Re: [Dcrup] I-D Action: draft-ietf-dcrup-dkim-usage-02.txt

Scott Kitterman <sklist@kitterman.com> Mon, 12 June 2017 14:32 UTC

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From: Scott Kitterman <sklist@kitterman.com>
To: dcrup@ietf.org
Date: Mon, 12 Jun 2017 10:32:51 -0400
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Subject: Re: [Dcrup] I-D Action: draft-ietf-dcrup-dkim-usage-02.txt
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On Monday, June 12, 2017 08:20:18 AM Martin Thomson wrote:
> On 12 June 2017 at 04:25, Murray S. Kucherawy <superuser@gmail.com> wrote:
> > On Sun, Jun 11, 2017 at 1:40 AM, Martin Thomson <martin.thomson@gmail.com>
> > 
> > wrote:
> >> https://github.com/kitterma/draft-ietf-dcrup-dkim-usage/pull/1
> >> 
> >> I think that I got the essence of your changes there.  And it's a lot
> >> shorter.
> > 
> > Can you explain what you mean by "rely on"?
> 
> Sure, that's probably a bit of an imprecise way of saying that a
> verifier needs to insist on receiving a valid rsa-sha256 signature and
> that it can't just fall back to rsa-sha1.

The way to do that would be to remove it from the protocol, which you seem to 
oppose, so I'm confused.

> > So by that logic, you can put a 1156-bit key in a record now without
> > changing anything.  For anything bigger you will need multiple
> > character-strings in the TXT field which I believe is one of the things
> > John says doesn't fly in current provisioning software.
> 
> 1156 is better, but I was hoping for a bit higher than that.  I would
> include text that suggests this (explaining the limitations you set
> out regarding the whitespace and so forth), but note that the
> additional security margin is, well, marginal.

1024 is operationally very common.  In the end, if it matters, I don't mind 
adding this discussion, but I also don't see any reason to set the floor 
higher than 1024 (I don't think there's enough security advantage in 1156 over 
1024 to matter).

Scott K