phc-discussions - Re: [PHC] Let's not degenerate when if the PRF is too narrow for desired output

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Date: Wed, 17 Apr 2013 11:26:11 +0200
From: Simon Josefsson <simon@...efsson.org>
To: Jeffrey Goldberg <jeffrey@...dmark.org>
Cc: "discussions\@password-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Let's not degenerate when if the PRF is too narrow for desired output

Jeffrey Goldberg <jeffrey@...dmark.org> writes:

> This is mostly me whining. I just got bitten by the fact that PBKDF2
> does weird stuff if you ask for more derived data than is natural for
> the PRF you give it.

Perhaps this can be converted into a requirement for a nicer KDF: the
cost to compute one bit of the output should be as high as computing the
entire output?

/Simon