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Date: Mon, 4 May 2015 21:09:27 +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 07:32:37PM +0200, Stefan.Lucks@...-weimar.de wrote:
> On Mon, 4 May 2015, Solar Designer wrote:
> 
> >I just recalled an earlier discussion in here where Christian Forler
> >felt it was OK for Catena to absolutely rely on salt uniqueness for a
> >feature, and I felt otherwise:
> >
> >http://thread.gmane.org/gmane.comp.security.phc/612/focus=659
> >
> >While uniqueness isn't randomness, the similarity is in strong reliance
> >on a property of the salts.
> >
> >I think this shows my pragmatism.  When this kind of reliance isn't
> >absolutely required for a security feature (there was another way to
> >specify the feature in question), I am against it.  When there's a
> >tradeoff between a not-yet-practically-relevant weakness and a currently
> >practically relevant one, I may well choose to accept the former and
> >mitigate the latter.
> >
> >Arguably, this also shows Catena team's inconsistency. ;-)
> 
> I see the smiley, but I don't get the joke.

The smiley means that I am not being entirely serious about that
statement.  I realize that it's a stretch.  Especially now that I found
you actually have uuid as a separate input from salt.

> This is wrong for two reasons:
> 
> Firstly, there is a simple logical implication, which holds for all salts 
> of decent sizes (like 128 bit for PHC):
> 
>     If the salt is random
>     then it is unique (except with negligible probability).
> 
> Thus, by relaxing the requirement (from random to unique) we get a 
> stronger scheme. And by inverting the requirement (from unique to random) 
> you get a weaker scheme.

As I wrote, "the similarity is in strong reliance on a property of the
salts".  I didn't mean to imply it's a similarly strong property.  You
are correct that it isn't.

> You try to generate a counterexample to the logical implication

No.

> by discussing 12-bit salts from old unix crypt. But this is 2015 and PHC, 
> unix crypt is history.

I referred to the historical example because you (quite reasonably)
insist on salts only being expected to possess whatever properties they
possessed historically.

I am opposing you in this thread for the sake of achieving a balanced
discussion of the topic, by putting our arguments to test.

> Secondly, Christian has actually been pointing out the development for 
> encryption and authenticated encryption. Initially, many (mostly 
> un-authenticated) encryption schemes assumed a random "initial value". 
> Later, authors of encryption schemes just assumed unique nonces. By 
> weakening the schemes, they got stronger cryptography. Currently, the 
> discussion among cryptographers is about robust schemes: Even if you 
> assume a unique nonce for authenticated encryption, does it make sense to 
> maximise the remaining security you can preserve if nonces are 
> accidentally reused?

I don't see what you're arguing with here.  My point in that old
discussion I referred to was precisely that Catena's hash encryption
feature could be made more robust by not relying on unique IDs (and
instead using a block cipher).  Catena team opted for simplicity vs.
(theoretical) robustness.  This may be reasonable, since simplicity (not
needing an extra crypto primitive, nor building an own one out of a
hash) may result in more robust implementations.

The "inconsistency" I mentioned is that Catena team does not always opt
for greater (theoretical) robustness.

> Of course, the Catena team consists of different human beings which may 
> have different opinions on some issues, or even change their opinion over 
> time. Maybe, that is what the smiley was about?

I did have this thought too, but the smiley is primarily about the
statement not being entirely serious.

Alexander

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