On Wed, Mar 26, 2025 at 09:42:44AM -0700, Eric Rescorla wrote:
> I believe the following paragraph should have a reference to DKIM,
> which also provides a signature over the message.
>
> Very high confidence in the authenticity of a message and its
> originator lies only in end-to-end methods involving the message
> bodies, such as those that use digital signatures on the original
> message (see RFC 1847 [27] and, e.g., Pretty Good Privacy (PGP) in
> RFC 9580 [51] or Secure/Multipurpose Internet Mail Extensions (S/
> MIME) in RFC 8551 [47]).
>
> I recognize that the security properties of DKIM are slightly harder to
> state, in part because it does not provide end-to-end signatures, but
> it does in fact provide some level of authenticity for the message,
> up to the claim of the sender (in the signed From field), and in practice
> the mail server operator will often be able to control who gets
> a credential for a given user at that domain. However, DKIM is by
> far the most common mechanism by which emails are signed, so
> I think it needs to be mentioned, though I don't think a lot of detail
> is required. Perhaps something like:
There are a few important DKIM caveats that are complex to detail in the
SMTP specification.
- DKIM keys are by design short-term, required to persist only while
the message is in transit. The selector can be rotated
frequently, and old keys discarded. So a stored DKIM message
cannot generally be expected to be validatable later.
- The keys are typically not obtained over a secure channel.
- The attested identity is that of the domain, not the particular
sender, and in many cases the authenticity of the localpart of
the address is not known to or attested by the sending system.
So I'd be reluctant to drag DKIM into the SMTP specification, since its
main purpose is enabling use of domain, rather than IP reputation for
message scoring by MTAs and anti-spam filters, rather than actually
authenticating the message author to a recipient.
--
Viktor.
--
last-call mailing list -- last-call@xxxxxxxx
To unsubscribe send an email to last-call-leave@xxxxxxxx
