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Date: Fri, 7 Mar 2014 16:20:59 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Are password trailing 0's a problem?

On Fri, Mar 7, 2014 at 6:23 AM, CodesInChaos <codesinchaos@...il.com> wrote:
> I like putting PBKDF2 in between the two parts of HKDF:
>
> hash1 = HKDF-Extract(salt, password)
> hash2 = PBKDF2(hash1)
> hash3 = HKDF-Expand(hash2, purpose, length)
>
> Using HKDF-Expand avoids the large cost increase for larger outputs
> and supports a purpose string.
> Using HKDF-Extract avoid the collisions (both the >64 and the 0
> padding variants) because it uses the salt not the password as HMAC
> key.

This is cool.  Thanks for the pointer to HKDF.

> Of course it still leaks information about the length of the password,
> but that's unavoidable if one doesn't limit the maximal password
> length.
>
> If you want to apply a pad, consider a removable padding, like PKCS#7
> instead of null padding.
>
> Even without a padding, an implementation can always choose to
> implement the initial hash step in constant time for all inputs
> shorter than a threshold.
> For example you could always use 5 SHA-256 compressions to avoid side
> channels on passwords shorter than 300
> or so bytes using proper constant time selection to choose one. The
> only problem with that is that many implementors won't bother.

It does still potentially leak the password length, and there's zero
effort in HKDF_Extract to avoid it, and in fact it bypasses HMAC's
attempt to at least hide the length of a password shorter than 64
bytes.  Grr...

So, what I coded today is HKDF_Extract and HKDF_Expand, but before I
pass the password to HKDF_Extract, I first pad it to 1024 bytes, and I
return an error if the password is longer.  I think this is better
than what I had with PBKDF2.  As for your recommendation on padding, I
think I'm OK, because I add the password length to the info parameter,
so I just zero-pad.  Then I HKDF_Expand to the user's desired hash
length, and pass this to my memory-hard hash function.  I'm not so
sure that doing my own padding is a good idea... why didn't they do it
in HKDF?  It seems really frightening to be rolling-my-own when HKDF
seems to have become a standard in 2010, and should already deal with
all my concerns.  Am I missing something?

Bill

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