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)

f(x) = x b mod n

• 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

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