The very basic test I use is what's in the From: address. That's the thing
that's pretty universally displayed and one that users are most likely to
grok. Anything beyond From, and you've probably lost at least half of
the user population at least.
So mailing lists preserve the original From: and perhaps add a Sender:.
If I had to chose a signature which was most relevant for _display_ purposes
(note that display purposes is but _one_ reason for signatures), I'd
like to
be able to say something good about the From address that most users
grok. It's unclear that I'd want to say something nice about the Sender or
any other third party signature because it may confuse users that the
mere presence of a signature imputes some trust, where it actually needs
to be considered on a case by case basis. I for one would only render the
sender: as "verified" both if it verified correctly as well as it being
on some
known whitelist of domains that I trust.
So the bottom line is that a valid third party signature from, say, a
mailing
list is not safe a priori. It requires special handling by the ultimate
receiver
in the form of a trust relationship with that domain which needs be done
out of band. The same is not of a first party signature because you're only
vouching for yourself which is already as good you trust that domain (or
not).
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I'm not sure what the right answer is just yet, but I'll offer some of
my thoughts FWIW.
I think the fundamental question is whether the mailing list is the
originator of the messages its members receive or whether the original
author is. This question has come up in other contexts before, and I
don't think it's ever been answered satisfactorily. A quick search
through DKIM archives seems to indicate that this question has come up
there too, and I think answering it will help us understand what we
should be doing here.
On Feb 1, 2007, at 3:00 PM, Mark Sapiro wrote:
Consider that while Mailman doesn't do all of these things to every
message, it can do any of the following:
[munge the original message]
From the DKIM FAQ:
- What is the purpose of DKIM?
DKIM lets an organization take responsibility for a message. The
organization taking responsibility is a handler of the
message, either as its originator or as an intermediary. Their
reputation is the basis for evaluating whether to trust the
message for delivery.
I think you can make a legitimate case that Mailman is the originator
of messages its members receive. The message is certainly different
than the one the original poster sent to the list, and Mailman is
clearly an intermediary. Perhaps the message has only been munged in
very trivial ways, but it's also possible to munge it in ways that
could potentially be viewed as spammy. For example, what if a site
decides to put some advertisements in the footer?
If you take this view then it seems reasonable to say that it is the
mailing list's system that "take[s] responsibility for a message."
Sure, the mailing list system could verify the DKIM headers on the
message it receives, but ultimate, it is up to the mailing list system
to decide whether that message (or some derivative of that message)
gets transmitted to its recipients.
Or looked at another way, if I send a message through a mailing list,
I wouldn't want to vouch for whatever comes out the other end because
I don't know what they're going to do to my original content. Maybe
then, it's correct for the DKIM signature on the copied message to be
broken because what recipients got was /not/ the message I sent, and I
don't know how it was munged. But that view implies that I am the
originator of the recipient's message. I am, sort of, but also sort
of not.
I'm not convinced that DKIM is really designed to handle the mailing
list use case. It seems to me that it was designed to handle
point-to-point messages, not messages that flow through an
intermediary, because it's not an enveloping system. Contrast that
with S/MIME or OpenPGP. I can sign the message I send from my mailer
and that could be preserved through the transformations that Mailman
performs, with Mailman wrapping my original in its own signature if it
wanted to.
Practically speaking, if we can't come up with a consensus on the
interpretation of which "organization [should] take responsibility
for" the actual message that recipients receive, then what would be
the right thing to do? (Note that this answer is different depending
on whether we're talking about Mailman 2.1 or some future version.)
When this came up before I statement my preference not to make a
"strip DKIM headers" selectable by the list owner. I still prefer
this for Mailman 2.1 because doing so would clearly be a new feature.
Maybe a future version could treat the DKIM header the way it treats
the RFC 2369 headers, with a separate selector for List-Post.
Ideally, we'd have a more general way to decide which headers get
cleansed and which new ones get added. But that's for the future.
One elaboration you /might/ be able to get away with in Mailman 2.1
occurs to me as I look at Mark's list:
- Add text to the beginning of the message body (msg_header)
- Add text to the end of the message body (msg_footer)
- Remove text from the beginning of the message body (Approved: line)
- Add additional MIME parts to a multipart message (msg_header,
msg_footer)
- Convert a single part message to multipart in order to add
msg_header/msg_footer
- Remove parts from a multipart message (content filtering)
- Convert an HTML part to plain text (content filtering)
- Decode a base64 or quoted-printable encoded part and perhaps
re-encode it with a different encoding.
- Change or delete various headers including Subject:, To:, From:
- Replace some MIME parts with URLs of where they were stored and
flatten the entire message into a single plain text message (scrubber).
- Probably other things I'm overlooking.
If you could identify the message transformations that break the
signature, then you could remove the signature. If the signature of
the outgoing message were still valid because Mailman didn't touch any
part of the message affecting the signature, then you could keep it.
The implementation of this would be fairly simple; the hard part is
writing the code to verify a DKIM signature and parse the selectors
(IIUC the specification) to figure out which of the above
transformations would break the signature. That might be enough to
not do it in Mailman 2.1.
I'm not sure how much I like that anyway, so comments are definitely
welcome. After mulling over this post for an hour ;) I'm starting to
believe that it's the mailing list system that needs to vouch for the
messages its recipients receive. Of course, it could be Mailman doing
the DKIM signing, or it could be Mailman's outgoing MTA, etc. But,
ISTM Mailman is ultimately deciding what goes into the list copy, so
it is responsible for it.
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