What approaches and mechanisms are used for user authentication in systems?

Different approaches and mechanisms are used to implement user authentication in systems:

• Password authentication: the user enters a login and password, and the system verifies their correctness.
• Multi-factor authentication (MFA): in addition to the password, an additional factor is required (SMS code, code generator app, biometrics).
• Token-based authentication: after successful login, the user is issued a token (e.g., JWT), which is used for subsequent requests.
• OAuth / OpenID Connect: authentication through third-party services (Google, Facebook, etc.), which simplifies login and enhances security.
• Sessions: the server stores the authentication state of the user, usually via cookies.

Example of a simple token authentication over HTTP:

POST /login
{
  "username": "user",
  "password": "pass"
}

Response:
{
  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI..."
}

Subsequent requests:
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI...

The choice of mechanism depends on security requirements, convenience, and system architecture.