um noise and eavesdropping. Their next step is to effect "information reconciliation." Alice selects a random subset X of n bits from her sequence A, sends X to Bob over a public channel (assumedly error-free) and also informs Bob which bits she has selected. Bob receives a sequence Y, which he is then able to compare with the corresponding n-bit selection from his A' sequence. Bob then deduces the error syndrome E (as defined by Y = X + E), deducts the number of errors e and tells Alice the value over the public channel. If e is over a certain commonly agreed threshold t, the QKD protocol is immediately aborted. In the favorable case (e t), Alice and Bob use a secret classical code, C1, having an error-correction capability of t, and unknown to Eve. This makes it possible for each to determine a common codeword W = X' = Y' corresponding to the correction of X and Y under this code. Yet it can be shown that Eve is able to possess partial information about W. Then the next step is for Alice and Bob to perform "privacy amplification," which consists of extracting from W a smaller secret key S of size m < n, based on their remaining n bits. How they achieve this and why, in this process, Eve's information about the reduced key S is lost Classical and quantum cryptography to an arbitrarily high level of confidence, is beyond the scope of this chapter to describe. The lesson to retain from the obligation for Alice and Bob to perform SKD is that QKD alone is not as "absolutely secure" a protocol as is commonly believed. 25.11 The B92 protocol A simpler version of the BB84 protocol, referred to as B92 (after its inventor, C. H. Bennet, and the year, 1992), uses only two reference qubits. Alice uses a random-bit generator to produce a single sequence A = (a1, a2,...) of N = (4 + A) n bits length. Then she produces an N-qubit block sequence 1/4), which can be expressed in the form: (25.26) For each cbit ak the corresponding single qubit (Vax) is generated according to the following convention: 140/=10) (25.27) On receivin