[cryptography] PBKDF2 + current GPU or ASIC farms = game over for passwords (Re: TLS2)

Wasa wasabee18 at gmail.com
Mon Sep 30 13:52:47 EDT 2013


On 30/09/13 16:43, Adam Back wrote:
> On Mon, Sep 30, 2013 at 02:34:27PM +0100, Wasa wrote:
>> On 30/09/13 10:47, Adam Back wrote:
>>> Well clearly passwords are bad and near the end of their life-time with
>>> GPU advances, and even amplified password authenticated key 
>>> exchanges like
>>> EKE have a (so far) unavoidable design requirement to have the server
>>> store something offline grindable, which could be key stretched, but 
>>> thats
>>> it.  PBKDF2 + current GPU or ASIC farms = game over for passwords.
>>
>> what about stronger pwd-based key exchange like SRP and JPAKE?
>
> what I mean there is a so far unavoidable aspect of the AKE design 
> pattern
> is that the server holds verifier v = PBKDF2(count,salt,password), so the
> server if hostile, or of even more concern an attacker who steals the 
> whole
> database of user verifiers from the server can grind passwords against 
> it. There is a new such server hashed password db attack disclosed or 
> hushed up
> every few weeks.
>
>> Passwords don't scale up and are very inconvenient, but are you sure 
>> your
>> argument "PBKDF2 + current GPU or ASIC farms = game over for passwords"
>> really holds?  what about scrypt?  And theoretically, you can always
>> increase the number of rounds in the hash...  I refer to this link too:
>> http://www.lightbluetouchpaper.org/2013/01/17/moores-law-wont-kill-passwords/ 
>>
>
> You know GPUs are pretty good at computing scrypt.  Eg look at litecoin
> (bitcoin with hashcash mining changed to scrypt mining, people use 
> GPUs for
> ~10x speed up over CPUs).  Litecoin was originally proposed as I 
> understood
> it to be more efficient on CPU than GPU, so that people could CPU mine 
> and
> GPU mine without competing for resources, but they chose a 128kB memory
> consumption parameter, and it transpired that GPUs can compute on that
> memory size fine (any decent GPU has > 1GB of ram and a quite nice 
> cacheing
> hierarchy).  Clearly its desirable to have modest memory usage on a 
> CPU or
> if it fill L3 cache the CPU will slow down significantly for other
> applications. 

depends on the context i guess. if you are using a smartphone to log-in 
your banking app, it does not matter so much if you slow down other 
_background_ apps.

> Even 128kB is going to fill L1 and half of L2 which has to
> cost generic performance.  Anyway in the bitcoin context that 
> coincidentally
> was fine because then FPGAs & ASICs became the only way to profitably 
> mine
> hashcash based bitcoin, and so GPUs were freed up to mine scrypt based
> litecoin.  Also for bitcoin purposes higher memory scrypt parameters
> increase the validation phase (where all full nodes check all hashes and
> signatures, a double SHA256 is a lot faster than a scrypt at even 128KB,
> changing that to eg 128MB will only make it worse.
>
> Also the PBKDF2 / scrypt happens on the client side - how do you think 
> your
> ARM powered smart phone will compare to a 9x 4096 core GPU monster.  Not
> well :)

i had this in the back of mind when I replied to ur email; so I tend to 
be on ur side here
How much would it help to delegate PBKDF2 / scrypt to smartphone GPU to 
break this asymmetry?

since SRP and JPAKE use exponent_modulo sort of computation rather than 
a hash, any idea how this impacts attackers?
how well can you paralellize a dictionary brute force for DL problem?
I'm not expert so glad to hear about it.

> So yes I stand by that.  One could use higher memory scrypt 
> parameters, and
> so the claim goes with memory bound functions that memory IO is less
> dissimilar between classes of machines than CPU speed (smartphone vs 
> GPU).
> However you have to bear in mind also that scrypt actually has CPU memory
> tradeoffs, known about and acknowledged by its designer.
>
> I believe its realtively easy to construct a tweaked scrypt that 
> doesnt have
> this problem.
>
> Also for the bitcoin/litecoin side of things I heard a rumor that 
> there were
> people working on a litecoin ASIC.  Bitcoin FTW in terms of proving the
> vulnerability of crypto password focussed KDFs to ASIC hardware. The 
> scrypt
> time memory tradeoff issue maybe useful for efficient scrypt ASIC design.
>
> But there is a caveat which is the client/server imbalance is related 
> to the
> difference in CPU power between mobile devices and server GPU or ASIC 
> farms. While it is true that moore's law seems to have slowed down in 
> terms of
> clock rates and serial performance the number of cores and memory
> architectures are still moving forward, and for ASICs density, clock 
> rates
> and energy efficiency are increasing, and thats what counts for password
> cracking.  But yes the longer term picture depends on the trend of the 
> ratio
> between server GPU/ASIC performance vs mobile CPU.  Another factor 
> also is
> the mobiles are more elastic (variable clock, more cores) but to get full
> perf you end up with power drain and people dont thank you for draining
> their phone battery.  It is possible for ARM eg to include an scrypt or a
> new ASIC unfriendly password KDF on the die perhaps if there was enough
> interest.  The ready availability of cloud is another dynamic where 
> you dont
> even have to own the GPU farm to use it.  You can rent it by the hour or
> even minute, or use paid password cracking services (with some disclaimer
> that it better be for an account owned by you).
>
> Anyway and all that because we are seemingly alergic to using client side
> keys which kill the password problem dead.  For people with smart 
> phones to
> hand all the time eg something like oneid.com (*) can avoid passwords 
> (split
> keys between smart phone and server, nothing on server to grind, even 
> stolen
> smart phone cant have its encrypted key store offline ground to recover
> encrypted private keys (they are also split so you need to compromise the
> server and the client simultaneously).  Also the user can lock the 
> account
> at any time in event of theft or device loss.

i like the idea. Any issue/complications with re-provisioning or 
multiple devices with same identity?

>
> Adam
>
> (*) disclaimer I designed the crypto as a consultant to oneid.



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