Date: Tue, 19 Feb 2013 01:48:19 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Q: Reference t_cost and m_cost parameters

On Mon, Feb 18, 2013 at 11:23:16AM -0800, Dennis E. Hamilton wrote:
> For the candidate reference-implementation API, the t_cost and m_cost
> parameters are apparently unitless values related to time cost and space
> (memory cost).
> 
> Is there some standard definition for these, so that one can substitute
> implementations but not need to know the implementation to know the
> consequences?  
> 
> Or is it presumed that a candidate specify how these are interpreted

I think it's the latter.

I expect PHC submissions to differ in their abilities a lot, including
what ranges of cost settings they work for, and with what granularity.
In some cases, the two cost settings might not be entirely independent -
e.g., in scrypt both N*r and p contribute to its running time.

I actually tried bringing 3 different underlying password hashing
schemes to the same cost scale, in terms of CPU time, in phpass.  I did
so by adopting bcrypt's costs as the input values, and translating them
to cost settings resulting in comparable CPU time usage on current CPUs
and with current code for BSDI's extended DES-based hashes and for
phpass' own "portable" hashes.  This works fine, but I think in PHC
context it should be left for a higher level, if we like.

If we try to insist that e.g. t_cost is expressed in microseconds on a
specific CPU when using a specific compiler/version and m_cost e.g. in
kilobytes, this means that PHS() will be a wrapper function doing the
necessary translation (and returning an error code for value
combinations that its underlying code can't get sufficiently close to?)
I'm not sure whether we want that.

> and once a particular function is "standardized" as *the* PHS, the
> interpretation of the parameters will be fixed?

I guess the PHS() interface might end up being for PHC use only, not for
actual use of the winning password hashing scheme(s).  (Chances are
that there will be more than one as I do not expect that a single one
will meet all of our wishes at once.  Maybe two.)  I think each password
hashing scheme, besides the PHC-mandatory PHS() interface, will also
provide its own APIs and indicate which one (or several) are recommended
for use.

> By fixed I mean that the salt and the cost parameters would be either stored
> or otherwise preserved between creation of a hash and recomputation of the
> hash for authentication of a password.  

Oh, that's a different aspect from what you wrote in the previous few
paragraphs.  Yes, it is expected that the parameter values, except for
secrets, will be stored along with the hashes or with the encrypted
data (if the scheme is used as a KDF).  This is beyond scope of PHC,
although we may discuss it in here.

I expect that once we get closer to the end of first year, we'll need to
have crypt(3) encoding syntax defined for the finalists, although this
is not mandated by PHC terms.  This implies storage of non-secret
parameters along with the hashes (inside those strings).

> I am thinking of the case where the authentication may be done by different
> systems than the one that originally derived the hash, as when using
> password-based encryption of documents in an interchange situation or in a
> long-lived personal archive, longer-lived than the computer used to create
> the password hash originally.  In this case it may be necessary to
> communicate the salt and the cost parameters along with the derived hash
> value.

Of course.  This is how it's being done already.

Alexander

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