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Date: Sun, 9 Mar 2014 07:48:55 -0400
From: Bill Cox <>
Subject: Re: [PHC] Are password trailing 0's a problem?

On Sat, Mar 8, 2014 at 11:14 PM, Solar Designer <> wrote:
> On Sat, Mar 08, 2014 at 10:59:58PM -0500, Bill Cox wrote:
>> However, while this flaw is certainly not a critical weakness, and
>> obviously Escript in compatibility mode needs to continue using PBKDF2
>> as-is, for non-compatible mode, or entirely new password hashing
>> systems, should we live with such flaws or try to eliminate them?
> We should try to eliminate them.
> Regarding your idea re: all PHC submissions doing it in the same way,
> I'm not so sure.  In each case, the choice might be affected by other
> crypto primitives and constructions that a given PHC submission would
> use anyway, so that its total complexity is less.  For escrypt, this is
> a reason for its non-compat modes to reuse PBKDF2 anyway, just working
> around its shortcomings.
> Alexander

I agree that they wont all be the same.  Still, hacking our own will
invite FUD-mongers... let me demonstrate :-}

Assume an attacker knows you XOR the password length on the output of
PBKDF2, and doesn't know your password, but for some reason has the
ability to append an arbitrary number of 0's to it before it's hashed
by PBKDF2, and he can see the resulting hash of PBKDF2 after you XOR
in the password length.  By seeing the result hash for two logins with
one extra 0's appended for one of them, he just XORs the two hashes
together and can see if the LSB of the password length is 0 or 1,
because if it was a 1, then the 2nd bit will be a 1 in the XOR of the
hashes.  With a few such hashes, he can find the exact password

That's a dumb attack, but do you want people worrying about it?  You
might be better off just using PBKDF2 with it's known input collision
problem.  It is hard to create FUD over an algorithm that has years of
use behind it.

I think a few of the other PHC entries will also have to defend
against FUD as a result of to hacking around PBKDF2.  Catena has a
good KDF at the start, better than PBKDF2, IMO, but if I want to be a
PITA and create FUD about Catena, I think I can be pretty effective

Here's the code (slightly compacted by me):

  uint8_t t[5] = {0xFF, tweak_id, lambda, hashlen, saltlen};
  __Hash1((uint8_t *) data, datalen, x);
  __Hash4(t, 5, x, H_LEN, (uint8_t *) pwd,  pwdlen, salt, saltlen, x);
  memset(x+hashlen, 0, H_LEN-hashlen);

Oh help, the sky is falling!  This KDF will end the world!  Well,
maybe not, but here's some FUD I could spread.

First, I can't figure out what 0xFF and tweak_id are for.  They aren't
used in any way other than in the "tweak" structure t.  If I wanted, I
might hack Blake2 (Catena has it's own copy) to provide a back-door
when the input starts with 0xFF, <my 8-bit secret>, 3, 64, 16.  OK,
that's pretty dumb, but what else if the 0xFF for?  Maybe it's clear
to the PHC judges, but it's not to me.

He does two hashes.  In the first, he hashes the data.  This is to
compress the data to a small size, since it can be any 32-bit length,
so I understand why he makes this call.  Unfortunately, this call
could leak sensitive timing information if the lower level hash
function runs in time proportional to the data size.  If a user passes
a server's master key as "data", he's going to get a huge boost in
security.  An attacker with the password database wont be able to do
anything without the master key, so the master key might be a higher
value target than the password.  If this call runs in time
proportional to the data length, how do we know an attacker can't
detect it?  This is a weakness, as far as I can tell.

In the second hash, he hashes the password directly, without padding,
leading to a potential leaking of the password length to an attacker.
We could easily add padding, but then we'd also have to hash in the
password length, or we'd wind up with input collisions just like HMAC
and systems that rely on it including PBKDF2 and Scrypt.  Adding the
password length to t along with padding the password would fix that.

His call to __Hash4 seems well thought out to me.  If he had not
hashed the salt size, then I could easily generate collisions, because
__Hash4 very likely just concatenates the input and hashes it.  With
salt and password being adjacent, I could just move bytes back and
forth between the password and salt, and get the same resulting hash.
Catena didn't make this mistake, but they also didn't hash the
password length, but did hash the salt length - the minimum required
amount to avoid this mistake.

I don't know why he's not concerned about leaking timing information,
but I also can't figure out why the author of HMAC (which does padding
to hide timing information) would later be an author of HKDF, which
does no padding.  Have security guys done a group-think session and
decided that padding was a bad idea?  Is running in constant time no
longer important?  If so, why are they not concerned about an attacker
who monitors the power rails, which easily can show what routines are
executing, and for how long?

In 1989, I debugged CPUs in a group at HP that built the Spectrum CPU,
and that thing had an embedded feature that allowed monitoring of the
on-chip power rails and gave us plots of voltage vs time.  It had to
be a repeating waveform to work, but in 2014, Intel might have one
that can give you power rail waveforms in one go.  Even if they have
old-school tech, I can still take that snapshot if I can get a user to
somehow call the KDF repeatedly.  I know with a scope probe on a power
rail, I could have told you exactly what part of the code was
executing during boot.

Finally, there's no provision for clearing the password or data.  I
would be pretty mad if my password just sat around in memory for 5
seconds while I hashed many GiB of memory.  In the Blake2 code, I see
they are paranoid about securely clearing buffers as soon as possible,
using their "secure_zero_memory" function, which I copied.  Even
zeroing memory is apparently hard.

Anyway, I think this stuff is hard enough that I don't want to roll my
own  I'd much rather collaborate with the other PHC guys.


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