How to Implement a Secure 'Remember Me' Feature in PHP?

Enhanced User Experience: Implementing a Secure "Remember Me" Feature in PHP

In the realm of user convenience, the "Remember Me" checkbox offers a seamless login experience by persisting a user's authentication between sessions. To ensure its security, understanding the best practices for storing cookies in a user's browser is crucial. Inspired by a trusted blog post, let's explore a robust implementation for this feature.

Core Structure

A separate database table, auth_tokens, provides the foundation for storing authentication information. It comprises fields for selector, token, user ID, and an expiration timestamp. Separating selector and token enhances security by preventing timing attacks during SELECT queries.

Initiating Remember Me

Upon successful login and activation of "Remember Me," the following operations take place:

1. Generation of random selector (12 characters) and authenticator (33 bytes).
2. Setting a cookie named 'remember' with the combined selector and base64-encoded authenticator.
3. Insertion of selector, hashed authenticator, user ID, and expiration timestamp into the auth_tokens table.

Re-Authentication on Page Load

In the absence of an active session and the presence of the 'remember' cookie, the following actions are performed:

1. Extraction of selector and authenticator from the cookie.
2. Database query to retrieve the corresponding row using the selector.
3. Comparison of the retrieved hashed authenticator with the database-stored hash using the secure hash_equals() function, protecting against timing attacks.
4. If the comparison succeeds, setting the session user ID and regenerating the login token.

Implementation Details

The selector utilizes 9 bytes of random data, providing a high level of collision resistance. The authenticator uses 33 bytes, ensuring its unpredictability. Storage of a hashed authenticator mitigates user impersonation risks.

The separation of selector and authenticator ensures constant-time database lookups, safeguarding against timing-based attacks.

By adhering to these principles, the "Remember Me" feature becomes an integral part of a secure and hassle-free user experience.

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