Example
f(x)=gx mod p for a large prime pExample
f(x) = x2 mod n when n=pq and p and q are unknown.
Definition
A function f:A -> B is called weakly collision free if for essentially
all x in A, it is computationaly infeasible to find a y in A with y not
equal to x and f(x) = f(y).
Definition
A function f:A -> B is called strongly collision free if it is computationally
infeasible to construct x and y in A with x not equal to y and f(x) = f(y).