The existence of public key cryptography was first postulated in print in the fall of 1975 by Whitfield Diffie and Martin Hellman. The two researchers, then at Stanford University, wrote a paper in which they presupposed the existence of an encryption technique with which information encrypted with one key could be decrypted by a second, apparently unrelated key. Robert Merkle, then a graduate student at Berkeley, had similar ideas, but due to the vagaries of the academic publication process Merkle’s papers were not published until the idea of public key encryption was widely known.
Since that time, a variety of public key encryption systems have been developed. Unfortunately, there have been significantly fewer developments in public key algorithms than in symmetric key algorithms. The reason has to do with the way that these algorithms are designed. Good symmetric key algorithms simply scramble their input depending on the input key; developing a new symmetric key algorithm simply requires coming up with new ways for performing that scrambling reliably. Public key algorithms tend to be based on number theory. Developing new public key algorithms requires identifying new mathematical problems with particular properties.
The following list summarizes the public key systems in common use today:
A system for exchanging cryptographic keys between active parties. Diffie-Hellman is not actually a method of encryption and decryption, ...