Why Does Python\'s `hash()` Function Produce Different Results Between Sessions?

Why Python's hash Function Produces Variable Results Between Sessions

In Python 3.3 and beyond, the built-in hash() function generates distinct hashes for identical strings in different sessions. This behavior stems from a design choice to protect against malicious input exploiting collision vulnerabilities.

To prevent attackers from overloading the application with colliding keys, Python utilizes a random seed that varies across sessions. This offset ensures unpredictability, thwarting attackers' ability to craft collisions.

Developers can override this default behavior by setting the PYTHONHASHSEED environment variable. A fixed positive integer value will set a specific seed, while setting the value to 0 will disable the offset entirely.

Python 2.7 and 3.2 do not enable this feature by default. However, Python 3.3 onwards incorporates it to enhance security.

The implications of this variable hash behavior extend beyond Bloom Filters. It affects the order of elements in sets, dictionaries (in Python 3.5 and earlier), and other mapping structures. Python provides no guarantees regarding this ordering, which can vary based on insertion, deletion, and the random hash seed.

For stable hash implementations, consider using the hashlib module, which provides cryptographic hash functions. The pybloom project relies on this approach for reliable hashing.

It's worth noting that storing the hash offset is impractical due to its complex structure. However, this added complexity also hinders attackers from exploiting timing attacks to determine the offset.

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