Idea: uses trapdoor one-way function.
EK should be easy to compute
DK (inverting EK ) should be hard
=> EK should be a one-way function
Example of possible one-way function:
n=pq (p,q two large prime numbers)
Don’t want EK to be one-way from Bob’s point of view => Bob must possess trapdoor, secret information that permits easy inversion of EK