On Thu, Oct 27, 2022 at 4:16 PM Douglas Foster < dougfoster.emailstanda...@gmail.com> wrote:
> Murray raised the issue of a signature which produces PASS, but lacks > trust because it is constructed with weak coverage, such as omitting the > Subject or including an L=valuie clause. > > DKIM was designed to be flexible so that it could be used for many > purposes. DMARC is a specific purpose and therefore it needs a more > specific definition of what a signature should and should not contain. I > am proposing that we ensure that all signatures used for DMARC follow a > content standard so that all compliant signatures are equally trustworthy. > > For DMARC, an aligned DKIM PASS should preserve the originator's content, > identity, and disposition instructions. Any header that might > legitimately be added or removed by a downstream MTA should not be included > in the original DKIM signature, as these are likely to produced false DKIM > FAIL. > > Here is a first-pass list of headers that meet these objectives: > > Date > To > From > Subject > Body (absence of L=value) > Reply-To > In-Reply-To > Authenticated-As > > This feels like a layering violation to me, if we accept the model that DMARC is a layer atop DKIM. Also, DKIM already provides advice of this nature. RFC 4871 actually listed the header fields we thought SHOULD be signed, but this was removed in RFC 6376 in favor of more general guidance to select header fields that preserve the intent of the message. I think that's enough for DMARC. Finally, "Authenticated-As" isn't a known header field (or, at least, it's not in the registry). -MSK
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