A Methodology for Quantum Risk Assessment

  • Dr. Michele Mosca
  • John Mulholland
A lineup of padlock icons on a screen, most blue and unlocked, and one red and locked.


Until recently, quantum computing was often viewed as a capability that might emerge in some future time, almost something that properly belongs in a science fiction novel. The science behind these computers originates in the physics of quantum mechanics, which presents a fundamental change in our understanding of the universe. The concepts can be difficult to reconcile with the world we live in; for example, in a quantum computer a bit can somehow be both zero and one at the same time! Even many physicists have had trouble with these revolutionary ideas, but experiments and observations have validated quantum theory and its underlying principles are evident in common devices such as lasers and transistors. These technologies only hint at the full promise of quantum, but considerable work is still required before we can build a true quantum computer.

This milestone is probably a decade or more in the future, but the work is being pursued by many researchers around the globe since quantum technology promises advantages in a variety of areas such as sensors, communications, optics and computation. In 1994, mathematician Peter Shor described an algorithm which enables quantum computers to solve extremely difficult mathematical problems, such as factoring very large numbers. Such problems are essentially unsolvable using today’s computers. So difficult are these problems that they have become the mathematical underpinning of the most commonly used security systems on the Internet. Modern public-key cryptographic systems provide security for virtually all sensitive communications on the Internet, including banking, email and web site access.

Once effective quantum computers are available, they will essentially eliminate the cryptographic strength of these public-key cryptosystems. More traditional shared-key cryptosystems (such as AES) will also be affected, reducing their effective security strength to roughly half of what we would consider it to be today.

This will have a devastating effect on the systems used to protect electronic communications and digital transactions. Most secure internet processes rely on protocols that employ public-key cryptography, including those used to secure web sites, for banking transactions, secure email and digital signatures. There are few businesses or individuals who could be confident of their cyber security profile once the quantum computing era arrives.