Comment on “Exposed-Key Weakness of αη”

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 and on an invalid statistical independence assumption. We generalize earlier work on nonrandom ciphers to random ciphers like αη and show that expressions of the kind given by Ahn and Birnbaum can at best be interpreted as lower bounds on the average number of spurious keys seen by the eavesdropper and thus cannot in principle establish insecurity of the system.