public function get_user_public_key( $user_id = 0 ) {
    global $wpdb;

    if ( empty( $user_id ) ) {
      return '';
    }

    $cache_key       = md5( 'edd_api_user_public_key' . $user_id );
    $user_public_key = get_transient( $cache_key );

    if ( empty( $user_public_key ) ) {
      if ( edd_has_upgrade_completed( 'upgrade_user_api_keys' ) ) {
        $user_public_key = $wpdb->get_var( $wpdb->prepare( "SELECT meta_key FROM $wpdb->usermeta WHERE meta_value = 'edd_user_public_key' AND user_id = %d", $user_id ) );
      } else {
        $user_public_key = $wpdb->get_var( $wpdb->prepare( "SELECT meta_value FROM $wpdb->usermeta WHERE meta_key = 'edd_user_public_key' AND user_id = %d", $user_id ) );
      }
      set_transient( $cache_key, $user_public_key, HOUR_IN_SECONDS );
    }

    return $user_public_key;
  }