Can you describe the main types of user authentication methods?

The main types of user authentication methods include:

1. Password verification — the most common method, where the user enters a login and password, which are compared with those stored in the database.

2. Multi-factor authentication (MFA) — combining multiple factors, such as password + SMS code or a code generator app.

3. Token-based authentication — for example, JWT (JSON Web Token), where a token is issued after successful login and used for subsequent requests.

4. Biometric authentication — using fingerprints, facial recognition, and other biometric data.

5. Authentication via external providers (OAuth, OpenID Connect) — login through third-party services like Google, Facebook.

6. Certificate-based authentication — using digital certificates to verify identity.

Example of a simple password check in C#:

public bool Authenticate(string username, string password) {
    var user = userRepository.FindByUsername(username);
    if (user == null) return false;
    return VerifyPassword(password, user.PasswordHash);
}