Birthday Attack - Understanding The Problem

Understanding The Problem

As an example, consider the scenario in which a teacher with a class of 30 students asks for everybody's birthday, to determine whether any two students have the same birthday (corresponding to a hash collision as described below; for simplicity, ignore February 29). Intuitively, this chance may seem small. If the teacher picked a specific day (say September 16), then the chance that at least one student was born on that specific day is, about 7.9%. However, the probability that at least one student has the same birthday as any other student is around 70% (using the formula for n = 30).