/di-skrēt lȯ-gə-ri-t͟həm prä-bləm/
Given integers a, n and x, find some integer m such that am mod n = x, if m exists. Modular exponentiation, the am mod n part, is straightforward (and special purpose chips are available), but the inverse problem is believed to be very hard, in general. Thus it is conjectured that modular exponentiation is a one-way function.