phc-discussions - Re: [PHC] blakerypt sequential memory-hard function

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Message-ID: <1436630008.20131206184537@gmail.com>
Date: Fri, 6 Dec 2013 18:45:37 +0100
From: Krisztián Pintér <pinterkr@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] blakerypt sequential memory-hard function

Dennis E. Hamilton (at Thursday, December 5, 2013, 5:16:02 PM):
> The advantage is that the Fibonacci recurrence provides a nice way
> of deriving k on the fly by seeing how much time is being taken in
> real time, and incrementing by the next step if a target threshold
> has not been reached.

this is a recurring theme, and i never was a great fan of that. now i
took some time to think through why.

conclusion

determining the proper cost parameters should not be a part of the
actual algorithm, and should come from outside. the parameters must be
provided in an explicit format, that is, number of cycles, number of
memory blocks, etc. it allows equivalent forms, like 2^t time, but
disallows time in milliseconds.

rationale

1. theoretical stand: separate things should be separate. it is
conceptually different to determine the parameters and using them. use
small code blocks, avoid swiss army knife antipattern.

2. many times we want to calculate the hash on different platforms,
including the case of switching platforms in the future. it is unwise
to lock the parametrization to the current platform.

3. if we always use fixed parameters, hashing the password the first
time and checking a password does not differ at code/circuit level.
this simplifies the code.

4. we lose nothing with taking fixed parameters. we can at any time
offer the user a separate "benchmark" option. together with a table of
preset values for different platforms.

the above is an opinion. don't get offended, discuss!