"Public" key ciphers rely on the recent (a few decades old) discovery of encryption functions with the following characteristics:
This set of characteristics dramatically changes the security model in comparison with secret key ciphers. Some application that wants to receive confidential messages could advertise a "public" encryption key to the world, while retaining and keeping secret the private decryption key. Senders would encrypt the data under this public encryption key. Only the intended recipient, who has the private decryption key, can then read it.
This cipher eliminates the problems associated with securely distributing symmetric keys among many entities in a large system. Only one public-private key pair is required for each entity and purpose because everyone can safely know all of the public keys. Unfortunately, two new problems arise with this technique:
Real-world use of asymmetric ciphers also requires padding methods because asymmetric ciphers usually operate on a block of data whose size may depend on the key size. For the specific asymmetric algorithms described in Chapter 18, the padding method is described or referenced there.