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

Nico Williams <> Wed, 15 April 2015 20:52 UTC

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Date: Wed, 15 Apr 2015 15:52:00 -0500
From: Nico Williams <>
To: Benjamin Kaduk <kaduk@MIT.EDU>
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Subject: Re: [kitten] WGLC on draft-ietf-kitten-aes-cts-hmac-sha2-06
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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:

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.

Pending a re-review of SP800-38A+ (I think I did it last time around),
I'm OK with the contents of this I-D.