ve missed the global application context. Quantum cryptography In this section, and the following ones, we shall venture into the intriguing and tricky subject of quantum cryptography (QC). After having reviewed the vast array of possi- bilities offered by classical cryptosystems (DES, AES, PKC) and being convinced that a would-be "Eve" would face extreme difficulties in code-breaking attempts, we may first wonder about or challenge the usefulness of QC approaches. It is possible to answer such a question straight away. First, from any current knowledge, it must be stated that QC is not a means of encrypting and decrypting information; rather, it is a means of distributing secret keys over a public channel; moreover, such an approach, referred to E. Desurvire, Wiley Survival Guide in Global Telecommunications, Broadband Access, Optical Components and Networks, and Cryptography (New York: John Wiley and Sons, 2004), Ch. 3, pp. 345-477. Classical and quantum cryptography as "quantum key distribution" (QKD), is provably, absolutely secure according to the laws of quantum mechanics. The above important statements will serve as a conceptual background for the fol- lowing description of QC and QKD and avoid raising other expectations. Here, we shall not venture into the debate of assessing whether or not absolute security (vs. that offered by PKC, for instance) is at all required in any secret-key exchange, or its impact on global communications security. The purpose of this chapter is only to convey the principle of how QKD works and not to try measuring the pros and cons of the approach from industrial and realistic network-environment standpoints. This being said, provable absolute security in the process of key exchange may only be seen as a desirable "extra" feature, especially if proven practical to implement and compatible with higher-layer network standards. The main difficulty is that academics involved in PKC may fail to recognize several other forms of network-security threats, particularly at the level of A