MX dot was (Re: [ietf-dkim] TXT wildcards SSP issues
mike at mtcc.com
Wed Jun 6 15:17:04 PDT 2007
Steve Atkins wrote:
> On Jun 6, 2007, at 2:41 PM, Jon Callas wrote:
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>> I have a huge fear that I am beating a dead horse down a rathole. I
>> also fear that I no longer understand what's being discussed.
>> However, I want to make a cryptographic observation.
>> If you create a suitably-sized RSA key, throw away the private key,
>> and put the public key in a DKIM selector, you have made a selector
>> that can't have mail signed from it (or if you want to be really
>> anal, forging a signature for that selector is equivalent to breaking
>> that key).
>> If you then say, "I sign all mail" for any domain pointing to that
>> selector, you've effectively made a cryptographically enforced no-
>> mail, no-use, etc. domain using the existing Tinkertoys.
>> In short -- saying "I sign everything" with a non-existent or bogus
>> key is the same thing as saying, "You'll never see a valid one of
> Yup. I think everyone (mostly) understands this. The entertainment
> arrives when you want to be able to have a single policy record
> refer to a large number of subdomains via wildcard.
> The line of reasoning goes something like this:
> 1. We can already say "NO MAIL" any time we can say
> "NO UNENCRYPTED MAIL".
> 2. We kinda know how to say "NO UNENCRYPTED MAIL"
> for a single domain.
> 3. There are a few tricky issues with being able to say
> "NO UNENCRYPTED MAIL" across large numbers
> of domains via wildcard.
> 4. But the vast majority of cases where we'd want to say
> "NO UNENCRYPTED MAIL" for a wildcard all we really
> want to say is "NO MAIL".
> 5. So if we can say "NO MAIL" for a wildcard via some other
> protocol we don't need to be able to say "NO
> UNENCRYPTED MAIL" via wildcard, and can avoid the
> tedium referred to in point 3.
> I don't believe point 4 is entirely valid, myself, but that's
> the line of reasoning, and why people are bringing up
> wildcards and "NO MAIL" in the same discussion.
But other that, I think you've exactly got right about what is being
thrown around here. And I agree that #4 is not valid: for one thing,
wildcards will not cover nodes that have other resource records (eg,
an A record).
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