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Date: Sun, 11 Aug 2013 19:07:18 -0700
From: "Dennis E. Hamilton" <dennis.hamilton@....org>
To: <discussions@...sword-hashing.net>
Subject: RE: [PHC] Interdependence of t_cost and m_cost parameters

My concern is that if all of these variations are all called PHS, and platform libraries determine which implementation and approach is used, there is no rational way to determine the parameters across such variations.   
 
If they are only named PHS for the purpose of being submitted as PHC candidates for evaluation, and not intended as substitutable, that would set my mind at ease.
 
-   Dennis
 
From: Peter Maxwell [mailto:peter@...icient.co.uk] 
Sent: Sunday, August 11, 2013 14:31
To: discussions@...sword-hashing.net
Subject: [PHC] Interdependence of t_cost and m_cost parameters
 
 
 
On 11 August 2013 19:51, Dennis E. Hamilton <dennis.hamilton@....org <mailto:dennis.hamilton@....org> > wrote:
I'm always having difficulty understanding the interdependence that might exist between t_cost, m_cost and, to a related degree, also outlen.
 
In an obvious sense, this is going to vary algorithm-by-algorithm.  Hypothetical algorithm A might operate on a trade-off basis between time & memory cost while another, B, may be designed in such a manner that the two are entirely orthogonal.
 
I'd suggest the important thing is that the parameters are fully documented and an evaluation of potential issues addressed by each designer, e.g. it is one thing for a defender to be able to set a memory:calc trade-off, it is an entirely different kettle of fish for the attacker to be able to change their route of calculation to set the memory cost to nil at the expense of increased time (which is usually far preferably for the attacker).
 
 

Here are some considerations I have.  I don't know how these fit with what others assume the interdependencies might be, or whether the interdependencies are simply left wide open, including when and where step functions arise.

 - Dennis

MY APPRAISAL OF THE SITUATION:

It is generally easy to have t_cost be directly related to a linear work factor apart from any step functions that may arise in conjunction with outlen.  Alternatively, a PHS could amortize t_cost when there is some outlen-related timing-step increase.  There is such an increase with PBKDF2, for example.  While the t_cost doesn't have to adjust for differences in terms of processor performance, it would be useful to have the effect be smooth, whatever the relationship is.
 
Again, I'd suggest this be left to each designer but to be decided upon and fully documented.  For example, I would not personally create a t_cost with a linear relationship, Moore's law is exponential in growth so it seems more natural to pick a logarithmic scale for t_cost.
 
Similarly, I'm personally not particularly concerned about t_size being impacted by outlen as it isn't a data-dependent parameter and, irrc, you can have a valid submission to the PHC with a fixed output length of 16 bytes.
 
 

Amortizing t_cost against m_cost is tricker.  That is, it is difficult to work out the combinations if m_cost also has step-function costs.  It is also not clear what the difference between m_cost 1 and m_cost 2 might be and whether a given implementation has step functions here too (in which case t_cost could be amortized once again).
 
Again, would this not vary with each algorithm?
 
 

Note 1: I am also assuming that, whatever the m_cost is, with the other parameters fixed, the result should be cryptographically unrelated to a result produced for any other m_cost value.  The prospect of t_cost, m_cost password-hash collisions with all other parameters the same must be negligible.
 
That would seem a sensible stipulation.  Perhaps have the parameters form part of the input to the actual password hash function?
 
 
 

Note 2: A requirement that I have for iterative password-hashing procedures is that I can have a version that will operate with a specific time threshhold and then report the t_cost that was achieved along with the resulting hash.  That becomes the known t_cost value for verification.  I've not considered m_cost in practical cases that have come up.  (Given a threshold t in milliseconds, the resulting t_cost
value is one for which the hash is produced in the approximate interval [t, 3t/2].)
 
Well, m_cost is in a naive manner more amenable, for most algorithms I'd imagine the storage space required for m_cost would be easy to calculate.  What is less obvious is the implication of using, say 64Mb, for a hash calculation... what impact does that have on other parts of the system, do we have to start thinking down the line re potential paging issues (parts of hash calculation ending up in swap), will the attacker's technology improve to handle much larger amounts of memory efficiently/at low cost?
 
 

Note 3: If I wanted variable m_cost control, I'm thinking that the procedure in (Note 2) would take an m_cost suggestion and that procedure would deliver the t_cost and m_cost values that were used in a generation that satisfied the specified time threshold.  That seems definitely trickier.  Interesting, but trickier.
 
In a practical sense, it's not particularly tricky: like you intimated, fix m_cost at the desired storage requirement then specify your desired time-limit and run the hash function enough times to calculate a median value for t_cost that centres the execution time around your time-limit.
 
The problem is understanding the impact on aspects such as CPU cache, virtual memory, system bus, number of hash invocations per unit time, what else is running on the system, etc, then balancing that against attackers' resources.
 
 
 

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