Introduction to Quantum Cryptography

In this chapter we consider the problem of amplifying secrecy, or uncertainty. This is the problem of privacy amplification: given a partially secret string, how do we make it almost-perfectly secret? This task forms one of the key building blocks in the protocol for quantum key distribution that we develop in later chapters. It can be solved using a fundamental object from the theory of pseudorandomness called a randomness extractor. We introduce an extractor based on hashing and show that it can be used to perform privacy amplification.