rcuits. In contrast, classical cryptography can pretend to globality - the key requirement of truly seamless network security - while lacking the "provable invulnerability" of the quantum cryptosystem. 25.13 Is quantum cryptography "invulnerable?" In this concluding section, I shall analyze how a malicious Eve may be able to tamper with Alice's and Bob's secure photon communication (or QKD) channel and, hence, threaten the concept of absolute secrecy in key exchange and message communication. Assume, first, that a malicious Eve puts a "wire tap" somewhere inside Alice and Bob's photon communication channel. To access the information originally destined for Bob, Eve must perform photon measurements. But like Bob, she does not know, for each of the incoming photons, which measurement basis, X or Z, could be the correct one. Eve may perform the measurement anyway, but she now has to generate new photons in replacement, SO that Bob does not notice anything. But this is where Eve is tricked: she does not know in which basis-state Alice generated the input photon (X or Z), SO she has to make a guess with a 50% chance of being right. When Bob measures Eves' photon substitute, he also has a 50% chance of choosing the right measurement basis. At the end of the process, when Bob communicates his measurements to Alice over the public channel, it immediately appears that he has been only 25% successful! And this is the signature of Eve's wiretap. Eve may try to outsmart Alice and Bob by not measuring all of Alice's photons, but only a few at random, in the hope that her tampering is not detected, and retrieve at least some part of the secret key. But even this approach cannot succeed. This is because Alice will immediately detect that Bob does not get the right proportion of correct bits. The widely known conclusion is that it is not possible to eavesdrop photon communication channel without Alice and Bob becoming immediately aware of it. To remain undetectable, however, Eve may choose instead to tamper with only a few photo