ABSTRACT
Entire messages can be enciphered with RSA, but it is a slow algorithm. The competition, in the late 1970s, was the Data Encryption Standard (DES), which was a thousand times faster. Yet, for DES, a key had to be agreed on ahead of time. So, what’s the solution? Which of these two systems should be used? The answer is both! Loren M. Kohnfelder suggested a hybrid system in his 1978 undergraduate thesis, written while he was studying electrical engineering at MIT.* Whitfield Diffie recalled ten years later how this was “hailed as a discovery in its own right.”†
Here is how RSA and DES can be combined:
1. Generate a random session key, K, which will only be used once. 2. Use K to encipher the message with DES (or any other traditional
symmetric cipher). 3. Encipher the session key using RSA, and send it along with the
ciphertext.
