Hi,
On Tue 23/Nov/2021 00:28:01 +0100 Wei Chuang wrote:
1. Ale's draft suggests not reversing all possible transforms, and rather
focuses on a subset caused by mailing lists that are reversible
* Could ARC be suitable for those other scenarios? Could we expect that
forwarders that do more substantial irreversible rewriting such as modifying
URLs in a spam/phishing filter MTA, already have a strong relationship with the
receiver? Presumably, might they be trusted by the receiver and their ARC
result could be used?
Sure. Note that if the receiver trusts the MLM, simply recognizing it would be
enough to pass DMARC per the "mailing_list" policy override. ARC additionally
provides the ability to learn the authentication status of the message when it
was received by the MLM. That way, reputation can be reckoned with great
precision.
2. Footers must only be added with as a) append on single text/plain part b)
mime part appended to multipart/mixed c) mime wrap where a footer is added in a
new multipart/mixed.
* It's not very clear to me how Ale's draft handles the b) and c) scenario.
(There is mention of "reason="transformed"", but this still seems incomplete)
I saw that Murray has a draft draft-kucherawy-dkim-list-canon
<https://datatracker.ietf.org/doc/html/draft-kucherawy-dkim-list-canon> that
identifies addition of new mime parts that could be helpful there.
I tried and implemented Murray's draft, but it requires that MLMs declare which
transformation they do. Since they don't, you need a pre-parser that guesses
the transformation type. That's the difference between the two drafts.
If there are two top-level MIME parts, the transformation must be (c), because
no one writes a MIME structure with just one part. Otherwise it's (b).
3. Footers added to text/plain must be identified with at least four "_" as a
separator.
* Would the DKIM length "l=" field be helpful? Understood there are abuse
risks.
Yes, l= could be a useful hint.
The risk of l= is that an attacker could exploit a poor HTML interpreter to add
a part that completely hides the original content when rendered. Requiring
attachments to be plain text avoids that risk.
4. "quoted-printable encoding must not be used for... single-part text/plain
messages, as it is impossible to guess original soft line breaks after re-encoding"
* Are you suggesting quoted printable encoding aren't fully reversible?
Actually, could the RFC2045 canonical encoding of the message be used as the
source for doing the DKIM content hashing? This would bypass having to worry
about additional transfer re-encodings by forwarders.
Mailman can copy MIME structures without changes, but simple text is often
re-encoded. Many messages on this list are converted to base64. If the
original text was quoted printable, its form depends on the agent. An agent
can choose where to break lines, whether to encode some characters or represent
them as ASCII, whether to break lines at column 76 or, to increase readability,
at white spaces. That can vary too widely.
5. Finding the original FROM by looking at From, Author, Original-From,
X-Original-From, Reply-To, and Cc.
* Can this be standardized to a fixed location such as Author? (Sorry I'm
unfamiliar with the discussion on Author)
That's exactly the purpose of Author. However, no one is using it yet.
6. Subject
* Agreed that some simple heuristic as proposed in the draft is a good
approach. Perhaps the original subject suffix length also might work here too.
I don't get this, I'm afraid. What is the subject suffix length?
Best
Ale
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