phc-discussions - Q: Reference t_cost and m

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Date: Mon, 18 Feb 2013 11:23:16 -0800
From: "Dennis E. Hamilton" <dennis.hamilton@....org>
To: <discussions@...sword-hashing.net>
Subject: Q: Reference t_cost and m_cost parameters

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

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.  

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.

 - Dennis