I've set up K-9 mail + OpenKeychain <https://k9mail.github.io/documentation/security/pgpmime_current.html> on my up-to-date Google Pixel smartphone (Android v9). When I try to open a PGP encrypted email message in K-9, I get a modal dialog titled "Allow access to your key?" and a message, "This key is not available. To use it, you must import it as one of your own!" The PGP key of the *sender* is listed as the "Requested key". The sender's key is very definitely available and verified in OpenKeychain. I can decrypt the email in both Flowcrypt and in Evolution.
The fix had nothing to do with the PGP key of the sender; I had in fact mistakenly deployed a stripped version of my own key that didn't include the subkey used to encrypt the email, so verifying the PGP key of the sender was a "red herring." This looks like a usability bug to me, but I don't know which app is actually producing the misleading message. Does the information about the requested key coming from K-9? -- You received this message because you are subscribed to the Google Groups "K-9 Mail" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
