Subject: Re: [Ietf-krb-wg] krb-wg report

From: Larry Zhu <[email-masked]>
Date: Fri, 27 Mar 2009 07:56:16 -0700

Most folks interpretrate "truncate to multiples of 1byte" in rfc3961 as no truncation needed. I.e. One would be kind to start with the assumption that rfc3961, our group prduct, is not completely silly. I thank you for that. This will be clarified.
Sent from my Windows Mobile® phone.

-----Original Message-----
From: Simon Josefsson <[email-masked]>
Sent: Friday, March 27, 2009 1:48 AM
To: Larry Zhu <[email-masked]>
Cc: [email-masked] <[email-masked]>
Subject: Re: krb-wg report

Larry Zhu <[email-masked]> writes:

> We then discussed an issue involving the RFC3961 PRF for AES. We found
> that all current implementations truncate the output to multiple of
> AES cipher block size 16 bytes while the specification in RFC3961 does
> not truncate.

That's wrong.  RFC 3961 always truncates.  For AES it truncates the hash
output to 1 byte and for DES3 it truncates the hash output to 8 bytes.

/Simon

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From: Simon Josefsson <[email-masked]>
Date: Fri, 27 Mar 2009 17:11:33 +0100

Larry Zhu <[email-masked]> writes:

> Most folks interpretrate "truncate to multiples of 1byte" in rfc3961
> as no truncation needed.

Are you saying that you implement RFC 3961 without truncation?

If so, it seems more difficult to use MIT-AES-PRF as a quick-fix because
(if I understand Sam's proposal correctly) will truncate to the
underlying cipher's block length.

/Simon

> I.e. One would be kind to start with the assumption that rfc3961, our
> group prduct, is not completely silly. I thank you for that. This will
> be clarified.  Sent from my Windows Mobile® phone.
>

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From: Greg Hudson <[email-masked]>
Date: Fri, 27 Mar 2009 13:25:57 -0400

> Are you saying that you implement RFC 3961 without truncation?

My recollection from the WG audio is that MS does not implement PRF at
this time.