Recall that when Whitfield Diffie and Martin Hellman first explained the idea of public-key ciphers, they did not suggest an actual type of public-key cipher. What they did was explain how mathematical operations that are easy to do but difficult to undo could be used to create public-key ciphers. Operations that are easy to do but difficult to undo are sometimes called one-way functions. We have already seen an example of a one-way function, the one that forms the basis of RSA ciphers: given a pair of very large prime numbers p and q, it is easy to form p · q, but given only the result of this multiplication, it is in general very difficult to find p and q.
Although Diffie and Hellman did not suggest an actual type ...
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