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

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Date: Thu, 25 Jun 2015 18:40:27 +0200 (CEST)
From: Stefan.Lucks@...-weimar.de
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Why protect against side channel attacks

On Thu, 25 Jun 2015, Ben Harris wrote:

> On 25 Jun 2015 9:07 pm, <Stefan.Lucks@...-weimar.de> wrote:
> >
> > If we would assume the salt to be secret, we should not call it a "salt".
> To avoid confusion, it would then deserve to be called a "key".
> 
> I'm curious if there was any period of time since the inception of the salt
> that it was considered "public".

For a security engineer, there is a huge gap between considering something 
"not public", and assuming something "secret".

> But no, the salt is better considered as "sensitive" and treated in the same
> respect as the password hash.

Agreed. But if the salt where "secret", one would have to generate it from 
some high-entropy random source -- as you would do for cryptographic keys.

I have seen plenty of implementations using low entropy sources to 
generate "random" salts. Thus, when knowing user i's salt, you have a 
decent chance to guess user (i+1)'s salt.

This appears to be OK ... except when the salt is *secret*.

Stefan

--------  I  love  the  taste  of  Cryptanalysis  in  the morning!  --------
www.uni-weimar.de/de/medien/professuren/mediensicherheit/people/stefan-lucks
----Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany----