RSA is an extremely useful algorithm; however, a full-fledged messaging protocol will generally involve a number of elements beyond RSA itself. Popular programs like PGP, GPG, and Lotus Notes combine a number of algorithms to form a total e-mail security system. In outline, the programs mentioned have pretty much the same elements. Let's take a look at what these elements are and how we might hypothetically build our own e-mail security protocol.

One important thing we have not yet mentioned about RSA is that it is quite slow in practice. As a mathematical abstraction, RSA looks like a good way to encrypt a message, but in real-life applications, we just do not have the CPU time to spare for RSA. Directly encrypting a message with RSA is likely to be approximately 100 times as slow in software as encrypting with DES (and DES is not a particularly speedy algorithm). By combining bits and pieces of several algorithms, we can create a practical program with desirable performance and security characteristics.