phc-discussions - Re: [PHC] Opinions sought on whether a specific side-channel leakage is ok.

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Message-ID: <CAOLP8p57FgCqeYCXuMvBGuUc3h2m=L9Lk6DOP-G-jeEoiXPkYw@mail.gmail.com>
Date: Mon, 27 Jan 2014 12:03:57 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Opinions sought on whether a specific side-channel leakage
 is ok.

I've considered this scheme.  The problem I run into is that very weak
passwords are still guessable, unless I force the user to wait an
obnoxious amount of time.  For example, a password with only 12 bits
of entropy could be guessed using the same machine as the user in just
over an hour, if I limit the runtime to 1 second.  Also, users who do
care about their password strength are typically the users who would
want a full second of password hashing to protect it.  So, I came to
the conclusion that 1-ish seconds is around the right number.

As for the side-channel exposure, it bothers me some.  It could allow
him to attack only the lowest complexity hashes.  If he threw out 9
out of 10 based on complexity, he'd save himself 10X on compute
effort.

Bill