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Date: Sat, 18 Jan 2014 17:41:06 +0400
From: Solar Designer <>
Subject: cost upgrades (Re: [PHC] Scripting memory (not so) high vs Catena in PHP (with optimizations))

On Mon, Jan 13, 2014 at 06:24:40PM +0100, Christian Forler wrote:
> On 13.01.2014 11:55, Solar Designer wrote:
> > I think this is just one of many issues/drawbacks with supporting and
> > using hash upgrades.  Another/worse drawback is that area-time cost for
> > upgraded hashes is lower than it can be for same-duration new hashes.
> > 
> > The hash upgrading idea would have been great in 1990s, when we were not
> > trying to use much memory.  These days, it is not as great.  (Yet I had
> > it on my slides from PHDays 2012 and later.)
> I respectfully disagree. :-)

Thank you for commenting on this.

To clarify: I still think that being able to upgrade hashes without
knowing the passwords is a valuable feature, but it does involve
non-trivial tradeoffs, making the feature somewhat less valuable.

> Thanks to Moore's law you have to upgrade your security parameters at
> some point in the future (Just compare a Amiga 2000 vs current COTS PC).
> Without CI-updates, todays password hashes will become easy prey for
> future (say 2030) state-of-the-art password-cracking frameworks. It is a
> common wisdom that from time to time security parameters has to be updated
> Do you really recommend use todays security parameters in a world where
> computers have both 2^10 times more memory and computational power?
> The problem of inactive users will not vanish,

I agree with you on this.  And yes, it's primarily for the inactive
users, whose accounts are nevertheless still active and/or who
potentially/presumably are reusing the same or similar passwords
elsewhere (with those accounts still active, or encrypted data still
relevant to attackers).

> and I think the PHC candidates should address this issue. :-)

If someone were to submit a compute-only candidate (not trying to use a
configurable amount of memory), then I'd definitely expect it to support
the (time only) cost upgrades.  Such a candidate would unlikely be
selected among the "winners", though, maybe unless it targets and "wins
for" certain low-memory-and-never-growing use cases (do those exist?)

With builtin support for cost upgrades in a memory-hard password hashing
scheme, ignoring the shortcut wiping idea I had mentioned before for
now, we have to choose between two non-perfect options:

1. Significantly lower area-time cost of upgraded hashes as compared to
same-duration newly computed hashes - or, depending on the design and
interface, even of possible initial hashes as well, as compared to
same-duration non-upgradable hashes.

2. High granularity of possible hash upgrades - or, depending on the
design and interface, even of possible initial settings as well.

If the granularity is 2x, then one upgrade gives us AT cost of
2*Norig^2 = Nnew^2/2 instead of Nnew^2, so twice lower.  Two upgrades
(2x time each, 4x time total) give us 2*Norig^2+(2*Norig)^2 = 6*Norig^2,
as compared to (4*Norig)^2 = 16*Norig^2, which would be possible without
upgrades, so 6/16th of what we could have.  With many upgrades, I think
this converges to 1/3 of AT cost for non-upgradable hashes.

Higher granularity improves this, but allows only for less frequent
upgrades.  Could be an acceptable tradeoff, though.

And there's the design and interface issue I alluded to above.  If both
never-upgraded and already-upgraded hashes are of exact same type (no
flag is set on upgrade, etc.), then Norig above has to be very small (it
has to correspond to the quickest hash we're able to compute with the
specified scheme, anywhere).  This means that even never-upgraded hashes
suffer from the lower AT cost (than would be otherwise possible for
their computation time), and the possible initial settings suffer from
the forcibly higher granularity.  This may be addressed by treating
never-upgraded and already-upgraded hashes differently, setting a flag
(encoded along with the hashes) on upgrades, so that the "pipe
narrowing" and the higher granularity are only enabled when they have to
be for the already performed upgrade and for possible further upgrades.
Unfortunately, this adds complexity.

With even more complexity, it is possible to do better: record each
upgrade's N along with the hash.  Then no extra "pipe narrowings" and no
higher granularity than is absolutely necessary for the actual upgrades.
Drawback: even higher complexity.

These are some nasty tradeoffs.

As you're well aware, hash upgrades are also possible without having
them built into the password hashing scheme.  Yes, this is not as clean
and is unlikely to be available on specific deployments without needing
code changes.

Now, to your example with year 2030.  A lot might change by then.
Suppose PBKDF2' were defined in 1999 to support cost upgrades (and this
wouldn't even have the drawbacks mentioned above since PBKDF2 is
compute-only anyway).  This means that technically we could upgrade
existing PBKDF2' hashes by merely adding iterations now.  Would we do
that e.g. shortly after PHC concludes (hopefully giving us a good,
modern password hashing scheme)?  No, if practical on a given deployment
(and I recognize that it's an "if"), I'd rather wrap those PBKDF2' hashes
in the modern hash.  Are you guessing that a significantly better scheme
won't appear by 2030 (to justify the switch) or that e.g. Catena would
be just good enough not to bother switching?  That's actually quite
possible, given how bcrypt is still good enough for many uses now.

Overall, I am not sure.  It's a somewhat valuable feature vs. some
combination of sometimes-lower security and added complexity.  With high
complexity, there may be no security drawbacks for cases when the
feature is not used (or hasn't been used yet).  Maybe we should explore
that option, then see if the added complexity is acceptable.


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