 critical is the communication success? In situations of conflict, where all the communications means (civilian or military) may be disabled, denied, or destroyed, there must always remain one way or another for communicating critical information. The cryptosystem must be able to borrow multiple, if not redundant, paths, just as with the Internet protocol. It must also be able to reach Alice or Bob anywhere they may happen to be, supposedly not in a predefined place. Notwithstanding its inherent strength, QKD remains a point-to-point, local cryptosystem whose extension at global scales and possibilities for path redundancy seem impractical. Furthermore, a classical communication channel is always required for Alice and Bob to compare measurements and agree on the secret key. The main assumption of QKD is that such a channel is always available, and resilient against any form of attack, and in realistic conflict situations this fact cannot be taken for granted! Finally, it is important to stress that despite the availability of provably-secure QKD protocols, the core cryptosystems eventually used in any classical message/ciphertext channels (DES, AES, and future upgrades) remain 100% exposed to conventional attacks (cryptanalysis, code-cracking .). Thus, channel security ultimately rests upon the classical notion of "code invulnerability", which represents a "reasonable conjecture" within a cryptosystem's lifetime. This discussion leads to the closing conclusion that despite its awesome conceptual elegance, quantum cryptography (or QKD) only represents a supplemental technique of information protection, to be situated somewhere within the grander domain of global network security, where there exists no such a thing as "absolute" confidence in any cryptosystems. Appendix A (Chapter 4) Boltzmann's entropy Task 1 Show that the number of ways W of arranging N particles into m boxes with populations N is given by Let us proceed as follows: we first apply the property according to which the number of ways of selecting n