phc-discussions - RE: [PHC] Why protect against side channel attacks

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Date: Thu, 25 Jun 2015 00:55:49 +0000
From: Marsh Ray <maray@...rosoft.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] Why protect against side channel attacks

> To recover from a cache-timing attack where the attacker has the salt and usernames, you can simply require users to reset passwords, just like we do normally.

I think you massively underestimate the amount of disruption this cause large service providers. I would think that many sites would see a double-digit percentage of their users just drop out and never come back.

> I am confused why a company would more easily learn of a password database leak than a cache timing attack combined with leaking the salt database.

SQL logs?

> They're passwords are only compromised if an attacker already breached the salt database

Again, these cache timing side channels tend to leak the salts too.

> If an attacker learns that somewhere in the world, an unknown user logged in at exactly 10:38AM, then the system is "broken".

Often the attacker has a position on the network near a targeted user and is able to observe (if not control) precisely the time that the correct password is received by the server.

This paper https://www.usenix.org/system/files/conference/foci12/foci12-final8.pdf outlines these scenarios a bit in the context of timing and deanonymization attacks.

-          Marsh

From: Bill Cox [mailto:waywardgeek@...il.com]
Sent: Wednesday, June 24, 2015 5:33 PM
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Why protect against side channel attacks

On Wed, Jun 24, 2015 at 5:18 PM, Greg Zaverucha <gregz@...rosoft.com<mailto:gregz@...rosoft.com>> wrote:
Yes I (and others, like you Krisztián) have understood the technical mechanics of how the attack would work for a while.  But I hadn't thought through the whole scenario that I described in my email, and the part that was new to me was that there isn't a good way to recover from this type of attack...

To recover from a cache-timing attack where the attacker has the salt and usernames, you can simply require users to reset passwords, just like we do normally.  This gives you a new password/salt database, which should be better protected in the future.

I am confused why a company would more easily learn of a password database leak than a cache timing attack combined with leaking the salt database.  The usual signal a company notices first is a ton of complaints from users with PWNed accounts.

Bill

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