In the previous chapter we saw some fundamental limitations of perfect secrecy. In this chapter we begin our study of modern cryptography by introducing the weaker (but sufficient) notion of computational secrecy. We will then show how this definition can be used to bypass the impossibility results shown previously and, in particular, how a short key (say, 128 bits long) can be used to encrypt many long messages (say, gigabytes in total).
652.8 KAT i