What is a hash collision?
A hash collision is two different inputs with the same hash. Learn why collisions break MD5 and SHA-1 and why SHA-256 is safe.
A hash collision happens when two different inputs produce the same hash digest. Because a hash maps infinite inputs to a finite output, collisions must exist mathematically — the question is whether an attacker can find one on purpose.
Why collisions matter
A secure hash makes finding a collision computationally infeasible, so a digest reliably identifies its input — the basis of signatures and integrity checks. Once collisions become practical (as with MD5 and SHA-1), an attacker can craft a malicious file with the same hash as a trusted one, forging signatures.
The birthday problem
Collisions appear far sooner than intuition suggests: for an n-bit hash, a collision becomes likely after roughly 2^(n/2) attempts, not 2^n — the "birthday bound." That is why hash length matters and why 128-bit MD5 fell first. SHA-256’s 256 bits keep the birthday bound comfortably out of reach.
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