Why Does the Python 3.3 `hash()` Function Produce Different Results for the Same String?

Python 3.3 Hash Function Discrepancies: Unveiling the Security Mechanism

In Python 3.3, the hash() function has been observed to return varying results for the same string across different sessions. This seemingly enigmatic behavior is rooted in a deliberate security mechanism implemented to thwart denial-of-service attacks.

To understand this mechanism, it is essential to recognize that Python utilizes a random hash seed that is set at startup. By incorporating this offset into hash calculations, attackers are deprived of the ability to design keys specifically intended to cause collisions.

To illustrate, consider the hash value for the string "235":

>>> hash("235")
-310569535015251310

Upon starting a new Python console, the hash value changes:

>>> hash("235")
-1900164331622581997

This variability serves as a protective measure against attackers who might exploit the worst-case performance of dict insertions, leading to O(n^2) complexity. As a result, attackers cannot predict which keys will collide and induce denial of service.

However, it is noteworthy that the offset does not merely entail a simple addition or subtraction. It comprises a prefix and a suffix, both of which are unpredictable and constantly changing. This complicates the storage and utilization of the offset.

Alternatively, for applications requiring a more stable hashing mechanism, one can explore the hashlib module, which offers robust cryptographic hash functions. It is the preferred choice in the pybloom project for its reliability.

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