code: func (tc *TeleportClient) u2fLogin(ctx context.Context, pub []byte) (*auth.SSHLoginResponse, error) {
  	// U2F login requires the official u2f-host executable
  	_, err := exec.LookPath("u2f-host")
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	password, err := tc.AskPassword()
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	response, err := tc.credClient.SSHAgentU2FLogin(
  		ctx,
  		tc.Config.Username,
  		password,
  		pub,
  		tc.KeyTTL,
  		tc.CertificateFormat)
  
  	return response, trace.Wrap(err)
  }
docs: // directLogin asks for a password and performs the challenge-response authentication