phc-discussions - Re: [PHC] IETF draft

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Date: Wed, 11 Dec 2013 11:11:10 +0100
From: CodesInChaos <codesinchaos@...il.com>
To: Poul-Henning Kamp <phk@....freebsd.dk>
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] IETF draft

On Tue, Dec 10, 2013 at 8:06 PM, Poul-Henning Kamp <phk@....freebsd.dk> wrote:
> If the database is stolen, any accidental salt+password collisions
> with previous collections would be useful.
That's why we use salts with sufficient entropy, at least 64 bits,
preferably 128. At that
point salt collisions are extremely rare and don't offer a practical
advantage to the attacker

> This was the major attack vector on the DES based unix crypt(3),
Which resulted from their insufficient 12 bit salt.

> My 1994 proposal:
>
>         DB(salt, hash(site_key + salt + password))
>
> Frustrates that attack, even if the site_key is also stolen, because
> there will not be any salt collisions -- as the site_key is
> effectively a part of the salt.
Why bother frustrating an already irrelevant attack?

> That is sort of what you propose, so your proposal is not
> without merit, it just doesn't adds a different kind of security
> than the IETF draft and my 1994 prior art.
If you use the key as input to the slow hash, you can't store it on a
low power device,
which is one of the advantages of encryption. I'd like to store the
key in a HSM which can't
afford to compute a proper slow hash like scrypt.

You can also gain this advantage by computing a cheap MAC of the slow hash.
MAC(site_key, slow_hash(salt, password))

But then you loose the re-encryption advantage. For example consider a
leak of the config file
containing the key without leaking the db. With encryption, you can
simply decrypt the database
and re-encrypt using a new key. With a MAC or pepper you can't.