On Fri, Sep 17, 2010 at 2:24 PM, silky <[email protected]> wrote: > On Tue, Sep 14, 2010 at 10:26 AM, silky <[email protected]> wrote: > > [...] > > > > The cookie might have the hashed result of an SSN. Shouldn't, but might. > > > > I don't think it's hashing that is at risk (they mention AES). I think > > the attack is that you can prepare an invalid encrypted message, and > > brute-force-ish ask ASP.NET to decrypt it, and based on it's answers > > you can get closer to getting the key that the other .NET process is > > using. So, assuming this is so, you should never report a > > cryptographic failure (though, it's still implied, because you don't > > get what you want, so ...). But then again, I know nothing of the > > attack and I'm not an expert, this is just my guess. > > > > The moral is probably to not forget that bruteforce-style attacks are > > still legitimate. > > Details: > http://www.gdssecurity.com/l/b/2010/09/14/automated-padding-oracle-attacks-with-padbuster/ > (different tool but probably similar approach). > > From there it seems that we can conclude what we thought initially: do > not send back .net exceptions for cryptography errors (always > something generic like "invalid username/password combination"). > > Also, a general throttling/blocking of repeated invalid attemps > (perhaps somewhat-exponentially slowed as n increases) is appropriate > (there are other risks associated with doing this; i.e. inconvenience > for users via a DoS style attack on accounts, but you can at least > consider it and other similar approaches).
Sorry to double-post, but I should comment on what I would consider the "real" solution to be: Don't decrypt data you personally didn't encrypt. That is, consider using a HMAC: http://en.wikipedia.org/wiki/HMAC That is, you should ensure you are the person who encrypted the given message. But also, I'm no expert so I hope someone can make a definitive statement on the matter (i.e: Microsoft). -- silky http://dnoondt.wordpress.com/ "Every morning when I wake up, I experience an exquisite joy — the joy of being this signature."
