I find it hard to believe that you couldn’t retrofit Signal into an email app. Encrypted comms is quite good at sitting on unencrypted untrusted channels and signal is already asynchronous.
The challenge is that there’s no money to be made so you’d need a non profit like Signal to do it to disrupt that industry.
The challenge is that there’s no money to be made so you’d need a non profit like Signal to do it to disrupt that industry.