为何用composer安装laravel时要验证？

我使用了中国镜像源，是按composer官方的命令操作的。

安装laravel时报如下错误，为何？以前在我另一台电脑安装时没问题的，现在换了台电脑。就出现这个问题如何解决？

[www@localhost composer]$ composer global require "laravel/installer"
Changed current directory to /home/www/.config/composer
Using version ^2.0 for laravel/installer
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
                                             
    Authentication required (packagist.phpcomposer.com):
      Username: 
      Password: