Hi - crypto team lead here. I'll hijack this comment to try to explain what Proton Bridge is intended to do, and why it doesn't work the way OP wants.
Bridge is a proxy which hosts a local IMAP and SMTP server, and takes "normal" unencrypted and unsigned messages from desktop MUAs like Thunderbird, signs and encrypts them, and then sends them out. Note that this requires changing the MIME message somehow.
OP writes:
> Everything was great until I decided the other day that I’d also like to do PGP signing on my outgoing messages.
The "intended" way to do this is enable the setting in Proton Mail that says "Sign external messages" :) That way, Bridge will sign them for you. (Internal messages are always signed.)
> Tough luck, bucko, we’re the SECURE email company, you’ll upload your private key to our servers and you’ll like it!
FWIW, private keys are stored encrypted on the server, we don't have access to them.
But yes, the entire goal of Proton is to handle PGP for you, without having to set up PGP encryption and signing manually on all of your devices. I know that the HN audience is fully capable of doing so, but our goal is to make it easier for everyone else :)
> It’s absurd that there’s no way to disable this, no option to tell Proton “if you see a multipart/signed or multipart/encrypted message, just leave it the hell alone.”
IMO, if we see a multipart/signed message, we should still encrypt it whenever possible, not leave it alone. But note that normally in OpenPGP, signing and encrypting is a single operation. It's possible in PGP/MIME to sign a message first and then encrypt it, but we don't support sending that way at the moment, though we could of course add that in the future. But in any case, that's the reason we currently recommend signing using Bridge rather than manually using gpg or similar.