Breaking “Unbreakable” Codes

Dorian Bouchet

Saswato R. Das

Researchers in Europe in the field of quantum cryptography have demonstrated for the first time that it should be possible? with the help of satellites? to communicate across thousands of kilometers using unbreakable codes whose security is guaranteed by the laws of quantum physics. For many business or government uses, the codes must be usable between cities and continents, but quantum cryptography machines today are limited to about 150 kilometers by the length of individual optical fibers and the loss of photons within them.

ARTUR K. EKERT

Hoi‐Kwong Lo

ABSTRACT Q uantum mechanics can revolutionize the art of code-making by breaking standard encryption schemes and by making unbreakable codes. In this document, I survey the motivation, power and limitation of quantum code-making. * Keywords: Quantum computing, quantum information, quantum cryptography, security, key distribution,cryptography 1. INTRODUCTION Human desire to keep written messages secret is almost as old as writing itself. Indeed, cryptography—the artof code-making—dated back to the Babylonians. For thousands of years, code-makers and code-breakers have engaged in an escalating arms race with each other.' Cryptographers do not sleep well. Every innovation in code-making has so far been defeated by new innovation in code-breaking with disastrous consequence tothe unsuspecting users of those insecure codes. For instance, Queen Mary of Scotland was executed by QueenElizabeth following the decoding of encrypted communications between Queen Mary and her conspirators whichprovided the irrefutable proof of her quilt in plotting against Queen Elizabeth. In the Second World War, thebreaking of the legendary German Engima code by the Allied code-breakers contributed significantly to theoutcome of the war. In cryptography, one must expect the unexpected and even the unthinkable.Q uantum mechanics has made a recent entry in this life and death struggle between code-makers and code-23 In this document, I will address the following frequently asked questions:(1) What quantum code-breaking can do?(2) What quantum code-making can do?(3) What quantum code-making cannot do?

Noriaki Horiuchi

Arindam Mitra

A simple unbreakable cipher system is presented which uses the concept of "pseudo-bits" that has never been used in either classical or quantum cryptography.