm=md5,'
+                        'response="05d233c1f0c0ef3d2fa203512363ce64"',
+    'method': 'REGISTER',
+    'uri': 'sip:5000@10.10.26.12',
+    'username': '5000',
+    'password': 'sangoma',
+    'response_with_qop': '7aafeb20b391dfb0af52c6d39bbef36e',
+    'response_without_qop': '05d233c1f0c0ef3d2fa203512363ce64'
+}
+
+
+def test_with_qop():
+    authenticate = auth.Auth.from_authenticate_header(
+        AUTH['auth_with_qop'],
+        AUTH['method']
+    )
+    assert authenticate.validate_authorization(
+        authenticate,
+        password=AUTH['password'],
+        username=AUTH['username'],
+        uri=AUTH['uri']
+    )
+
+
+def test_without_qop():
+    authenticate = auth.Auth.from_authenticate_header(
+        AUTH['auth_without_qop'],
+        AUTH['method']
+    )
+    assert authenticate.validate_authorization(
+        authenticate,
+        password=AUTH['password'],
+        username=AUTH['username'],
+        uri=AUTH['uri']
+    )