Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06

Nico Williams <nico@cryptonector.com> Wed, 15 April 2015 20:27 UTC

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Date: Wed, 15 Apr 2015 15:27:26 -0500
From: Nico Williams <nico@cryptonector.com>
To: Greg Hudson <ghudson@mit.edu>
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Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
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On Thu, Apr 09, 2015 at 08:11:50PM -0400, Greg Hudson wrote:
> On 04/09/2015 06:21 PM, Michael Jenkins wrote:
> > Bit string lengths and bits-o'-security: When this draft was originally
> > introduced to kitten, it was stated that it was to support
> > draft-burgin-kerberos-suiteb
> > <http://www.ietf.org/mail-archive/web/kitten/current/msg03802.html>. So
> > there's an implicit presumption that one is working in one of two modes,
> > 192-bit or 128-bit security, and consequently using either SHA-384 or
> > SHA-256, respectively. 
> 
> However, SHA-384 truncated to 192 bits does not have a collision
> resistance strength of 192 bits; it has a collision resistance strength
> of 96 bits.  This is described in detail in SP 800-107 section 5.1,
> which is referenced by FIPS 180-4 section 7.

(We're talking about the HMAC being truncated.  And we're after forgery
and valid MAC distinction attacks, not collision resistance.)

> I don't think any meaningful review would deem
> 192-truncate(HMAC-SHA-384(k, m)) to be stronger than
> 192-truncate(HMAC-SHA-256(k, m)) by any metric.

(provided that k is 192-bit)

I suggested that prudence would.  But I was wrong.  Recall that Bellare
and Mihir proved HMAC is secure if the hash function is a PRF, and this
does not depend on collision resistance.

AES256-HMAC-SHA256-192 works.

Nico
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