diff --git a/sleekxmpp/features/feature_mechanisms/mechanisms.py b/sleekxmpp/features/feature_mechanisms/mechanisms.py
index b480d5be..9f3c47f6 100644
--- a/sleekxmpp/features/feature_mechanisms/mechanisms.py
+++ b/sleekxmpp/features/feature_mechanisms/mechanisms.py
@@ -44,15 +44,16 @@ class FeatureMechanisms(BasePlugin):
     }
 
     def plugin_init(self):
-        if not self.use_mech and not self.xmpp.requested_jid.user:
-            self.use_mech = 'ANONYMOUS'
-
         if self.sasl_callback is None:
             self.sasl_callback = self._default_credentials
 
         if self.security_callback is None:
             self.security_callback = self._default_security
 
+        creds = self.sasl_callback(set(['username']), set())
+        if not self.use_mech and not creds['username']:
+            self.use_mech = 'ANONYMOUS'
+
         self.mech = None
         self.mech_list = set()
         self.attempted_mechs = set()
@@ -92,11 +93,7 @@ class FeatureMechanisms(BasePlugin):
         values = required_values.union(optional_values)
         for value in values:
             if value == 'username':
-                result[value] = self.xmpp.requested_jid.user
-            elif value == 'password':
-                result[value] = creds['password']
-            elif value == 'authzid':
-                result[value] = creds.get('authzid', '')
+                result[value] = creds.get('username', self.xmpp.requested_jid.user)
             elif value == 'email':
                 jid = self.xmpp.requested_jid.bare
                 result[value] = creds.get('email', jid)
@@ -106,13 +103,13 @@ class FeatureMechanisms(BasePlugin):
                 else:
                     result[value] = None
             elif value == 'host':
-                result[value] = self.xmpp.requested_jid.domain
+                result[value] = creds.get('host', self.xmpp.requested_jid.domain)
             elif value == 'realm':
-                result[value] = self.xmpp.requested_jid.domain
+                result[value] = creds.get('realm', self.xmpp.requested_jid.domain)
             elif value == 'service-name':
-                result[value] = self.xmpp._service_name
+                result[value] = creds.get('service-name', self.xmpp._service_name)
             elif value == 'service':
-                result[value] = 'xmpp'
+                result[value] = creds.get('service', 'xmpp')
             elif value in creds:
                 result[value] = creds[value]
         return result
diff --git a/sleekxmpp/features/feature_mechanisms/stanza/auth.py b/sleekxmpp/features/feature_mechanisms/stanza/auth.py
index 7b665345..6b6f85a3 100644
--- a/sleekxmpp/features/feature_mechanisms/stanza/auth.py
+++ b/sleekxmpp/features/feature_mechanisms/stanza/auth.py
@@ -40,7 +40,7 @@ class Auth(StanzaBase):
         if not self['mechanism'] in self.plain_mechs:
             if values:
                 self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
-            else:
+            elif values == b'':
                 self.xml.text = '='
         else:
             self.xml.text = bytes(values).decode('utf-8')
diff --git a/sleekxmpp/util/sasl/mechanisms.py b/sleekxmpp/util/sasl/mechanisms.py
index 55ae44dd..80cb7219 100644
--- a/sleekxmpp/util/sasl/mechanisms.py
+++ b/sleekxmpp/util/sasl/mechanisms.py
@@ -467,7 +467,8 @@ class DIGEST(Mech):
             'qop': self.qop,
             'digest-uri': quote(self.digest_uri()),
             'response': self.response(b'AUTHENTICATE'),
-            'maxbuf': self.maxbuf
+            'maxbuf': self.maxbuf,
+            'charset': 'utf-8'
         }
         resp = b''
         for key, value in data.items():
@@ -480,7 +481,7 @@ class DIGEST(Mech):
             if self.cnonce and self.nonce and self.nonce_count and self.qop:
                 self.nonce_count += 1
                 return self.respond()
-            return b''
+            return None
 
         data = self.parse(challenge)
         if 'rspauth' in data: