Thinking about list footers

Murray S. Kucherawy writes:
As Mark and Barry point out, the MUAs-for-people-most-vulnerable-to- email-fraud often handle them poorly.
Also, the last time partial signatures came up, it was pointed out that there are *no* MUAs that differentiate between signed parts and unsigned parts. You don't get a warning when your eyes move from a signed part to an unsigned part or vice-versa the way you do when following a link from an HTTP URL to an HTTPS URL in a browser. The DKIM advocates have not liked the idea of signatures that don't apply to the whole message at all.
And along those lines, do any MUAs do useful things with the various List-* fields, other than permitting one to sort on them?
I think many do. I had a proposal some years back that I discussed with the Mozilla people. The idea was to devise an algorithm for MUAs that would get rid of Reply-To munging in most cases, and an optional header field that would allow lists to express a preference. They thought it would be nice if someone would write up a document but weren't much interested in helping or implementing, they thought their products already did a good job. There were some refinements but the basic idea was
- If there is a Reply-To:, use that address, otherwise
- if there is a List-Post:, use that address, otherwise,
- reply to the address in From:.
The optional field (I even forget the name, it was something like List-Prefer-Reply) allowed giving From: priority over List-Post:.
I'm pretty sure that Thunderbird, Mutt, and Emacs/Gnus implement either a "smart" reply algorithm or or a reply-to-list function. Other Emacs-based MUAs probably do, and it would be trivial to add in most cases. I don't know about KMail, Sylpheed, and Evolution.
On the other hand Windows and web-based MUAs didn't do much useful at the time, and probably don't now, either.

On Sat, May 31, 2014 at 4:30 AM, Stephen J. Turnbull <stephen@xemacs.org> wrote:
All true, but that's mostly specific to MUAs. There's nothing saying a filter of some kind could do something special with appended content when it senses a message that's bigger than what was signed. The library in OpenDKIM does make it easy to spot these, for example, and can tell you stuff like which header fields were added or modified and in what way, or how much of the content was signed and how much wasn't.
We didn't intend for this to be used by MUAs, however, so to some degree they're doing what we expected.
The reason I asked is that there's a proposal for a DKIM canonicalization that could survive modifications if the modifications are entirely in new MIME parts. Thus, if an MLM altered the message strictly by adding parts, the added parts could be easily isolated by this method, and the remainder verified against an author signature that should still validate (modulo Subject field changes). So you'd have a DKIM signature from the author domain that validates on the original author content (the final content minus the added part), and a DKIM signature from the list domain that validates on the modified content. I'm trying to figure out if that would be useful at all, but it sounds like MUAs are the showstopper there.
-MSK

On Sat, May 31, 2014 at 4:30 AM, Stephen J. Turnbull <stephen@xemacs.org> wrote:
All true, but that's mostly specific to MUAs. There's nothing saying a filter of some kind could do something special with appended content when it senses a message that's bigger than what was signed. The library in OpenDKIM does make it easy to spot these, for example, and can tell you stuff like which header fields were added or modified and in what way, or how much of the content was signed and how much wasn't.
We didn't intend for this to be used by MUAs, however, so to some degree they're doing what we expected.
The reason I asked is that there's a proposal for a DKIM canonicalization that could survive modifications if the modifications are entirely in new MIME parts. Thus, if an MLM altered the message strictly by adding parts, the added parts could be easily isolated by this method, and the remainder verified against an author signature that should still validate (modulo Subject field changes). So you'd have a DKIM signature from the author domain that validates on the original author content (the final content minus the added part), and a DKIM signature from the list domain that validates on the modified content. I'm trying to figure out if that would be useful at all, but it sounds like MUAs are the showstopper there.
-MSK
participants (2)
-
Murray S. Kucherawy
-
Stephen J. Turnbull