谈何容易！ In order to design a workable public-key cryptosystem,we must figure out how to create a system in which we can reveal a transformation P()without thereby revealing how to compute the corresponding inverse transformation S40.This task appears formidable,but we shall see how to accomplish it. 或者换句话说： The encoding function f must be relatively easy to compute,but f-1 must be extremely difficult to compute without some additional information,so that someone who knows only the encrypting key cannot find the decrypting key without prohibitive computation.谈何容易！ The encoding function f must be relatively easy to compute, but f-1 must be extremely difficult to compute without some additional information, so that someone who knows only the encrypting key cannot find the decrypting key without prohibitive computation. 或者换句话说：