phc-discussions - Re: [PHC] Upgrade HKDF to HKDF2?

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Date: Mon, 10 Mar 2014 19:04:26 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Upgrade HKDF to HKDF2?

I have converted TigerPHS (as I'm now calling it) to use the reference
version of HKDF-SHA256.  It's better than I thought.  For one thing,
it has an API for adding multiple sets of key data, so I can add user
supplied "data" there.  I like the API, and I think I'll add a similar
API to TigerPHS in case a user has multiple pieces of "data" he'd like
to add.

The only issues I still see with HKDF are minor:

1) Output length limited to 255*32 (for SHA256).  Not perfect, but I
can live with it.
2) No padding on sensitive inputs, and it relies on the underlying
hash function for that.  However, I can recommend to users that they
have a fixed max password, like 256 bytes, zero it initially, and pass
the whole thing to my PHS.  That's more work for the user, but it's
even more secure than a pad.

Bill