You are not taking the entire hash, you are only taking the first 33 bits of the hash. Since there are only about 8.5 billion different values for the 33 bits a

You are not taking the entire hash, you are only taking the first 33 bits of the hash. Since there are only about 8.5 billion different values for the 33 bits and there are about 7 billion people, the odds are astronomically low that each of those 7 billion people will receive a different one of those 8.5 billion possibilities. This is the birthday paradox with instead of 365 days you have 2^33 possible answer values and instead of 23 people you have 7 billion people. I leave it as an exercise for the reader to fill in these values into one of the formulas to calculate the probability of successfully giving each person a unique 33 bit answer: http://en.wikipedia.org/wiki/Birthday_problem http://en.wikipedia.org/wiki/Birthday_problem