ns amongst a long sequence. She may even occasionally luck out in intercepting and re-emitting some of the photons in the right bases. This approach is referred to as a man-in-the-middle attack (MIMA), see more later. It may be justly argued that MIMA would not give Eve sufficient information to figure out the whole key. However, the key is weakened to some extent, and this fact justifies the need for SKD. Thus, contrary to widespread acceptance (except in the expert community), QKD alone is not absolutely secure, while QKD + SKD provably is, as far as the key-exchange protocol is concerned. See further for more discussion on how such a notion as "absolute security" may be otherwise challenged. 25.13 Is quantum cryptography "invulnerable?" If Eve is capable of eavesdropping Alice's and Bob's public communication channel, can she derive any useful information about their secret key? At a first level of analysis, the answer is definitely no. All the information that Eve has access to is ZXZX from Bob's side, and YesNoNoYes from Eve's side. Such information does not reveal in any way the outcome of Bob's valid measurements (+1 eigenvalues, or 1 and 0 bits). The fact that Eve knows which of Bob measurements were right does not tell her, in any way, which are the measured and valid bits. At a second level of analysis, however, the tapped information makes it possible for Eve to analyze the possible algorithms according to which Alice and Bob choose their random basis and measurement sequences. Thus, rather than trying to wiretap the key, Eve may figure out Alice's and Bob's random- sequence generation algorithms and, thus, reconstruct or predict, with some potential success, the key exchange. Should Eve access the technology used by Alice and Bob for random-key generation, she does not need to leavesdrop their photon communication channel anymore, having a powerful tool for key attacks on the classical DES and AES ciphers. This is referred to as random-number generator attack (RNGA). A second possibility for Eve is to