phc-discussions - Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel Attacks

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Date: Mon, 4 May 2015 14:19:38 +0200 (CEST)
From: Stefan.Lucks@...-weimar.de
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel
 Attacks

On Thu, 30 Apr 2015, Bill Cox wrote:

> I consider garbage collector attacks to be a more common side-channel than cache-timing.  Yescrypt does a good job overwriting early memory
> fast, so that there's less chance of a devistating garbage collector attack.  I prefer what I did in TwoCats, which minimizes early garbage
> collector attacks by using a Catena-style increasing garlic, up to some reasonable size.

Either attack cache timing and garbage collector, is unlikely to happen, 
and either will eventually catch people not aware of these issues.

I'd consider a password hash function vulnerable to either a time bomb -- 
most of the time, nothing happens, but eventually, someone gets killed.

> As for side-channel resistance, Lyra2 (especially if we enhance it's 
> early walk pattern) has a cache-timing resistant first loop, 
> significantly reducing the damage from a successful cache-timing 
> attack. 

Agreed. Cache-attacks don't allow to significantly speed-up "conventional" 
attacks (by more than a factor of two or four or so).

There is still the issue of enabling an offline-attack, where it would be 
impossible otherwise, because the password file has not been compromised. 
Alexander rightfully pointed out that the adversary needs to know the salt 
for that kind of attack, but see my response to his post.

> Cache timing attacks remain a laboratory experiment,

Not really. See, e.g, <https://eprint.iacr.org/2014/435.pdf>, which 
appears to be a seriuos practical threat to virtual servers.

> Argon2i or Catena as a sole winner would be much worse for defense against these real-world off-line attacks than Scrypt. 

While I don't quite agree with your usage of "much", this is actually the 
reason why I would prefer more than one winner.

> However, I would prefer to have two winners, one from the fast 
> large-memory multi-threaded group (Argon2d, Lyra2, and Yescrypt), and 
> one that is cache-timing attack resistant (either Catena or Argon2i).  
> We could recommend the cache-timing resistant algorithm for algorithms 
> running on VMs which may also be running an attacker's code, and the 
> unpredictable algorithm for full-disk encryption, dedicated 
> authentication servers, and other cases where cache-timing attacks are 
> not likely.

Agreed!

Stefan

------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
uni-weimar.de/de/medien/professuren/mediensicherheit/people/stefan-lucks
--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--