Re: [dmarc-ietf] ARC-Seal is meaningless security theatre

[Bron Gondwana <[e-mail]>]

On Sat, 12 Aug 2017, at 10:04, Dave Crocker wrote:
> On 8/11/2017 4:54 PM, Bron Gondwana wrote:
>> On Sat, 12 Aug 2017, at 03:22, Dave Crocker wrote:
>> 
>> I'm just picking out the key quote here:
>> 
>>> On 8/7/2017 4:22 PM, Seth Blank wrote:
>>> 
>>>   When validating an ARC signed message, one verifies the latest AMS>>>   (which must validate), and *the entire chain* of ARC Seals, not
>>>   only>>>   the latest. This guarantees you a list of all message
>>>   signatories ->>>   the chain of custody we're talking about.
>> 
>> Yes, I follow this bit, but then...
>> 
>>>   When evaluating the chain for final receipt, there are two
>>>   states to>>>   worry about as a matter of local policy: 1) you trust all the
>>>   signatories on the chain 2) there is an untrusted signatory on the>>>   chain
>> 
>> Which is why it's a bad idea to sign if you're not modifying, because>> then everybody has to trust you or their chain breaks, even
>> though you>> didn't do anything which required signing.
> 
> I don't have an opinion about whether this conclusion is correct, but> I'm quite certain it a type of consideration that needs to be
> fundamental, to recommendations about usage.  Who should do what, and> why?  What are the upsides of their doing or not?  Downsides?
> 
> 
> 
>>>   Without the ARC Seal this determination is not possible and
>>>   there is>>>   no way to evaluate the ARC chain for delivery as a final receiver.>> 
>> And this is the crux of our disagreement.  Seth thinks it's
>> necessary to>> do more than signing a statement that you believed the message was
>> authenticated when you got it, in a way that the next hop can verify>> your signature over your own Authentication Results plus the
>> content of>> the message.  I disagree.
>> 
>> I'm proposing exactly the same stragety DKIM uses, just with
>> series of>> signed "chain of custody" statements rather than the DKIM signature
>> having to align with the sender domain.
> 
> by 'strategy DKIM uses' what do you mean exactly?  I'm guessing
> you mean> having the signature cover more of the header and all of the body, but> please confirm or clarify.

Sorry - yes, to clarify...

DKIM signs the entire body plus parts of the header.

In the strategy I am proposing, site "X" modifying a message in transit
(e.g. a mailing list) would add their own DKIM-like header (ARC-Message-
Signature is a perfectly fine name for it) which signed the new "body
and parts of the header", including a statement that site "X" had
verified the message authentication before modifying it (ARC-Authentication-
Results is a perfectly fine name for that statement).
That gives a complete chain of custody.

Bron.

--
  Bron Gondwana, CEO, FastMail Pty Ltd
  [e-mail]