Comment on "Exposed-Key Weakness of Alpha-Eta" [Phys. Lett. A 370 (2007) 131]

We show that the insecurity claim of the AlphaEta cryptosystem made by C. Ahn and K. Birnbaum in Phys. Lett. A 370 (2007) 131-135 under heterodyne attack is based on invalid extrapolations of Shannon’s random cipher analysis and on an invalid statistical independence assumption. We show, both for standard ciphers and AlphaEta, that expressions of the kind given by Ahn and Birnbaum can at best be interpreted as security lower bounds.