Extra requirements: - Not trusting one company/hosting provider (like keybase/AWS). - Works for periodically or totally offline signing and verification. - No

Extra requirements: - Not trusting one company/hosting provider (like keybase/AWS). - Works for periodically or totally offline signing and verification. - Not be an ephemeral record (like a tweet) or undated. - Not require memorisation of very long passphrases. - Not be subject to mass compromise (i.e. like a bug in a Bitcoin wallet code on android).