3

A Secret to Share

In the last chapter, we saw that in order to communicate securely over an insecure channel, Alice and Bob need a shared secret (or possibly more than one secret) that is only known to them. Once this is given, they can use cryptography to protect their communication against both passive attackers such as Eve and active attackers such as Mallory.

In cryptography, that shared secret is called a key, and we have seen that you can use a secret key K to establish a secure channel between Alice and Bob. But how do you actually generate a secure cryptographic key? How long should it be, and, perhaps most importantly, how can Alice and Bob agree on a key in a secure manner? In this chapter, we will try to provide a brief overview of these issues without becoming too engrossed in the mathematical details, which will be covered later.

In this chapter, we will cover the following topics:

• What is a key and what is a keyspace?

• What is the length of a key and how long should a...