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Hash Generator

Glossary

Birthday Attack

A birthday attack uses probability to find hash collisions in about 2^(n/2) tries instead of 2^n — why a 256-bit hash gives only ~128-bit collision security.

A birthday attack is a generic method for finding hash collisions that is far faster than brute force. It takes its name from the birthday paradox: in a room of just 23 people, there is a better-than-even chance two share a birthday, because the number of pairs grows quadratically.

Applied to an n-bit hash, an attacker who computes about 2^(n/2) digests has a good chance that two of them collide — rather than the 2^n work needed to hit one specific target. This square-root speedup is why:

  • Collision resistance for an n-bit hash is only about n/2 bits.
  • A 256-bit hash like SHA-256 targets ~128-bit collision security, which is why 256-bit outputs are the modern baseline.

The birthday bound is a property of all hash functions — it does not mean a function is broken. Practical breaks (MD5, SHA-1) are much faster still. See how cryptographic hashing works.