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

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Message-ID: <218AE73F98E99C4C98AF7D5166AA798E09091853@TK5EX14MBXC287.redmond.corp.microsoft.com>
Date: Wed, 17 Apr 2013 15:18:30 +0000
From: Marsh Ray <maray@...rosoft.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: RE: [PHC] Let's not degenerate when if the PRF is too narrow for
 desired output

From: Steve Thomas [mailto:steve@...tu.com] 
Sent: Wednesday, April 17, 2013 9:51 AM
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Let's not degenerate when if the PRF is too narrow ...
> The problem is that people don't know that PBKDF2 is for generating
> keys and not password hashes. Even Microsoft gets this wrong:
> http://hashcat.net/forum/thread-1752-post-9981.html#pid9981

RFC 2898 explicitly endorses use of PBKDF2 for password hashing:
https://tools.ietf.org/html/rfc2898#page-5 :
> It is expected that the password-based key derivation functions may
> find other applications than just the encryption and message
> authentication schemes defined here. [...]
> Another application is password checking, where the output of the key
> derivation function is stored (along with the salt and iteration
> count) for the purposes of subsequent verification of a password.


Also, on https://tools.ietf.org/html/rfc2898#page-5 :
> For instance, one might derive a
> set of keys with a single application of a key derivation function,
> rather than derive each key with a separate application of the
> function. The keys in the set would be obtained as substrings of the
> output of the key derivation function.

I'd argue that neglecting to mention that keys derived in this way
can be selectively created by an attacker sometimes using *less work*
than the defender's "single application of a key derivation function"
is a flaw in PBKDF2.

- Marsh