对不同站点之间的请求进行加密以进行跨域身份验证

There are two sites. The main site, where there is an authentication logic for users and second site that works as an additional service for main site and has its own database, code, technology etc. Both sites are own by different teams. Second site should be able to authenticate users, but has no access to database and code of the main site and is hosted on the different domain. There are two sites. The main site, where there