diff --git a/offlineimap/head/offlineimap/imapserver.py b/offlineimap/head/offlineimap/imapserver.py
index 9981faf..bc254cb 100644
--- a/offlineimap/head/offlineimap/imapserver.py
+++ b/offlineimap/head/offlineimap/imapserver.py
@@ -108,18 +108,9 @@ class IMAPServer:
         ui = UIBase.getglobalui()
         challenge = response.strip()
         ui.debug('imap', 'md5handler: got challenge %s' % challenge)
-        while len(challenge) < 64:
-            challenge += "\0"
 
-        if len(challenge) > 64:
-            challenge = md5.new(challenge).digest()
-
-        ui.debug('imap', 'md5handler: post-processed challenge is %s' % \
-                 repr(challenge))
-        msg = self.getpassword()
-        reply = hmac.new(challenge, msg)
-        retval = self.username + ' ' + \
-                     reply.hexdigest()
+        passwd = self.getpassword()
+        retval = self.username + ' ' + hmac.new(passwd, challenge).hexdigest()
         ui.debug('imap', 'md5handler: returning %s' % retval)
         return retval