Laws of Physics Say Quantum Cryptography Is Unhackable. It’s Not


The beam splitter in a quantum cryptography unit created for the European Union’s Integrated Project Qubit Applications. Photo:Anders Sandberg/Flickr

In the never-ending arms race between secret-keepers and code-breakers, the laws of quantum mechanics seemed to have the potential to give secret-keepers the upper hand. A technique called quantum cryptography can, in principle, allow you to encrypt a message in such a way that it would never be read by anyone whose eyes it isn’t for.

Enter cold, hard reality. In recent years, methods that were once thought to be fundamentally unbreakable have been shown to be anything but. Because of machine errors and other quirks, even quantum cryptography has its limits.

“If you build it correctly, no hacker can hack the system. The question is what it means to build it correctly,” said physicist Renato Renner from the Institute of Theoretical Physics in Zurich, who will present a talk on calculating the failure rate of different quantum cryptography systems at the 2013Conference on Lasers and Electro-Optics in San Jose, California on June 11.

Regular, non-quantum encryption can work in a variety of ways but generally a message is scrambled and can only be unscrambled using a secret key. The trick is to make sure that whomever you’re trying to hide your communication from doesn’t get their hands on your secret key. Cracking the private key in a modern crypto system would generally require figuring out the factors of a number that is the product of two insanely huge prime numbers. The numbers are chosen to be so large that, with the given processing power of computers, it would take longer than the lifetime of the universe for an algorithm to factor their product.

But such encryption techniques have their vulnerabilities. Certain products – called weak keys – happen to be easier to factor than others. Also, Moore’s Law continually ups the processing power of our computers. Even more importantly, mathematicians are constantly developing new algorithms that allow for easier factorization.

Quantum cryptography avoids all these issues. Here, the key is encrypted into a series of photons that get passed between two parties trying to share secret information. The Heisenberg Uncertainty Principle dictates that an adversary can’t look at these photons without changing or destroying them.

“In this case, it doesn’t matter what technology the adversary has, they’ll never be able to break the laws of physics,” said physicist Richard Hughes of Los Alamos National Laboratory in New Mexico, who works on quantum cryptography.


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