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Date: Fri, 14 Mar 2014 23:42:41 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] "Why I Don't Recommend Scrypt"

On Fri, Mar 14, 2014 at 12:42:47PM +0100, Thomas Pornin wrote:
> On Fri, Mar 14, 2014 at 12:11:05PM +0400, Solar Designer wrote:
> > Thank you for your opinion.  Does this imply that you'd like the PHC to
> > introduce a new scheme that would use CPU only (not RAM)?
> 
> Actually, I expect PHC to ultimately provide a portfolio of "deemed
> good" schemes, somewhat like eSTREAM (and unlike AES or SHA-3).

Right, but I think that ideally this portfolio should be small.

> In
> particular, there are extra features that any specific algorithm may or
> may not provide, e.g.

I realize you're merely providing examples, yet I'd comment on them:

> the ability to increase the work load (iteration
> count) without having access to the source password,

Maybe we should ask for this to be added during the "tweaks" phase, if
it's not implemented in a submission that gets selected as a finalist,
yet the feature is deemed desirable.  I think that this property alone
is not a sufficient reason to have an extra scheme in the portfolio.

> or a "fast path"
> allowing much faster computation of a given hash if a specific secret
> key is known.

As I mentioned before, I think a shortcut like this could be used for
more effective memory cost upgrades than what e.g. Catena provides.
It'd be curious to see that implemented.  As to simply "faster"
computation (and thus "slower" after the secret is wiped), this isn't
worth the complexity and added risks (e.g. of imperfect wipe), I think,
because the same may be achieved by simply adding iterations.  This
trick is beneficial specifically for memory cost upgrades, I think.

> Or even more advanced features such as the ability to
> delegate computation to an external untrusted system.

This is an interesting topic, which somehow we haven't touched on this
list yet, although at least Jeremy Spilman and I thought of it before.

Arguably, it falls in the same category as hash upgrades and client-side
pre-hashing, because it can also be implemented on top of an existing
password hashing scheme lacking this feature as a built-in one -
although the resulting hashes are then incompatible with those produced
by the hashing scheme on its own (that is, when it's applied directly to
a plaintext password and its result is stored as-is).  I agree that
having features like that defined within the password hashing scheme
itself may be beneficial.  Moreover, there are probably ways to
implement this feature as a built-in better than it'd be implemented
externally, by introducing blinding with some per-hash-computation
randomness rather than merely by relying on fixed secrets (the
difference is in how much metadata is leaked to the external party).

> Some candidates
> may appear to be extremely good in the CPU+RAM tunable usage area while
> lacking some of these extra features, so a portfolio will probably be a
> more appropriate result.

I think some of the features you mentioned aren't very difficult to add
to any scheme, so it may be better to do that during the "tweaks" phase
(or before) than to increase the number of schemes in the portfolio.
Of course, the author(s) of that otherwise "extremely good" schemes
would need to be willing to make or accept such additions.

> It also implies that there is room for improvements for CPU-only (or
> CPU-and-small-RAM-only) schemes.

Definitely!

> E.g. with bcrypt you cannot increase
> the iteration count in an "offline" way (you have to start again from
> the source password),

True, although this can be done in a higher-level algorithm running on
top of bcrypt.  Of course, it's no longer bcrypt proper, then - and yes,
it'd be nice to have this built-in.

> there is no "fast path",

No need for it if the memory cost is not upgradable.

> the output is 192 bits only,

184, and that's more than enough.  Yes, PHC stipulates 256+ bits for
extra safety margin and for use as KDF.  But for password hashing
bcrypt's 184 bits is no problem.

> the input password length is limited

Unfortunately, yes.

> (nominally 56 bytes, in

The actual limit is 72 chars.  56 is an error in the bcrypt paper, and
it's repeated in the scrypt paper and elsewhere.

> practice more often 55 or 51 characters, depending on the implementation
> library),...

Huh?!  I've never seen implementations of bcrypt like that.  Have you?
Got any pointers?

You might be referring to some implementations of Blowfish, but are
those relied upon by any implementation of bcrypt?  If so, that's a bug,
and it's not bcrypt proper.  But like I said, I am unaware of such
implementations.

I actually thought of making a PHC submission based on bcrypt,
correcting the issues/limitations you mentioned and making a few other
enhancements (including possibly making the memory size tunable) - but I
didn't feel it'd be worth introducing yet another password hashing
scheme to this world.

> PBKDF2 avoids some of these issues (e.g. it does not limit
> input or output length) while adding a few extra ones (PBKDF2+SHA-256 is
> GPU friendly; computing time is proportional to the output length;...).

Right.  Even worse, PBKDF2's computing time is proportional to the
output length only for the defender, but not for the attacker - if the
attacker has the derived key (which is the case when PBKDF2 is directly
used for password hashing, and the hashes are then attacked).  This is
mitigated by setting dkLen to at most the underlying hash's output size.
Not great.

> Now it is very fine that some PHC candidates will use RAM and be tunable
> in that respect. This is a research area that is in need of deep
> exploration, and there are contexts where gobbling up RAM by the
> gigabyte is the right thing to do for password hashing (e.g. when
> deriving the symmetric key for disk encryption).

Agreed, and it's not only for the "by the gigabyte" case.  I think the
whole range of memory cost settings from KB to multi-GB may be considered.

> I don't think that there will be ONE winner. I don't thinkg that there
> should be one and only one winner

I also don't think so, but I also don't think the portfolio should be
large.  There has to be justification for adding each additional scheme
to the portfolio, and maybe such justification should include rationale
on why those same additional features or whatever were not added to one
of the other schemes in the portfolio instead.  "The authors didn't do
it at the time of submission" might not be sufficient justification -
it'd only explain how it happened in the PHC process, not excuse us for
subjecting the industry to an unnecessarily large number of "approved"
password hashing schemes.

I think we might need a round of cross-pollination, as someone (I don't
recall who) called it when I suggested it in here a few months ago.

Alexander

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