Date: Mon, 4 May 2015 19:39:39 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel Attacks

On Mon, May 04, 2015 at 02:37:48PM +0200, Stefan.Lucks@...-weimar.de wrote:
> On Thu, 30 Apr 2015, Solar Designer wrote:
> >While you're mostly correct, you're somehow omitting important detail.
> >As discussed before, salting may defeat side-channel attacks on password
> >hashes, as long as the salts are not known/predictable by the attacker.
> >In your example, "without knowing the password file, Mallory would"
> >typically also not know the salts, so would not be able to mount a
> >side-channel attack.
> 
> Agreed. I should have been more precise here.
> 
> Having spent much of my academic life on performing cryptanalysis, i.e., 
> on attacking schemes, the idea to rely on the randomness of the salt 
> doesn't quite convince me, though. I understand the "random" salt mostly 
> as a nonce, i.e., a value that is supposed to never repeat. You can safely 
> use a low-quality entropy source, such as your system clock, to derive 
> your "random" values. You can even use a counter, together with (a hash 
> of) your sever ID.

That's the traditional understanding, yes.  A decade ago or so I would
have been mostly with you on it.  Now I'm not so sure.

As the security community eventually started to recognize timing attacks
including on password hash comparisons as something to actually consider
and deal with, it became just too convenient to realize that
cryptographically random salts may prevent those.  The same applies to
cache-timing attacks on password hashes themselves.  And these
maybe-vulnerabilities are not new: bcrypt has been around since 1997,
and strcmp() is being used to compare crypt() hashes since 1970s.  As it
happens, some systems have already been using cryptographically random
salts for a while now (IIRC, OpenBSD has been since the introduction of
bcrypt in 1997), preventing these attacks in that way (even if the
attack were possibly not understood back then).  This is why I said
"maybe-vulnerabilities".

Now our options are: use solely hashes and comparison functions that are
side-channel safe(*) even without cryptographically random salts, or/and
require cryptographically random salts.  The latter hardens existing
software too - such as uses of strcmp() on password hashes in lots of
existing software.

(*) Not only constant-time, but also including approaches such as
https://www.isecpartners.com/blog/2011/february/double-hmac-verification.aspx
http://openwall.info/wiki/people/solar/algorithms/challenge-response-authentication#Timing-attacks
which can be easily applied to hash comparisons.

Another aspect is precomputation.  When salts are predictable, like in
your examples, an attacker may pre-hash a wordlist (or whatever) to the
expected salt(s) before actually obtaining a copy of the target system's
hashes.  Then they get usable cracked passwords sooner.  This is
probably not happening in practice yet(*), but it might be of comparable
likelihood to having a cache-timing attack mounted in practice.

(*) The situation with WPA-PSK SSIDs is similar, with rainbow tables
having been pre-generated for likely SSIDs, but it's not quite it yet.

> As long as the salt does not repeat, your password hashing scheme should 
> be OK. (And even very rare exceptions of the same salt for two independent 
> passwords would not harm too much.) But to defeat the kind of side-channel 
> attacks we talk about, the salt would now have to be unpredictable for the 
> adversary. I.e., you would need a high-quality random source for to 
> generate your salts -- pretty much the same thing you would use for 
> generating secret keys.
> 
> (Well, not quite. But close. You might be happy with, say, a 80-bit enropy 
> salt, where you would expect your 128-bit key to have full 128-bit 
> entropy.)

Yes, something like that.  Here's Marsh Ray's attack on 12-bit salts:

http://lists.randombit.net/pipermail/cryptography/2011-December/001866.html
http://lists.randombit.net/pipermail/cryptography/2011-December/001874.html

> >However, a relevant question is: are we focusing on avoiding unlikely 
> >yet potentially fatal special cases, or are we trying to reduce the 
> >percentage of passwords that will get cracked in a more likely case?  I 
> >expect that answers will vary.
> 
> Agreed. The tradeoff, potentially a few more bits of pseudo-entropy for 

Yes, and we definitely should realize that potential in order for use of
a cache-timing unsafe scheme to make sense.  This is a reason why e.g.
original Argon isn't a good choice.

> all cases versus an almost total loss for some cases. This is precisely my 
> reason I wanted to discuss this.

It might be useful to keep in mind that there's always some risk of an
attacker just hitting the right password in a few tries. ;-)  Unlikely,
sure.  Possible, yes.

I am not taking sides.  Rather, I am being pragmatic, and since I am
replying to you I am emphasizing a point of view different from yours,
to make this discussion useful.

At this time, going from bcrypt to Catena for password hashing is
usually a downgrade.  Things might or might not be substantially
different in 5+ years from now, as faster attacks on bcrypt become
generally available and higher cost settings become usable by more
systems (this will play in favor of Catena and other schemes well suited
only for higher memory usage).  Right now, the target performance
figures are often in the thousands of requests/second/machine(*), and
side-channel attacks are by far not a top threat.

(*) Facebook's use of scrypt at 16 MB (so probably only around 1000/s
per 16-core machine) is a notable exception here.  I think their running
of scrypt on "frontends" (whatever this means) is key to this.

Alexander

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