Public Key Ciphers
The ciphers we have discussed so far require a key for each pair of users. A secure method-such as the use of couriers with locked briefcases-is required for the exchange of these keys. Such methods of key exchange can be expensive, unreliable, and time consuming. If there are many pairs of users in a system, each using a diﬀerent key, the cost becomes prohibitive. In this chapter we describe cryptosystems that allow keys to be exchanged securely in the open.