First we refer to the result where it is shown that in the six-state variant of the BB84 protocol (Bru\ss, Phys. Rev. Lett., 1998), the mutual information between Alice (the sender) and Eve (the eavesdropper) is higher when two-bit probe is used compared to the one-bit probe and hence the two-bit probe provides a stronger eavesdropping strategy. However, from cryptanalytic point of view, we show that Eve has the same success probability in guessing the bit transmitted by Alice in both the cases of the two-bit and the one-bit probe. Thus, we point out that having higher mutual information may not directly lead to obtaining higher probability in guessing the key bit.
It is also explained in the work of Bru\ss~that the six-state variant of the BB84 protocol is more secure than the traditional four-state BB84. We look into this point in more detail and identify that this advantage is only achieved at the expense of communicating more qubits in the six-state protocol. In fact, we present different scenarios, where given the same number of qubits communicated, the security comparison of the four and six-state protocols is evaluated carefully.Category / Keywords: Advantage, BB84, Key Distribution, Optimal Eavesdropping, Quantum Cryptography Publication Info: Accepted in INDOCRYPT 2012 Date: received 30 Oct 2011, last revised 8 Nov 2012 Contact author: goutam paul at ieee org Available format(s): PDF | BibTeX Citation Note: This is a substantially revised version (with a slight change in title) that has been accepted for presentation in INDOCRYPT 2012. Version: 20121108:105927 (All versions of this report) Short URL: ia.cr/2011/588 Discussion forum: Show discussion | Start new discussion