s. Such a return loss, which expresses the probability that a single photon will be reflected might range Classical and quantum cryptography between 10-2 and 10-6. A "giant" light pulse, with 102-106 photons, may provide Eve some information about Alice's and Bob's choices of X and Z bases, as based on the polarization state (I, n, U) of the reflected pulse. Additionally, such a probing is performed at a wavelength different from that used by Alice and Bob, SO that Eve's probing action may remain essentially unnoticed. This type of "side-channel" attack illustrates that QKD's absolute security may be challenged by classical means, and that great care must be taken in any physical inplementation of QKD to eliminate the possibility of any side channels. Another form of attack, which is far more straightforward, is for Eve to sever the photon communication channel physically. This is referred to as the denial-of-service attack (DoSA). The contingency plan for Alice and Bob would be to resort to classical cryptosystems and network means, exposing themselves to ordinary forms of classical security attack. Finally, the weakest point in a quantum cryptosystem is not the link, which, as we have seen, cannot be tampered with, without triggering alarms, but the terminals themselves. Since these terminals must be connected to a network of some kind, they are potentially exposed to attacks, for instance "spy" viruses, which can detect the keys that are exchanged between Alice and Bob. Considering these possibilities, is it possible to state that a quantum cryptosystem is absolutely secure? The answer is yes, but only within a certain set of assumptions regarding the security of the other elements in which the cryptosystem is embedded. The worst situation for Alice and Bob would be to trust, in absolute confidence, a system that could be wired without their awareness. An element that remains central to the discussion about cryptosystem security is the criticality of the application: what information must be protected, and how