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Date: Fri, 7 Mar 2014 12:23:14 +0100
From: CodesInChaos <codesinchaos@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Are password trailing 0's a problem?

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.

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.

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