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Date: Wed, 24 Jun 2015 18:15:02 -0700
From: Bill Cox <>
To: "" <>
Subject: Re: [PHC] Why protect against side channel attacks

On Wed, Jun 24, 2015 at 5:40 PM, Peter Maxwell <>

> ​Yeah, but on balance a constant-time implementation is still preferable
> to even the remote possibility of opening up a new attack vector that
> hadn't existed before... would be somewhat embarrassing for the PHC if a
> few years down the line ​it turned out that password data could be
> recovered from a local timing attack that an unsalted md5 password hash
> isn't vulnerable to :-)

I think you have a common point of view, which is reasonable until certain
additional data is known.

​Admittedly, I'm still not convinced on the utility of most memory-hard
> PDFs anyway.  Put it this way, I'm working on a project just now that may
> end-up serving a reasonably high load of login requests and will choose
> something *far* more light-weight for the password hash than has been
> generally proposed on this list -- memory and compute cycles aren't free.
> You get far more bang for your buck by giving users a swift swift kick up
> the rear-end and forcing them to pick marginally better passwords.

First, forcing users to pick difficult to remember passwords doesn't work.
Just Google it.  There's lot's of research backing this up.  You can
require 10 character passwords, upper case, lower case, digits, and
punctuation, and what you get is a ton of Password1!  Worse, users wont
remember Password1!, because they'll be trying password, and when that
doesn't work, they'll reset their password and go through that 5-minute
e-mail process, just to choose Password1! again when they see all the crazy
rules.  They really enjoy doing that several times a week :-p

Regardless of the password rules, your median user will pick passwords that
are typically found in a dictionary containing somewhere from 2^21 to 2^26
entries.  There's no way to protect these users with just password hashing,
and there seems to be no good way to force these users to choose better

I recommend you avoid angering your users and marketing department, and
just allow users to use passwords they can remember.  If you need more
security, add layers, such as a pin, security question, track their MACs
and other signals, and for any real security, add a second factor, such as
an SMS message to their phone.

If you can't afford the compute effort required by a modern password
hashing algorithm like Scrypt, then secure your password hashes with
additional layers.  For example, consider keeping your salt separate from
your password hashes.  If you can afford a decryption oracle server and the
RPC latency, consider having a well secured oracle that hashes in it's
secret key into the password hash before writing it to the disk.  That way,
an attacker with the database also needs the oracle's key to make any use
of the database.

Or you can just do what everyone else always does, and attempt to make
users choose really strong passwords.  Good luck with that :-)

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