class Net::SSH::Authentication::Session

def default_keys

by system default.
Returns an array of paths to the key files usually defined 
def default_keys
  %w[~/.ssh/id_ed25519 ~/.ssh/id_rsa ~/.ssh/id_dsa ~/.ssh/id_ecdsa
     ~/.ssh2/id_ed25519 ~/.ssh2/id_rsa ~/.ssh2/id_dsa ~/.ssh2/id_ecdsa]
end