Let m be a message (or data) and h(m) the hash of m. Note that m can be of any length, but h(m) is of a fixed length. Then the hash function h must satisfy the following.

1. For any m, it is easy to compute h(m)
2. Given h(m), it is infeasible to find any message n such that h(n)=h(m)
3. It is computationally infeasible to find any two messages x and y such that h(x)=h(y) (i.e., collisions are hard to find)