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Date: Fri, 7 Mar 2014 17:59:14 -0500
From: Bill Cox <>
Subject: Re: [PHC] Are password trailing 0's a problem?

On Fri, Mar 7, 2014 at 4:58 PM,  <> wrote:
>> It seems really frightening to be rolling-my-own when HKDF
>> seems to have become a standard in 2010
> Hey, let's not be hasty. HKDF was first published in 2010, and the article
> (which can be seen on ) contains this
> sentence, right in the abstract:
>    The HMAC-based scheme presented here, named HKDF, is being standardized
> by the IETF.
> And a footnote on page 3 further qualifies that assertion as:
>    The proposed HKDF scheme is being standardized by the IETF as RFC 5869.
> Ok, so let's have a look at RFC 5869:
> And there we see that the RFC has category "INFORMATIONAL", and not at all
> Bottom-line: HKDF is not a standard in the IETF sense (so the assertion in
> the article is a bit misleading). Being published as a RFC is in no way an
> official blessing, asserting quality, or implying that you can get jailed
> for not using it. An "informational" RFC only means that the author could
> write in a coherent enough English some description of a technical element
> that was not immediately detected as flawed (I should know -- I myself
> wrote an informational RFC).

Thanks for that tip.  I'd not noticed the difference before between
informational and standards track on RFCs.

Here's another question: Is it worth worrying so much about looping
over the password in a way that might enable a side-channel timing
attack of some sort?  When we get the password, it comes with a
certain length, and I see no way to avoid reading all these addresses.
 An attacker might be able to detect the password length through a
cache timing attack where he sees how many addresses in sequence were
loaded into the cache, though it's probably at an almost useless
resolution of 64-bytes.

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