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Date: Fri, 7 Mar 2014 17:06:52 -0800
From: Andy Lutomirski <luto@...capital.net>
To: discussions <discussions@...sword-hashing.net>
Subject: Re: [PHC] Are password trailing 0's a problem?

On Fri, Mar 7, 2014 at 3:45 PM, Solar Designer <solar@...nwall.com> wrote:
> On Fri, Mar 07, 2014 at 03:30:37PM -0800, Andy Lutomirski wrote:
>> It seems odd to me that PBKDF2 is being used in any PHC proposals.
>
> For escrypt, this is in part because of the scrypt compat mode.
>
> PBKDF2-HMAC-SHA256 also has the advantage of being well accepted, so
> being able to say that cryptographic security of a new password hashing
> scheme is provably no worse than PBKDF2-HMAC-SHA256's may be helpful.
>
> BTW, Bill's choice of BLAKE2 for the sole cryptographic primitive, while
> sound technically, may be problematic for some prospective users.
>
>> AAUI PBKDB2 was intended as a password hashing algorithm,
>
> I'd say it was intended as a key derivation function, not as a password
> hash.  It has problems when used as the latter and the output size is
> naively set to be larger than the underlying hash's block size.
>
>> and it's not very good.
>
> Yes.  It is not.
>
>> The modern PHC candidates really want to use at as a
>> function that maps arbitrary-length strings to arbitrary-length
>> strings and that has "some of" the properties of a PRF, where "some
>> of" is possibly not very well thought through.
>>
>> Why not use a simple, modern primitive for this?  To me, the obvious
>> candidate is Keccak.  It's immune to generic attacks like this, which
>> was half the point of developing it in the first place.
>
> Maybe.  However, if I were to bring a new crypto primitive in, I'd
> prefer it to be suitable not only for the initial/final steps, but also
> for per-block data mixing.  Keccak favoring hardware implementations is
> not great for that.  (Unfortunately, BLAKE2 is not going to be SHA-3.)
>
>> ISTM that it
>> would be embarrassing for the PHC winner to have less resistance to
>> generic attacks than a hash function should provide.
>
> I agree.  So I am thinking of a suitably minimal change to scrypt's use
> of PBKDF2 so that the two problems pointed out in this thread today
> would not affect escrypt (except in scrypt compat mode).  Unfortunately,
> those minimal changes feel hackish.  For example, I could XOR the
> least significant byte of password length into each derived key block
> after the initial PBKDF2 call.  How does this sound to you?  (One byte
> should be enough since it contains all the info that is otherwise lost
> with padding.  And there's no endianness issue.)

Well, if people really want "at least as secure as PBKDF2" (sigh) and
scrypt compatibility (I've already expressed my opinion on that), then
how about something that's ugly but should be provably good:

MGF1-SHA256(password legnth || password || PBKDF2(whatever))

Benefits:

 - MGF1 is well studied, at least for some purposes.
 - I think that it can be proved that the MD construction is free from
generic attack when the length is prefixed (in a fixed-size or
prefix-free encoding)
 - You can use whatever PBKDF2 parameters you want, and the only way
it can be weaker than not using PBKDF2 at all is if PBKDF2 actually
conspires to *increase* the number of collisions in SHA-256.

Downsides:  I still think this is silly.  Crypto shouldn't be about
"this looks okay" these days.  I'd be happier with:

Keccak(reasonable params, PBKDF2(whatever))

--Andy

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