GATE CS 2023 Set 1 — Question 20
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Programming & Data Structures → Hashing → Hash Functions
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Question
An algorithm has to store several keys generated by an adversary in a hash table. The adversary is malicious who tries to maximize the number of collisions. Let be the number of keys, be the number of slots in the hash table, and .Which one of the following is the best hashing strategy to counteract the adversary?
Correct answer
(C) Universal hashing method.
Solution
In a scenario where an adversary can choose keys to intentionally cause collisions, deterministic hashing functions like the Division or Multiplication methods are vulnerable because the adversary can pre-calculate which keys will collide. Universal Hashing counteracts this by choosing a hash function randomly from a carefully designed family of functions at runtime. Because the specific function is not known to the adversary beforehand, they cannot reliably pick keys that will collide, ensuring that the expected number of collisions for any set of keys remains small ().
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