# Quantum Threat

Threat Posed by Quantum Computers to Blockchains

A quantum computing is a rapidly-emerging technology that harnesses the laws of quantum mechanics to solve problems too complex for classical computer.

Quantum computers are elegant machines, smaller and requiring less energy than supercomputers. The Quantum processor is a wafer not much bigger than the one found in a laptop. And a quantum hardware system is about the size of a car, made up mostly of cooling systems to keep the superconducting processor at its ultra-cold operational temperature.

Classical computing uses an array of transistors. These transistors form the heart of your computer (the CPU). Each transistor is capable of being either on or off, and these states are used to represent the numerical values 1 and 0. Binary digits’ (bits) number of states depends on the number of transistors available, according to the formula (2^n) + 1, with n being the number of transistors. Classical computers can only be in one of these states at any one time, so the speed of your computer is limited to how fast it can change state.

Quantum computers on the other hand, use what are termed quantum bits or ‘qubits’ which are represented by the quantum spin of electrons or photons. These particles are placed into a state called superposition, allowing the qubit to assume a value of 1 and 0 simultaneously, generally resulting in an exponential increase in computational power over their classical counterparts.

Quantum-resistance — also known as post-quantum, quantum-secure, and quantum-safe — are cryptographic algorithms that can fend off attacks from quantum computers.

The first quantum computing algorithm was published by Peter Shor in 1994 — three years before the first quantum computer was built. But the idea that quantum computers could solve problems traditional computers can’t was first put forward by Richard Feynman, Paul Benioff, and Yuri Manin in the early 1980s.

While the first quantum computer was built in 1997, the field became an arms race during the 2010s.

IBM unveiled the first quantum computer for scientific and commercial use — IBM Q System One — in January 2019. In October of the same year, Google made history by announcing they’d achieved quantum supremacy. Their quantum computer had solved a mathematical problem it would take a traditional machine 10,000 years to solve.

Researchers at the University of Singapore have said that Bitcoin’s cryptographic algorithm could be under threat by quantum computers as soon as 2027.

Most blockchains like Bitcoin, Ethereum use Elliptic Curve Digital Signature Algorithm (ECDSA) for public key cryptography. Using a quantum computer, Shor’s algorithm can be used to break ECDSA.

With Bitcoin, a private key is generated by an algorithm, this is run through the ECDSA algorithm to generate a public key, which is hashed to create a public Bitcoin address. Though, when a user makes a transaction the public key is revealed on the blockchain. A powerful quantum computer could reverse this process and derive the private key from a public key. And voila! Bitcoin will not be unhackable or secure anymore and that will be a death knell.

Two major quantum algorithms that threaten the current state of cryptography have already been developed: Grover's and Shor's algorithms.

Nexus has been designed not only to be quantum resistant today, but also to adopt quickly to innovations in quantum computers and post quantum algorithms. Quantum resistance is not only about selecting a secure post quantum algorithm and implementing it, lot of thought has gone into designing Nexus today and to be relevant in the future.

Nexus has taken a multi pronged approach to tackle quantum threat. Sigchain is one of the innovations which abstracts the private / public key pair from the account and cryptography, this enables the use of one-time keys, generated before the transaction and discarded after the transaction is confirmed. As the keys are not reused this reduces the attack window to 500 ms. Users have the option to use FALCON, a post quantum algorithm for key generation, which is includes as an option. This enables each user to use the cryptography of his choice to generate keys. In the future users will have multiple algorithms to choose for key generation.

The second is by creating cryptographic redundancy, using different algorithms for the various functions. We use Argon2 for generating the key and usernames, ECDSA & FALCON for public key generation, Keccak for register, transaction and block hashes, SABER for node key exchange. This reduces the danger when a specific algorithm becomes vulnerable and bring the network to a standstill.

The third is additional authentication for node handshake, which makes it impossible for any entity to carry out a man in the middle attack. All these features make it a real quantum resistant design not only for today but also for tomorrow.

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