What does increasing the hash output size affect in the context of birthday attacks?

• A. It increases the probability of collisions
• B. It reduces the space of possible hashes
• C. It increases resistance against collisions
• D. It makes hashing slower but no security impact

Answer: (D)

Increasing the hash output size expands the space of possible hash values, which directly strengthens protection against collisions. In a birthday attack, the chance of finding two different inputs that produce the same hash grows with the number of outputs you can choose from, and the work required to cause a collision scales roughly with 2^(n/2) for an n-bit hash. So, larger output size means more possible hashes and a higher work factor to find a collision, making collisions significantly harder and reducing their probability for a given set of inputs. The idea that hashing would be slower with no security impact isn’t accurate—the primary effect is improved collision resistance, with any speed difference being a secondary, implementation-dependent detail.