auth/auth_tkt.py: enable overriding digest algorithms

Currently, mod_auth_tkt supports also SHA256 and SHA 512 [1],
not just plain MD5.  Quoting:

----v----
The default is MD5, which is faster, but has now been shown to be vulnerable
to collision attacks. Such attacks are not directly applicable to
mod_auth_tkt, which primarily relies on the security of the shared secret
rather than the strength of the hashing scheme. More paranoid users will
probably prefer to use one of the SHA digest types, however.

The default is likely to change in a future version, so setting the digest
type explicitly is encouraged.
----^----

Thus, enable it also in this implementation so one can optionally switch
to a stronger secure hash.

Backward compatibility should be untouched as ``md`` is being passed
as a default kwarg.  The only change affecting external world is
a new parameter required at ``calculate digest`` (specifying the
digest to use), but as it has probably no use outside the module,
this is a non-issue.  Alternatively: another optional kwarg.

[1] http://linux.die.net/man/3/mod_auth_tkt

1 files changed, 27 insertions, 14 deletions

diff --git a/paste/auth/auth_tkt.py b/paste/auth/auth_tkt.py
index 8dfce00..a20e498 100644
--- a/paste/auth/auth_tkt.py
+++ b/paste/auth/auth_tkt.py
@@ -47,6 +47,8 @@ from paste import request
 from urllib import quote as url_quote
 from urllib import unquote as url_unquote
 
+DEFAULT_DIGEST = md5
+
 
 class AuthTicket(object):
 
@@ -55,8 +57,8 @@ class AuthTicket(object):
     the shared secret, the userid, and the IP address.  Optionally you
     can include tokens (a list of strings, representing role names),
     'user_data', which is arbitrary data available for your own use in
-    later scripts.  Lastly, you can override the cookie name and
-    timestamp.
+    later scripts.  Lastly, you can override the timestamp, cookie name,
+    whether to secure the cookie and the digest algorithm.
 
     Once you provide all the arguments, use .cookie_value() to
     generate the appropriate authentication ticket.  .cookie()
@@ -86,7 +88,7 @@ class AuthTicket(object):
 
     def __init__(self, secret, userid, ip, tokens=(), user_data='',
                  time=None, cookie_name='auth_tkt',
-                 secure=False):
+                 secure=False, digest_algo=DEFAULT_DIGEST):
         self.secret = secret
         self.userid = userid
         self.ip = ip
@@ -98,11 +100,12 @@ class AuthTicket(object):
             self.time = time
         self.cookie_name = cookie_name
         self.secure = secure
+        self.digest_algo = digest_algo
 
     def digest(self):
         return calculate_digest(
             self.ip, self.time, self.secret, self.userid, self.tokens,
-            self.user_data)
+            self.user_data, self.digest_algo)
 
     def cookie_value(self):
         v = '%s%08x%s!' % (self.digest(), int(self.time), url_quote(self.userid))
@@ -132,21 +135,22 @@ class BadTicket(Exception):
         Exception.__init__(self, msg)
 
 
-def parse_ticket(secret, ticket, ip):
+def parse_ticket(secret, ticket, ip, digest_algo=DEFAULT_DIGEST):
     """
     Parse the ticket, returning (timestamp, userid, tokens, user_data).
 
     If the ticket cannot be parsed, ``BadTicket`` will be raised with
     an explanation.
     """
+    digest_hexa_size = digest_algo().digest_size * 2
     ticket = ticket.strip('"')
-    digest = ticket[:32]
+    digest = ticket[:digest_hexa_size]
     try:
-        timestamp = int(ticket[32:40], 16)
+        timestamp = int(ticket[digest_hexa_size:digest_hexa_size + 8], 16)
     except ValueError, e:
         raise BadTicket('Timestamp is not a hex integer: %s' % e)
     try:
-        userid, data = ticket[40:].split('!', 1)
+        userid, data = ticket[digest_hexa_size + 8:].split('!', 1)
     except ValueError:
         raise BadTicket('userid is not followed by !')
     userid = url_unquote(userid)
@@ -158,7 +162,8 @@ def parse_ticket(secret, ticket, ip):
         user_data = data
 
     expected = calculate_digest(ip, timestamp, secret,
-                                userid, tokens, user_data)
+                                userid, tokens, user_data,
+                                digest_algo)
 
     if expected != digest:
         raise BadTicket('Digest signature is not correct',
@@ -169,15 +174,16 @@ def parse_ticket(secret, ticket, ip):
     return (timestamp, userid, tokens, user_data)
 
 
-def calculate_digest(ip, timestamp, secret, userid, tokens, user_data):
+def calculate_digest(ip, timestamp, secret, userid, tokens, user_data,
+                     digest_algo):
     secret = maybe_encode(secret)
     userid = maybe_encode(userid)
     tokens = maybe_encode(tokens)
     user_data = maybe_encode(user_data)
-    digest0 = md5(
+    digest0 = digest_algo(
         encode_ip_timestamp(ip, timestamp) + secret + userid + '\0'
         + tokens + '\0' + user_data).hexdigest()
-    digest = md5(digest0 + secret).hexdigest()
+    digest = digest_algo(digest0 + secret).hexdigest()
     return digest
 
 
@@ -234,6 +240,12 @@ class AuthTKTMiddleware(object):
         page will be shown as usual, but the user will also be logged out
         when they visit this page.
 
+    ``digest_algo``:
+        Digest object constructor compatible with named constructors from
+        ``hashlib``.
+        Defaults to ``md5``, as in mod_auth_tkt.  The others currently
+        compatible with mod_auth_tkt are ``sha256`` and ``sha512``.
+
     If used with mod_auth_tkt, then these settings (except logout_path) should
     match the analogous Apache configuration settings.
 
@@ -253,7 +265,7 @@ class AuthTKTMiddleware(object):
     def __init__(self, app, secret, cookie_name='auth_tkt', secure=False,
                  include_ip=True, logout_path=None, httponly=False,
                  no_domain_cookie=True, current_domain_cookie=True,
-                 wildcard_cookie=True):
+                 wildcard_cookie=True, digest_algo=DEFAULT_DIGEST):
         self.app = app
         self.secret = secret
         self.cookie_name = cookie_name
@@ -264,6 +276,7 @@ class AuthTKTMiddleware(object):
         self.no_domain_cookie = no_domain_cookie
         self.current_domain_cookie = current_domain_cookie
         self.wildcard_cookie = wildcard_cookie
+        self.digest_algo = digest_algo
 
     def __call__(self, environ, start_response):
         cookies = request.get_cookies(environ)
@@ -282,7 +295,7 @@ class AuthTKTMiddleware(object):
             # Also, timeouts should cause cookie refresh
             try:
                 timestamp, userid, tokens, user_data = parse_ticket(
-                    self.secret, cookie_value, remote_addr)
+                    self.secret, cookie_value, remote_addr, self.digest_algo)
                 tokens = ','.join(tokens)
                 environ['REMOTE_USER'] = userid
                 if environ.get('REMOTE_USER_TOKENS'):