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Re: [Curdle] Diffie-Hellman modulus sizing in Kerberos PKINIT

mbaushke ietf <mbaushke.ietf@gmail.com> Mon, 02 August 2021 20:09 UTC

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From: mbaushke ietf <mbaushke.ietf@gmail.com>
In-Reply-To: <jlglf5jlqgu.fsf@redhat.com>
Date: Mon, 02 Aug 2021 15:09:04 -0500
Cc: "Mark Baushke (ietf)" <mbaushke@gmail.com>, curdle@ietf.org, kitten@ietf.org
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References: <jlglf5jlqgu.fsf@redhat.com>
To: Robbie Harwood <rharwood@redhat.com>
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Subject: Re: [Curdle] Diffie-Hellman modulus sizing in Kerberos PKINIT
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Hi Robbie,

A 3k group is 128 bit of security strength and I have no issues selecting such to be either SHOULD or MAY.

For example, the 3072-bit MODP group 15 is reasonable for use with 128 bit symmetric ciphers such as aes128-{ctr,gcm} or chch20-poly1305.

For the rest, yes, I agree.

        Enjoy!
        -- Mark

[Sent from my iPhone -- Please pardon
any auto-fix created typos.]


> On Aug 2, 2021, at 12:14 PM, Robbie Harwood <rharwood@redhat.com> wrote:
> 
> "Mark Baushke (ietf)" <mbaushke@gmail.com> writes:
> 
>> Hi Robbie,
>> 
>> I made a few mistakes in my last message.
> 
> No worries.  To be sure I understand, though, please let me know if this
> matches what you're suggesting:
> 
>    - 1k: MUST NOT (matches the draft)
>    - 2k: SHOULD NOT (strengthen the draft's MAY)
>    - 4k: MUST (matches the draft)
>    - 6k/8k: MAY (not present in draft)
> 
> I'm probably fine to add larger groups like the 6k and 8k you suggest at
> MAY.  If I read right, Heimdal implements MODP 6k and 8k already.
> 
> For completeness: Heimdal also implements 3072-, 1636-, and 768-bit
> MODP.  It seems worth taking a position on those groups as well.  I
> imagine that position would be imagine would be:
> 
>    - 3072: MAY
>    - 1636: SHOULD NOT
>    - 768: MUST NOT
> 
> (though it may be cleaner to adjust the language to address sizes rather
> than specific groups at that point).
> 
> Be well,
> --Robbie

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