 is Z, and if b' = the measurement basis is X. In any case, there is a 50% chance of this measurement choice being "right" or "wrong," according to the description in the previous section. The outcome of Bob's measurement is, thus, a random sequence A' = (a', a'2, length = (4 + A) N. Using a public channel, either Bob announces to Alice his sequence of state- measurement bases (ZXZX...), or Alice announces to Bob her sequence of state- preparation bases (ZZXX.. The other person then tells which bits are to be discarded, according to the fact that either measurement or preparation was made in the "wrong" basis. The wrong bits being discarded, the two bit sequences A (from Alice) and A' (from Bob) are, thus, reconciled. With a sufficiently large bit sequence length N = (4 + A) n, there is a high probability that there are at least 2n valid bits in A, A'. In the contrary event, the protocol session must be aborted and restarted with new random sequences A, B, B'. We may conclude too quickly that the above protocol wholly suffices to guarantee the strict equality of Alice's and Bob's bit strings, or A = A'. Indeed, there is yet a possibility that the quantum channel used by Alice and Bob is corrupted by the effects of random quantum noise and also Eve's tampering actions (e.g., suppressing, injecting, or modifying some of the qubits). In this last event, Eve is capable of obtaining partial information about the secret key and, therefore, a higher level of security is required from Alice and Bob. This is where the techniques of secret key distillation (SKD) through information reconciliation and privacy amplification come into the picture. It is beyond the scope of this chapter to venture into the mathematical details of SKD algorithms. Here, it will suffice to convey a rough idea of the underlying principle. At this point, Alice and Bob have validated two 2n bit sequences, A, A', but have no reason to believe that these do perfectly match. Further, they suspect that some of the bits might have been corrupted by quant