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Date: Wed, 24 Jun 2015 21:44:42 +0000
From: Marsh Ray <>
To: "" <>
Subject: RE: Why protect against side channel attacks

To Greg's excellent summary I would just add a couple of further points, probably all have been mentioned before.

While the salt isn't generally supposed to be public:

a.       The security benefit of a PHF comes only after there has been a breach, so design of a PHF ought to assume everything is known except for the secret password itself. That's like Kerckhoffs 2nd. But unlike Kerckhoffs, we cannot tolerate leaks of salt or hash either. Kerckhoffs doesn't apply nicely here because the 'secret' lives in a gray area of insufficient entropy.

b.      Cache timing side channels tend to leak information about the salt too.

> Suppose a slow-memory attack was demonstrated, and suddenly the attacker hash rate goes up by 10x

While this sounds dramatic, and is technically a cryptographic weakness, it's not as big of an impact as it sounds. It's a loss of 3.2 bits of security which could be approximately compensated for by increasing the minimum password length by a single character.

My feeling is that 10x is probably within the range of variation that will be seen in the field. For example, did you get a faster server and forget to re-tune your PHF parameters? Or did you tune your work factor to take 20 ms when you really could have afforded 200 ms? Well, by that measure you've lost 10x security.

I guess I'm saying that, while I don't think this will happen, if a 10x rate or power efficiency attack were discovered in the future personally I could still consider PHC to have been a success.

-          Marsh

From: Greg Zaverucha []
Sent: Wednesday, June 24, 2015 12:12 PM
Subject: [PHC] Why protect against side channel attacks

Some of the recent discussion has again turned to whether side-channel resistance is worthwhile.  Marsh and I were discussing side-channel protections a while back and there was one point that I found really compelling.  I've tried to reproduce it here, apologies if this was already discussed on the list.

Suppose you use password hashing function X in many products and services, in a range of contexts (on mobile devices, PCs, servers, in VMs, etc...).  Now suppose that cache-timing attacks against X are shown to be practical, someone has released an attack tool, and it's in the news under the name PasswordBleed, and someone has even made a logo for it.  The tool needs the salt and username.  It works by observing the cache access pattern of a victim process/VM, which narrows down the search space to a set of passwords small enough to attack online (i.e., the tool re-computes the hashes for passwords in a dictionary and compares their access patterns to the observed pattern, those that are close are kept). So no breach has occurred and no hashes were leaked.  (I realize that in most systems the salts aren't public, but I think we all agree that X should not get weaker if the salt is known).

How do you respond/recover in this situation?  Which passwords were compromised? I think the only way to be sure is to have all users change their passwords (and replace X), unless you can have confidence that at no point an attacker had the opportunity to observe the side channel.  But the attack leaves no trace, and would likely only affect operation of the system in a very minor way.  Against this type of attack, storing the password in the clear would have been better (since at least it would *require* a breach of the password file).

Now suppose X was side-channel resistant, and consider an algorithmic advance for cracking X.  Suppose a slow-memory attack was demonstrated, and suddenly the attacker hash rate goes up by 10x.  In this case the response is much easier - since you can be sure that only accounts where the hashes leaked need password resets.  The unbreached hashes can be re-hashed with updated parameters for X, or with a new algorithm Y.  There's a period when the defense in the event of a breach is lower than desired, but a breach must occur for it to be a risk.

To summarize, hashing with a function that has side channels introduces a new attack vector, and can put passwords at risk even without a breach.


PS - I think the attack tool described would work equally well against a design like Lyra2 that only does password-dependent accesses part way through the computation.

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