8 Comment on “ Exposed - Key Weakness of αη ” [ Phys . Lett . A 370 ( 2007 ) 131 ]

We show that the claim of insecurity of the Y-00 (or αη) cryptosystem made by C. Ahn and K. Birnbaum in Phys. Lett. A 370 (2007) 131-135 even under ciphertextonly heterodyne attack, as well as the unfavorable security comparison between αη and an additive stream cipher for all kinds of attacks, is based on invalid extrapolations of Shannon’s random cipher analysis to two concrete ciphers. We generalize earlier work on nonrandom ciphers to random ciphers like αη and show that arguments of the kind given by Ahn and Birnbaum can at best provide lower bounds on the average number of spurious keys seen by the eavesdropper and cannot even in principle establish insecurity of the system.