In this section, we assume the following about the adversary Eve:
Once one has chosen not to assume a completely unbounded adversary - as we do here - it is always debatable which restrictions one should place on Eve, and we do not claim to have the only reasonable answer to this. We have tried to be quite generous to Eve compared to today's technology, but at the same time we have tried to avoid assumptions that seem to us to be unnatural or unnecessarily generous. In particular, the first two assumptions are long shots given today's state-of-the-art cavity QED experiments, where quantum non-demolition measurements on single photons are carried out. However by development of the existing technology they may become feasible in the near future. We also do not expect Eve to perform collective attacks, which require a quantum computer of some sort. The third assumption denies Eve' ability to store photons in an unknown quantum state. Even though proposals for quantum memory exist in the literature, none of them seems to assume memory time longer than a fraction of a second. It is always possible to wait with the announcement of the basis for that long time and therefore we discard the quantum memory feature. Our last assumption is the main difference between our analysis and the less optimistic one from Brassard et al. , where it is assumed that Eve can somehow make Bob detect photons perfectly.
In the following, we analyze which attacks Eve may launch on various types of pulses sent by Alice, under the constraints we assumed. Then we put this together, to see what the maximal damage is that Eve can do. Of course, the goal of Eve will all the time be to obtain as much information as possible about the bits sent, while causing as little disturbance on the signal as possible. In particular, Eve must make sure that the error rate and size of the raw key is close to what Alice and Bob expects, since otherwise they will abandon this run of the protocol. For simplicity, we will focus on what happens asymptotically for long keys.
By the raw key, we mean in the following the bit string as sent by Alice after she has removed the bits for which Bob did not detect the corresponding pulses or measured in the wrong basis.