5.4.5 RSA Encryption Limitations

Every RSA initialization process requires the random selection of two very large primes, traditionally referred to as p and a, and computing n such that n=pq. Previously, we extolled the virtues of RSA over DSA, citing that RSA could be used for encryption and digital signature applications, while DSA was strictly for digital signature applications. In the real world, the encryption capabilities of RSA are rarely used for one simple reason: the length of the plaintext that can be encrypted is limited to the size of n. In fact, the real length is even smaller than n because of the overhead introduced by the algorithms. As a result, the predominate approach is to generate a random secret key and encrypt that key with the RSA keys. The message is then encrypted using a symmetric cipher with the generated secret key. The net-net is that two pieces of information are then sent over the wire. Let's talk about an example using Alice and Bob.