I'm building slides to talk about DKIM2 in a webinar shortly, and thinking about the crypto migration strategy.
The first bits make sense: a) define a new key type b) start signing with that in addition to the original algorithm c) ... time passes, everybody can verify both types d) you stop signing with the old algorithm The issue is, how do you know when it's safe to do "d"? If email silently disappears, then people will be very wary of removing the old message, and it's via multiple hops! So we really want some response DSN which explicitly says "your DKIM2 signature looked reasonable but we don't understand your algorithms". This would allow probing for issues with removing an algorithm, and an ability to re-send with the old algorithm added back in if the delivery failed. Including older algorithms as well isn't a security risk if we work from the basis of "if it fails the strongest algorithm you know about it's dodgy"; but we still want to remove excess DNS lookups and header junk over time. Bron. -- Bron Gondwana, CEO, Fastmail Pty Ltd / Fastmail US LLC [email protected]
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