> His point was that you pointed out a use case for some sort of cryptographic signing, not for (ab-) using DKIM for this purpose rather than what it was designed for.

First, thank you for the clarification.

Second, to answer tptacek's point, I understand that authenticating emails as a third party is an unintended side effect of the DKIM protocol. I understand that cryptographers would like people to move onto using other protocols for purposes like this. However, the suggestion that Google should periodically publish and rotate their secret keys, does not achieve this goal in any way. If Google were to do this, the webstore that you purchase items from, would not suddenly start using different protocols to authenticate purchase receipts, they would continue to send regular email... but those emails could no longer be authenticated. Or if we go back to the example in OP, the politician that's admitting to crimes over email, they're certainly not going to switch over to another method of documenting their crimes.

Edit: I was incorrectly using GPG as an example. I removed the incorrect example and let the point stand without it.