phc-discussions - Re: [PHC] Why protect against side channel attacks

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Message-ID: <CAOLP8p6p81KLgk9ttv=MV1jUdVZy6VwU2QZ+FoJa4JEWuiao4w@mail.gmail.com>
Date: Thu, 25 Jun 2015 11:52:11 -0700
From: Bill Cox <waywardgeek@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Why protect against side channel attacks

On Wed, Jun 24, 2015 at 7:17 PM, Peter Maxwell <peter@...icient.co.uk>
wrote:

> Ok, please enlighten us as to this "certain additional data".
>

I just meant the body of evidence that attempts to increase user password
entropy generally fail.  Having a minimum password length is not
unreasonable, but user password entropy increases only slightly with
required length.  I also meant the body of evidence that median user
password entropy is in the range of 21 to 26 bits generally, and that
common password policies do not push it out of this range.

It is in fact the case that no password hashing algorithm can well protect
these median-entropy passwords against an off-line attack.  With even a few
KiB of memory, we can improve over PBKDF2 (with Bcrypt for example), but
1ms of _any_ algorithm is not enough to slow down attackers much once they
have the password hash/salt database.  Your user passwords will be revealed
no matter what 1ms password hash you apply.  Therefore, addtional security
measures are required.  That's all I'm saying.

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