Unify authentication errors.

Give the same error if the username doesn't exist or if the password
is wrong.  If we deliver separate errors, we tell the attacker whether
they've picked a valid password or not.

Also, if username doesn't exist, hash the password anyway to avoid
this timing side-channel attack:

1. Invalid Username:

   A. User tries to log in with invalid username.
   B. User name is not found in database.
   C. Password is never hashed.

2. Invalid Password:

   A. User tries to log in with valid username.
   B. User name is found in database.
   C. Password is hashed.

Given that proper password hashing will take a minute, *not* hashing
the password takes so much less time that we've effectively indicated
to the attacker that the username didn't exist, regardless of the
error message.  This way, no such error occurs.

modules/installed/lib/auth.py

@@ -11,6 +11,7 @@
 import cherrypy
 import urllib, hashlib
 import cfg
+import random
 
 cfg.session_key = '_cp_username'
 
@@ -18,29 +19,28 @@ def check_credentials(username, passphrase):
     """Verifies credentials for username and passphrase.
     Returns None on success or a string describing the error on failure"""
 
+    start = time.clock()
+
+    if not username or not passphrase:
+        error = "No username or password."
+        cfg.log(error)
+        return error
+
     u = cfg.users[username]
-    if u is None:
-        cfg.log("Unknown user: %s" % username)
-        return u"Username %s is unknown to me." % username
-    if u['passphrase'] != hashlib.md5(passphrase).hexdigest():
-        return u"Incorrect passphrase."
 
+    elif u is None:
+        # hash the password whether the user exists, to foil timing
+        # side-channel attacks
+        hashlib.md5(passphrase).hexdigest()
+        error = "Bad user-name or password."
+    elif u['passphrase'] != hashlib.md5(passphrase).hexdigest():
+        error = "Bad user-name or password."
+    else:
+        error = None
 
-def check_auth(*args, **kwargs):
-    """A tool that looks in config for 'auth.require'. If found and it
-    is not None, a login is required and the entry is evaluated as a
-    list of conditions that the user must fulfill"""
-    conditions = cherrypy.request.config.get('auth.require', None)
-    if conditions is not None:
-        username = cherrypy.session.get(cfg.session_key)
-        if username:
-            cherrypy.request.login = username
-            for condition in conditions:
-                # A condition is just a callable that returns true or false
-                if not condition():
-                    raise cherrypy.HTTPRedirect("/auth/login")
-        else:
-            raise cherrypy.HTTPRedirect("/auth/login")
+    if error:
+        cfg.log(error)
+    return error
 
 def check_auth(*args, **kwargs):
     """A tool that looks in config for 'auth.require'. If found and it
@@ -60,8 +60,8 @@ def check_auth(*args, **kwargs):
                     raise cherrypy.HTTPRedirect("/auth/login?from_page=%s" % get_params)
         else:
             # Send old page as from_page parameter
-            raise cherrypy.HTTPRedirect("/auth/login?from_page=%s" % get_params) 
-    
+            raise cherrypy.HTTPRedirect("/auth/login?from_page=%s" % get_params)
+
 cherrypy.tools.auth = cherrypy.Tool('before_handler', check_auth)
 
 def require(*conditions):