phc-discussions - Re: [PHC] simplifying yescrypt

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Message-ID: <20141021020440.GA8628@openwall.com>
Date: Tue, 21 Oct 2014 06:04:40 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] simplifying yescrypt

More detail on one of the possible changes:

On Tue, Oct 21, 2014 at 05:58:37AM +0400, Solar Designer wrote:
> - Drop support for ROM access frequency mask.  In practice, this will
> mean no ROM-on-SSD support (only ROM-in-RAM will be supported).  The
> reason why I am considering dropping this is that in most cases where I
> would have recommended use of ROM-on-SSD I would _also_ recommend
> simultaneous use of ROM-in-RAM, and then it would need to be two ROMs,
> with different access frequency, resulting in even greater complexity.
> Perhaps just one mask (only for the ROM-on-SSD) would be enough even in
> that two-ROMs case, though.

My options are:

1. Leave things as-is, meaning that "large" yescrypt users (on dedicated
authentication servers) will have to choose either ROM-in-RAM or
ROM-on-SSD, but not both at once (well, except possibly by chaining two
invocations of yescrypt, which is a suboptimal hack).

2. Add support for two ROMs at once.

3. Drop support for controlling ROM access frequency (and thus for
ROM-on-SSD).

Obviously, only #3 fits the topic of this posting, but I'd like this
community's opinions on which option is best.  Technically, I'd be able
to introduce #1 or #2 in a superset of yescrypt later even if I go with
#3 now, so the question may be whether we want ROM-on-SSD considered a
supported feature in PHC context (in the PHC candidate or in a currently
existing superset of it).

For ROM-in-RAM, there's normally no reason to use a mask other than 1,
which means that 1/2 of reads (and obviously none of the writes) are
from ROM.

BTW, the current API is defined to be extensible to multiple ROMs:

typedef yescrypt_region_t yescrypt_shared1_t;
typedef struct {
	yescrypt_shared1_t shared1;
	uint32_t mask1;
} yescrypt_shared_t;

As you can see, it mostly already has the complexity that would be
associated with two ROMs.  yescrypt_shared_t struct would become e.g.:

typedef struct {
	yescrypt_shared1_t shared1, shared2;
	uint32_t mask1;
} yescrypt_shared_t;

On the other hand, removing the mask (and thus keeping just ROM-in-RAM)
would allow for:

typedef yescrypt_region_t yescrypt_shared_t;

and the extra struct would be gone.

Alexander