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Date: Sat, 31 Jan 2015 16:07:12 -0800
From: Bill Cox <>
To: "" <>
Subject: Re: [PHC] yescrypt pre- and post-hashing

On Sat, Jan 31, 2015 at 3:59 PM, Solar Designer <> wrote:

> This was believed to be just a peculiarity with no security impact.
> Well, due to a recent tweet/reminder by Juliano Rizzo, I no longer think
> so.  There was real security impact, even if in obscure cases.
> Specifically, if a site upgrades from raw SHA-256 to e.g. scrypt hashes
> (by re-hashing passwords as users log in), but does not securely wipe
> all copies of the SHA-256 hashes from all places, then for the (few) 65+
> char passwords those hashes can be used to log in to the service,
> without the attacker ever having to crack the plaintext passwords.  This
> was Juliano's original concern.
> I came up with two more:

Wow... that's frigthening!  So, any leaked SHA-256 hash DB of y 65+
characterpassword would be accepted as valid by PBKDF2-SHA256.  That's a
rare case, but still a hole we should fill, IMO.

Thanks for coming up with this elegant solution to the problem.


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