phc-discussions - Re: [PHC] An API for Efficiently Obtaining t

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Date: Sun, 11 Oct 2015 17:08:15 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] An API for Efficiently Obtaining t_cost

Some entries (I think including Yescrypt and TwoCats) do something similar,
and provide estimated parameters to meet your time goal.  These tools take
a max_memory and target_time parameters.  The more typical case is that we
cannot fill as much memory as you would like for a given target time. so
t_cost is set to the minimum value, and an m_cost parameter is suggested to
target the desired runtime.  In rare cases, you want to fill memory and
keep rehashing, in which case m_cost is set to max_mem and t_cost is
adjusted accordingly.

I have yet to see an actual deployment making use of more than the minimum
t_cost parameter in any application, so I'm not a fan of that parameter,
but it's there, and both m_cost and t_cost should be suggested for Argon2.
I suspect their team already has something like this in the works.  They
are welcome to look at my code in twocats-common.c to see what I did.

Bill

On Sun, Oct 11, 2015 at 1:52 PM, Taylor Hornby <taylor@....io> wrote:

> Hi,
>
> I'm not sure if it's the PHC's job to standardize on an API for the
> winning hash function, but I'm going to strongly argue that
> implementations MUST to provide an API similar to the following:
>
> int PHS_find_t_cost(
>         void *out,                 /* same */
>         size_t outlen,             /* same */
>         const void *in,            /* same */
>         size_t inlen,              /* same */
>         const void *salt,          /* same */
>         size_t saltlen,            /* same */
>         unsigned int target_msec,  /* target milliseconds */
>         unsigned int *t_cost,      /* output parameter */
>         unsigned int m_cost        /* same */
> );
>
> Calling this API should *efficiently* find the best t_cost for the
> given m_cost and target running time, on the current system under its
> current workload, while at the same time computing a hash. In other
> words, for a fixed m_cost, it should find the smallest valid t_cost
> such that the time taken is longer than target_msec milliseconds.
>
> The problem is that on memory-constrained devices, you might want to
> use as much memory you can, and then keep computing on that memory for
> a certain amount of time, say 5 seconds, when you don't know which
> t_cost to use for that amount of time, and you don't want to do trial-
> and-error or extrapolation from faster calls.
>
> I think this is an extremely important API to mandate, since if the
> developer needs it, and it isn't provided for them, they either have to
> do trial-and-error or heuristically guess-then-check t_cost which is
> not efficient, OR they have to modify or even re-implement the
> password-hashing library, likely losing important optimizations and
> thus giving the attacker an advantage in the process.
>
> It will also result higher t_cost being chosen in practice. Developers
> can hard-code the time cost in units of seconds, so that t_cost is
> always as high as it can be without impacting usability. Without this
> API, they'd have to run benchmarks on the system to select t_cost, or
> hard-code non-optimal costs, like the libsodium API [1,2,3] does.
>
> It's possible to efficiently provide this API for the iteration count
> in PBKDF2 as well as for 'p' in scrypt. I think it is also possible for
> Argon2.
>
> Even if we decide not to require this API, please consider adding it to
> your implementations.
>
> Thanks,
> -Taylor
>
> [1] http://doc.libsodium.org/password_hashing/index.html
> [2] https://github.com/jedisct1/libsodium/blob/e326ef90301cf570a5e6691c
> 46d4a1d37adc1e07/src/libsodium/include/sodium/crypto_pwhash_scryptsalsa
> 208sha256.h#L28
> [3] https://github.com/jedisct1/libsodium/blob/e326ef90301cf570a5e6691c
> 46d4a1d37adc1e07/src/libsodium/crypto_pwhash/scryptsalsa208sha256/pwhas
> h_scryptsalsa208sha256.c#L18
>
> p.s. Apologies if this has been discussed before. There's a lot on this
> list I haven't read.
>

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