Wordy answer but +1 on what a dkim sig means
Bill Oxley
Messaging Engineer
Cox Communications
404-847-6397
-----Original Message-----
From: ietf-dkim-bounces(_at_)mipassoc(_dot_)org
[mailto:ietf-dkim-bounces(_at_)mipassoc(_dot_)org] On Behalf Of Douglas Otis
Sent: Thursday, December 13, 2007 3:36 PM
To: Hector Santos
Cc: ietf-dkim WG
Subject: Re: [ietf-dkim] Issue #1524: Signature semantics
On Dec 12, 2007, at 2:56 PM, Hector Santos wrote:
Doug,
I would like to know one thing:
When does a signer expect when his signature to be broken?
When sending to a mailing list, would be one example. : )
or
When is it reasonable for a signer to believe his signature
can be broken? and if so, what does he expects to happen?
In the case of a "strict" assertion that "all messages are signed" is
reasonable. That verifiers MUST consider unsigned or broken
signatures having a From email-address within their domain as
"Suspicious" is not a meaningful assertion.
A domain can only assert what they DO, and not what others MUST THINK!
The "strict" assertion should be defined as "this domain signs all
messages originating from this domain" AND "this domain avoids
services that might damage DKIM signatures". Then the domain is
asserting what they DO, and not what the verifier MUST THINK.
I mean, after all, this is all about mail integrity and an attempt
at non-repudiation. It gets to a point where if a domain is going
to begin to digitally sign its mail, then there is a reasonable
expectation that it will be non-repudiated.
A valid DKIM signature ensures non-deniability or non-repudiation of
having handled the message. DKIM, by itself, makes no assertions
regarding the validity of message content.
By definition, non-repudiation occurs when all participants agree
that something is true.
A valid DKIM signature only means the signing domain has handled the
message. What do you think it means?
So given our current email infrastructure what steps are taken to
ensure full or partial non-repudiation?
Huh? Either the signature is valid or it is not.
There has to be some statements of facts, and in my opinion, a
domain signing his mail under a set of conditions he holds to be
true, can only non-repudiated if the receiver can hold him to these
expected true conditions.
SSP records are not signed. SSP records are published within DNS for
a duration of seconds or years. SSP can not offer non-deniability or
non-repudiation.
If the domain says "I'm the exclusive signer," than nothing should
repudiate that true condition of exclusivity.
You appear to be confusing a concept of non-repudiation with fairly
temporal operational assertion.
A domain may assert "this domain signs all messages" AND "this domain
avoids services that might break signatures". This assertion can not
ensure all messages received containing a From header email-address
within this domain are illegitimate or somehow in conflict with their
assertion. Dealing with email is _never_ that simple. These
assertions provide information that can be used to better evaluate
message content. These assertions should not be aimed at telling
verifiers what to DO or what to THINK, especially when such actions or
thoughts might be wrong!
If the domain expects a different set of relaxed conditions that can
be repudiated, then he really shouldn't be signing his mail or have
his signed by others and still expect verifiers to waste its time
with it trying to reach an impossible state of non-repudiation.
Any valid DKIM signature offers non-repudiation.
It can't be both ways Doug. Something is got to give here.
A DKIM signature does not ensure validity or conversely invalidity of
message content. Are you suggesting a domain signing a message with
"all" assertions (that essentially implies "this domain signs all
messages") must also authenticate identities associated with the "on-
behalf-of" email-address? In my view, authenticating identities
associated with email-address goes beyond what MTAs are currently
equipped to safely assure AND this WG's charter as well.
The only consideration a domain may wish to make might be to refuse
message submissions containing From headers where the foreign domain
has made a "strict" assertion. While this could be about "reputation"
when such act causes a domain to be removed from a list of acceptable
third-party signers, this should be ignored in order to resolve other
issues first. Messages containing From headers with email-addresses
of domains with "strict" assertions might cause acceptance problems.
These exceptions may also involve a signing domain with possible acts
of fraud. What should DKIM be saying about a signers obligations
regarding assurances of email-address ownership?
TPA-SSP provides a means for domains wishing to communicate with other
domains asserting "strict" and avoid interruptions created by dogmatic
policy handling. TPA-SSP can scale to accommodate a sizeable list of
domains which might become necessary to ensure uninterrupted
communications. In other words, without the TPA-SSP mechanism, a
"strict" assertion _will_ create problems that are either impossible
or unsafe to resolve using ad hoc key exchanges or DNS delegations.
-Doug
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