Securing 'pickle'

[Paul Rubin]

Dave Cole <djc at object-craft.com.au> writes:
>     def pickle_sign(self, text):
>         m = md5.new()
>         m.update(self.__secret)
>         m.update(text)
>         text = m.digest() + text
>         return text

Use instead:

     def pickle_sign(self, text):
         m = hmac.new(self.__secret)
         m.update(text)
         text = m.digest() + text
         return text

     def pickle_unsign(self, text):
         digest = text[:16]
         text = text[16:]
         m = hmac.new(self.__secret)
         m.update(self.__secret)
         m.update(text)
         if m.digest() == digest:
             return text
         raise CookieError # or something like that

Differences are: 
   
1) use hmac instead of md5, to prevent appending attack

2) raise an exception if authentication fails, and handle it of
   course.  The null string might be a valid cookie value and
   returning it on authentication failure lets attacker force return
   of a null string.

There's another issue, mentioned in other post: if you have several
pickles in separate cookies, you should sign them all together, not
use an independent signature for each pickle like your class does.

Say session 1 sets pickles A and B, and session 2 sets pickles C and
D.  With independent signatures, an attacker controlling both sessions
can send back A and C, or B and D, with results that might confuse
the application.