How is authentication and authorization implemented in the system? Explain the SSO and OAuth2 mechanisms.

Authentication is the process of verifying a user's identity, such as confirming their login and password. Authorization is the process of determining the user's rights, i.e., what they can do within the system after authentication.

SSO (Single Sign-On) is a mechanism that allows a user to authenticate once and gain access to multiple related systems without re-entering credentials. It is usually implemented through a centralized authentication server that issues a token or session accepted by all services.

OAuth2 is an authorization protocol that allows third-party applications to access a user's resources with limited permissions without sharing their password. OAuth2 involves roles: resource server (stores data), authorization server (issues tokens), client (application), and user. The client receives an access token after successful user authorization and uses it to access resources.

A simplified OAuth2 flow example:

1. The user authenticates via the authorization server.
2. The server issues an access token to the client.
3. The client uses the token to make requests to the resource server.

Thus, SSO simplifies logging into multiple systems, while OAuth2 securely delegates access to resources without sharing passwords.