The basic problems

We saw in Chapter 5 that the one-time pad is a cryptosystem that provides perfect secrecy, so why not use it? The obvious reason is that the key needs to be as long as the message and the users need to decide on this secret key in advance using a secure channel.

Having introduced public key cryptography in Chapter 7 one might wonder why anyone would want to use a symmetric cryptosystem. Why not simply use RSA or some other public key cryptosystem and dispense with the need to exchange secret keys once and for all?

The problem with this approach is that symmetric cryptosystems are generally much faster. For example in 1996, DES was around 1000 times faster than RSA. In situations where a large amount of data needs to be encrypted quickly or the users are computationally limited, symmetric cryptosystems still play an important role. A major problem they face is how to agree a common secret key to enable communications to begin.

This basic ‘key exchange problem’ becomes ever more severe as communication networks grow in size and more and more users wish to communicate securely. Indeed while one could imagine. Alice and Bob finding a way to exchange a secret key securely the same may not be true if you have a network with 1000 users.