Encryption for notification emails

[almereyda]

Description:

Synapse sends notification emails. These contain the plain text of a message, when it is unencrypted. They contain the phrase "An encrypted message." for encrypted ones.

It is imaginable that people put their GPG / S/MIME public Key / a reference to a fingerprint on a Keyserver into their profile and Synapse uses that to encrypt notification messages about encrypted messages.

This way people could keep a human-readable copy of the message, while privacy is still maintained.

Due to the complexity of key exchange and multiple data paths, we would probably have to assume a multitude of edge cases, in which things don't work or work differently than in others. This would need to be considered, e.g. optionally also continuing to send the "An encrypted message." plain text notifications, in case something breaks or to have usable notifications when the secrets are not available.