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

Benjamin Kaduk <kaduk@MIT.EDU> Wed, 15 April 2015 20:58 UTC

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Date: Wed, 15 Apr 2015 16:58:08 -0400 (EDT)
From: Benjamin Kaduk <kaduk@MIT.EDU>
To: Nico Williams <nico@cryptonector.com>
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Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
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On Wed, 15 Apr 2015, Nico Williams wrote:

> On Mon, Mar 30, 2015 at 12:40:40PM -0400, Benjamin Kaduk wrote:
> > This message begins the Working Group Last Call (WGLC) of "AES Encryption
> > with HMAC-SHA2 for Kerberos 5" <draft-ietf-kitten-aes-cts-hmac-sha2-06>.
> > The WGLC will last two weeks, ending on Monday, April 13th.  The draft is
> > available at:
> >
> > https://tools.ietf.org/html/draft-ietf-kitten-aes-cts-hmac-sha2-06
>
> To summarize the changes from RFC3962:
>
>  - CTS remains the same but is now given by a different reference
>    (SP800-38A+).  The motivation is clear (to have a NIST reference for
>    the cipher mode).
>
>    This should be the same as in RFC3962.
>
>    (Confounding is still used.)
>
>  - Use encrypt-then-MAC instead of MAC-then-encrypt.  +1 to that.
>
>  - SHA-256 is used at the 128-bit security level, instead of SHA-1, and
>    the HMAC output is truncated to 128 bits.  The keys for the HMAC are
>    128 bits at the 128-bit security level.
>
>  - SHA-256 is used at the 192-bit security level, and the HMAC is
>    truncated to 192 bits.  The keys for the HMAC are 192 bits at the
>    192-bit security level.
>
>    AES-256 is used at the 192-bit security level because AES-192
>    implementations are not as universally available as AES-256, or
>    something.  In any case, I've no objection.
>
>    I also do not object to the use of HMAC-SHA256-192 with 192-bit keys
>    at the 192 bit security level.

I think that some of these "256" should be "384", unless you have
progressed from summarizing the document to describing what you would like
to see...

-Ben