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In message <[email protected]
93.PROD.OUTLOOK.COM>, MCN-IETF <[email protected]> writes
>As I understand the recent discussion, if an intermediary receives a message
>for
>which the existing DKIM2 chain can no longer be continued, it may remove the
>previous DKIM2 information and start a new chain with m=1, i=1.
correct
>If that is correct, I am wondering what the final receiver can actually know
>about the history before the new i=1.
>
>For example:
>
>A (i=1) -> B (i=2) -> legacy intermediary -> C
> |
> DKIM2 chain lost
>
>C -> new DKIM2 chain (i=1) -> D -> Receiver
>
>When the Receiver verifies the second chain
there is no "second chain" ... if you do not remove the existing
DKIM2-Signature records then your message will be malformed and it is
most likely to be rejected
>, can it determine in any way that
>this message previously had a DKIM2 chain which was interrupted or discarded?
no ... and why would a receiver wish to know that, or indeed why would
it consider your reporting of this information to be valid ?
>Or, from the Receiver's point of view, is this indistinguishable from a
>message
>whose DKIM2 history genuinely started at C?
>In other words, does i=1 only mean:
>
>"this is the beginning of the DKIM2 chain that I can verify"
yes
>rather than:
>
>"this is the original DKIM2 origin of this message"?
if history is discarded then it is gone : the notion that someone can
attest to you that they can reveal the history of a message to you is
what underpinned the ARC experiment. We have found that this is not
valuable and so we are moving on
>If the previous chain is completely removed, I assume the Receiver cannot
>distinguish between a legitimate restart after a legacy intermediary and
>someone
>simply removing an existing DKIM2 chain and starting a new one. Is that
>correct?
absent any other meta-data and some fancy heuristics (or a signed
attestation from a notary passed out-of-band) yes
>There is also a related identity question. If a DKIM2 intermediary discards an
>existing chain and starts a new one at i=1, what authenticated identity, if
>any,
>can a Receiver associate with the original message, as opposed to the identity
>represented by the new i=1 signer?
there is no "original message" that the receiver can identify, because
there are no signed recipes to construct it
>I am mainly trying to understand exactly where the cryptographically
>verifiable
>history starts and, equally importantly, whether DKIM2 gives the Receiver any
>indication that some history existed before that point.
DKIM2 can claim that there was a message before the first
DKIM2-Signature was applied by providing recipes in the i=1 instance.
Whether that is of any practical use to anyone is still to be determined
by interworking experiments. I would suggest prioritising running code
so that you can fully participate in those experiments
- --
richard @ highwayman . com "Nothing seems the same
Still you never see the change from day to day
And no-one notices the customs slip away"
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