If you have a user's public key, you can verify they are who they say they are (ie. they hold the correspending private key):

The client takes their private key (defaults to ~/.ssh/id_rsa) and encrypts a random string as proof of work.

request = Sshkeyproof::Client.new key_file: "./id_rsa"

The server takes the request string and verifies it

s = Sshkeyproof::Server.new request

s.fingerprint # => public key SHA1 fingerprint

s.correct?(public_key)   # => true