A birthday attack is a bruteforce collision attack that exploits the mathematics behind the [[Birthday Problem]] in probability theory. This attack can be used to abuse communication between two or more parties The attack depends on the higher likelihood of collisions found between random attack attempts and a fixed degree of permutations (pigeonholes). With a birthday attack, it is possible to find a collision of a hash function with 50% chance in $\sqrt{2^n} = 2^{\frac{n}{2}}$, with $2^{n-1}$ being the classical [[Preimage Resistance]] security with the same probability.