I'm curious why you chose to use asymmetric crypto for this instead of symmetric (for example, encrypting a secondary key using the primary one, and then using that with an authenticated encryption mode like AES-GCM to give confidentiality/integrity).
Are there plans to have the protonmail service provisionally add contact info for the user, then have the user accept this by signing it with their private key?
I can only presume,but I believe they simply want it to integrate well with the existing protonmail public key system. Also,maybe they plan on letting protonmail users share or send contacts(non-repudiation and authentication).
Encrypted contacts uses PGP, which in turn uses symmetric crypto internally (the symmetric key is encrypted using the asymmetric private key). So it's both standards compliant and fast.
Are there plans to have the protonmail service provisionally add contact info for the user, then have the user accept this by signing it with their private key?