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Date: Sat, 4 Feb 2017 19:50:26 +0200
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To: Yasuo Ohgaki <yohgaki@ohgaki.net>
Cc: "internals@lists.php.net" <internals@lists.php.net>
Content-Type: multipart/alternative; boundary=001a113e026c9c641e0547b808dc
Subject: Re: [PHP-DEV] [Discussion] HKDF
From: narf@devilix.net (Andrey Andreev)

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Hi,

On Sat, Feb 4, 2017 at 1:01 AM, Yasuo Ohgaki <yohgaki@ohgaki.net> wrote:

> Did everyone understand why I propose salt as required parameter and
> specify optional salt explicitly?
>
>
I did, and I disagreed.


> HKDF w/o salt is OK, but with salt, it's much stronger than w/o it.
> In addition, most use case with PHP is something like as follows:
>
> 1. Get password hash for the user
> 2. Generate new key with 1 using HKDF
> 3. Use key produced by 2 for encryption/etc
>
>
No it's not. That's the first thing *you* could think of, searching for a
problem to solve with it.

If you search for it on GitHub, you'll see the most common scenario is to
derive a pair of keys for encryption and HMAC.
(yes, there are PHP projects using it)

Cheers,
Andrey.

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