Once the simple substitution cipher had been cracked, codemakers began to develop new, stronger ciphers. The history of cryptography is a process of evolution. Evolution is a wholly appropriate term, because the development of cryptography can be viewed in terms of an evolutionary struggle.


A cipher is constantly under attack from codebreakers. When the codebreakers have developed a new weapon that reveals a cipher's weakness, then the cipher is no longer useful. It either becomes extinct or it evolves into a new, stronger cipher. In turn, this new cipher thrives only until the codebreakers identify its weakness, and so on. This is analogous to the situation facing, for example, a strain of infectious bacteria. The bacteria live, thrive and survive, until doctors discover an antibiotic that exposes a weakness in the bacteria and kills them. The bacteria are forced to evolve and outwit the antibiotic, and, if successful, they will thrive once again and re-establish themselves. The bacteria are continually forced to evolve in order to survive the onslaught of new antibiotics.


This section includes three ciphers that are stronger than those previously described, but each was cracked in turn.