Re: [dmarc-ietf] Reversing modifications from mailing lists

[Wei Chuang <[e-mail]>]

On Mon, Nov 29, 2021 at 11:27 PM Murray S. Kucherawy <[e-mail]>
wrote:

> On Thu, Nov 25, 2021 at 12:07 AM Wei Chuang <[e-mail]> wrote:
>
>> Sorry I wasn't too clear here.  It's largely the same idea as the DKIM
>> body length "l=" field above except for reformulated for the Subject header
>> and its mailing list mutations.  The original sender would encode a length
>> of the original subject say "s.l=<value>".  A receiver would only hash the
>> right most "s.l=<value>" length string when validating a Subject hash from
>> the original sender.  This assumes that mailing lists may prepend a string
>> typically for identification.
>
>
> Seems to me that means I could insert anything I want before the last N
> octets of Subject -- say, a URI pointing you to an ad or other unsavory
> content -- and the original signature will verify.
>
> -MSK
>

Agreed the original signature will verify.  We could also encourage
resigning by forwarders as done in your list-cannon draft in section 5 and
with a new "s.l=" field value, in which case we can determine who added the
URI to the subject in the above example.  Partially signed messages i.e.
signed by the original sender but not the forwarder might be treated with
more suspicion, than messages with full coverage.

-Wei