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Security Fundamentals & Crypto · Intermediate

Hash Collision

When two different inputs produce the same hash digest.

Definition

Collisions always exist mathematically because outputs are fixed size, but a secure hash makes finding them computationally infeasible. Practical collision attacks broke MD5 and SHA-1 for use in signatures and certificates. Collision resistance strength is roughly half the digest length due to the birthday paradox.

Example

Attackers who find a collision can get a benign certificate signed and swap in a malicious one with the same hash.

Exam tip

Collision resistance of an n-bit hash is about 2^(n/2) operations.

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