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

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.

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.

But in practice, quantum cryptography comes with its own load of weaknesses. It was recognized in 2010, for instance, that a hacker could blind a detector with a strong pulse, rendering it unable to see the secret-keeping photons.

Renner points to many other problems. Photons are often generated using a laser tuned to such a low intensity that it’s producing one single photon at a time.

There is a certain probability that the laser will make a photon encoded with your secret information and then a second photon with that same information.

In this case, all an enemy has to do is steal that second photon and they could gain access to your data while you’d be none the wiser.

Alternatively, noticing when a single photon has arrived can be tricky. Detectors might not register that a particle has hit them, making you think that your system has been hacked when it’s really quite secure.

Smart grids need to react to changes quickly lest some part of the system get damaged from electricity overflows.

But traditional cryptography usually requires time and processing power to encrypt and decrypt the large numbers used as keys.

The computers used in such cryptography could drive up the price of a smart grid. Quantum cryptography, on the other hand, simply requires pushing around some photons and the computations for decryption are much less complicated.

Hughes and his collaborators have worked with the University of Illinois Urbana-Champaign to show that quantum cryptography was two orders of magnitude faster than conventional techniques in encrypting smart grid information.

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