Cryptographic scheme proposed by Bennett, Brassard, and Mermin [Phys. Rev.
Lett. {\bf 68}, 557 (1992)] is reformulated in a version involving two
polarizing Mach-Zehnder interferometers. Such a form, although physically
equivalent to the original one, makes its security explicit, suggestive and
easy to explain to non-experts.Comment: revtex, 4 pages, 1 ps figur
