How Unique Are PHP Session IDs, Really?

Unique Identifying Characteristics of PHP Session IDs

Conventional wisdom suggests that PHP session IDs are highly unique, essentially preventing multiple users from sharing the same ID. However, a deeper understanding reveals a different reality.

By default, PHP's session ID is generated using a hash involving information such as the current time, which may not be entirely unique. To enhance uniqueness, it is recommended to configure session entropy by setting entropy sources in the server's php.ini file:

ini_set("session.entropy_file", "/dev/urandom");
ini_set("session.entropy_length", "512");

These settings utilize the system's randomness to improve the uniqueness of generated session IDs.

Underlying Algorithm

Searching for "php_session_create_id" in PHP's codebase unveils the core algorithm used for ID creation. It operates as a Deterministic Finite Automata (DFA) random number generator seeded with the process ID (pid) and time information. This approach may not provide sufficient uniqueness from a security standpoint.

PHP 5.4.0 and Beyond

As of PHP 5.4.0, session entropy defaults to /dev/urandom, a more reliable entropy source. For PHP versions prior to 5.4.0, configuring session entropy manually is crucial. By adopting these measures, you can significantly enhance the uniqueness of PHP session IDs.

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