Email registration helper
My proposed idea for this: Make an SSL connection to smtp.gmail.com:587. Verify the certificate using the same pinned certificate used by Chromium from https://src.chromium.org/viewvc/chrome/trunk/src/net/base/transport_security_state_static.certs?view=markup (I think the order of that file is the same as
enum SecondLevelDomainName in https://src.chromium.org/viewvc/chrome/trunk/src/net/base/transport_security_state.cc?view=markup). Encrypt the message body (containing the client IP address and port) using a public key generated specifically for this registration method. A backend program retrieves and decrypts the messages and sends them to the facilitator.
Most people probably use Gmail using a browser rather than STMP, so this may be conspicuous. I wouldn't want to take a chance of using a browser and possibly someone's identifiable cookies. Another possibility is for users to send their client registration using plaintext, manually with their own email client. But this has the big downsides of allowing Google to see all the registrations, and also we don't want to know people's email addresses.