Re: Hash salt (was Re: BCRYPT - Why not using it?)

[Ron Johnson <ron.l.johnson@cox.net>]

On 04/06/2011 10:40 PM, Boyd Stephen Smith Jr. wrote:
> In<[🔎] 4D9D1B22.2010608@cox.net>, Ron Johnson wrote:
> > On 04/06/2011 08:19 PM, Aaron Toponce wrote:
> > > First, if you don't have the salt, but you do have the hash, then a
> > > rainbow table attack is completely pointless.
> >
> > The OS must store the salt somewhere, in order to correctly authenticate
> > the user when he logs in.  But I've never heard of /etc/hashsalt so what
> > am I misunderstanding?
>
> The value stored in /etc/shadow is both the salt + the encrypted
> salt+password.  This allows a process with read access to /etc/shadow to
> easily read the shadow, encrypt the salt + provided password, and compare the
> result to the encrypted salt+password.  The salt is randomly generated each
> time the password is set, and it (usually) different for each entry in
> /etc/shadow.

Is the salt just bits that are either pre- or suffixed to your password 
before being run through the hashing function?

> This increases the size of a rainbow table by a factor of 2^(bits in salt),
> effectively stopping the attack for all but the most high-profile target with
> just an 8-bit salt.  I'm not sure how many bits are used in a modern salt, but
> I think it is somewhere between 48-bits and 64-bits.

The first 3 characters of every hash in my /etc/shadow are the same. 
That's what, 24 bits?

> Salted MD5 is still considered secure, even with the known attacks against
> MD5.  Salted SHA1 has no attacks more effective than brute-force.  I'd like to
> believe that shadow passwords will more to SHA3 within 2-3 releases after SHA3
> is finalized.  At the current rate of attack improvements against MD5, that
> should be plenty of time.

But if you're machine is rooted then (besides having lots of other 
problems) the attacker has your system-wide salt.  (But the rainbow 
table would still be unimaginably huge...)

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