What a hash collision is
A hash collision occurs when two different inputs produce the same output from a hash function. A hash function takes data and maps it to a fixed-size value, but because many possible inputs are mapped into a limited range of outputs, different inputs can sometimes land on the same result. When that happens, it is called a collision. Collisions are a basic concept to understand when working with hashing.
Why collisions are unavoidable
A hash function maps a potentially huge or unlimited set of inputs onto a limited set of output values. Because there are more possible inputs than outputs, it is impossible to give every input a unique result, so some inputs must share. This means collisions cannot be entirely avoided in principle. Good hash functions aim to make collisions rare and spread out, but they can never eliminate them completely.
Collisions in hash tables
Hash collisions are especially relevant to hash tables, a data structure that uses hashing to store and look up data quickly. When two keys hash to the same location, the hash table must handle the collision so both can still be stored and found. Hash tables include strategies for dealing with collisions, allowing them to keep working correctly even when different keys produce the same hash.
Collisions and security
In security, hash functions are used in ways that depend on collisions being extremely hard to find on purpose. If someone could easily create two different inputs with the same hash, it could undermine systems that rely on hashes to verify data. For this reason, hash functions used in security are designed to make deliberate collisions impractical, and a function that fails at this is considered weak.
Managing collisions
Since collisions cannot be fully avoided, systems that use hashing are built to handle them gracefully. Data structures like hash tables use techniques to store multiple items that collide, and well-designed hash functions minimize how often collisions happen. Understanding that collisions are expected, and must be managed rather than wished away, is central to using hashing correctly.
Why it matters
Hash collisions are a fundamental aspect of how hashing works, affecting both data structures and security. Understanding what a hash collision is - two different inputs producing the same hash - clarifies why hash tables need collision handling and why security hashes must resist deliberate collisions. For anyone working with hashing, it is a key concept.
Related on Skillo
See also: What is a hash table? Explained, What is a data structure? Explained.
Sources
Published date reflects the original event date (2023-06-27). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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