phc-discussions - Re: Password hashing by itself is not enough

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Message-ID: <op.xvf12gvgyldrnw@laptop-air>
Date: Fri, 13 Mar 2015 09:18:12 -0700
From: "Jeremy Spilman" <jeremy@...link.co>
To: discussions <discussions@...sword-hashing.net>, "Justin Cappos"
 <jcappos@....edu>
Cc: polypasswordhasher-dev <polypasswordhasher-dev@...glegroups.com>
Subject: Re: Password hashing by itself is not enough

That all definitely makes sense, thanks!

>> In either case you need some admin (threshold account) input when the  
>> server first boots to ensure all users can immediate start logging in.  
>> In either >>case the secret can be stolen over the network from a  
>> compromised server.
>
> I think we likely get in our own way by saying administrator, non admin,  
> etc.  Really, if you have some reason to believe that an attacker cannot  
> simply >create as many accounts as they like, you can make all of the  
> accounts 'protector accounts' (or threshold accounts in our prior draft)  
> *so long as you >don't use ICB (partial verification) to allow logins  
> before a threshold is reached*.

I meant 'admin' in the case of the volatile pepper, or 'threshold account'  
in the case of PolyPass, sorry for any confusion.

> Some people have commented to us that a reasonably number of users  
> choose password or 123456, so this would be guessable.  For example, .9%  
> of the >RockYou users chose 123456.   However, since you have to guess  
> *which* accounts choose this password, the amount of time also becomes  
> massive.  >It is basically a .009^5 probability that each guess is  
> correct.  If you use tricks like key stretching to make the  
> recombination time long, then this operation >can also take a very long  
> time.

Key management / Protector account management concerns aside, I think it  
is reasonable to say the result (the resulting security posture) of a  
secret key and PolyPass are fairly similar if not exactly the same?

>
> Note, I copied the wrong cracking number from my previous email.  With a  
> threshold of 5 on RockYou, and an attacker checking for 123456, the time  
> >would be hundreds of years w/ stretch to 1 check per second.  (1 check  
> per second is sane because the defender will only need to do this  
> operation a >small number of times, often once, per reboot.)

That's a very nice feature, to be able to add latency / cost to the key  
recovery process independent of the latency / cost of a password  
verification after the key has been recovered! I will have to read up on  
how you imposed tunable latency on the Shamir secret recovery process.
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