phc-discussions - Re: [PHC] Client-side hashing (was side-channel stuff)

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Date: Fri, 24 Apr 2015 17:40:41 -0500 (CDT)
From: Steve Thomas <steve@...tu.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Client-side hashing (was side-channel stuff)

> On April 24, 2015 at 10:01 AM Sascha Schmidt <sascha.schmidt@...-weimar.de>
> wrote:
>
> Server relief, keyed hashing and client-independent updates are all
> important features. The problem is that one can only have two of the
> three at the same time. It maybe possible to get them to work
> together, but this will result in a lot of different code branches
> depending on the number of updates and so on.
>

You can have all three if you change "keyed hashing" to "server secret". They
achieve the same thing but server secret is more general. Anyways I like to
encrypt the hash, so that after a break-in you can rotate keys without user
input. To upgrade (if you are using an HSM) you need to copy all the hashes to
an offline computer so you can safely decrypt them. Also I consider server
secret to be outside of the PHC. Since encryption should be done on an HSM and
we can't have bindings for all current and future HSMs or whatever.

h = fastHash(password, salt)
for u = 1 to upgrade
{
  h = fastHash(slowHash(h, t_cost, m_cost))
  t_cost = increaseTCost(t_cost)
  m_cost = increaseMCost(m_cost)
}
return encrypt(h, key) // HSM? :)

P.S. For encrypt(), I'm a fan of ECB if h fits in the block size or null IV
CBC-CTS. These are the cheapest safe modes for this. You could do AEAD but
that's like using bazooka to kill a fly.