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By Kathleen Jordan
Posted on Oct. 31, 2018

The possibilities of blockchain having a sustainable impact on our future, are sprawled across the mainstream technology. Yes, we are trying not to define the potential of blockchain & cryptocurrencies within a certain specific periphery. That’s exactly what makes the perceptible interference of quantum computing the biggest threat further out in the horizon. And let us assure you, this is not just a random idea experts enjoy pondering over but a question looming.

Much of blockchain’s allure arises from its security benefits. The tech allows a ledger of transactions to be distributed between a large network of computers. No single user can break into and change the ledger. This makes it both public and secure. But combined with another emerging (and much hyped) technology, quantum computing, blockchain’s seemingly immutable ledgers would be under threat. Like blockchain, quantum computing has been making progress and headlines too.

The number of quantum computing companies and researchers continues to grow. And while there is a lot of focus on hardware, many are looking into the software as well.

Cryptography is a commonly debated topic because quantum computing poses a threat to traditional forms of computer security, most notably public key cryptography, which undergirds most online communications and most current blockchain technology.

**But first, let’s understand computer security in threads**

Public key cryptography uses a pair of keys to encrypt information: a public key which can be shared widely and a private key known only to the key’s owner. Anyone can encrypt a message using the intended receiver’s public key, but only the receiver can decrypt the message using her private key. The more difficult it is to determine a private key from its corresponding public key, the more secure the system.

The best public key cryptography systems link public and private keys using the factors of a number that is the product of two incredibly large prime numbers. To determine the private key from the public key alone, one would have to figure out the factors of this product of primes. Even if a classical computer tested a trillion keys a second, it would take up to 785 million times longer than the roughly 14 billion years the universe has existed so far due to the size of the prime numbers in question.

If processing power were to greatly increase, however, then it might become possible for an entity exercising such computing power to generate a private key from the corresponding public key. If actors could generate private keys from corresponding public keys, then even the strongest forms of traditional public key cryptography would be vulnerable. This is where quantum computing comes in. Quantum computing relies on quantum physics and has more potential power than any traditional form of computing. Quantum computing takes advantage of quantum bits or “qubits” that can exist in any superposition of values between 0 and 1 and can, therefore, process much more information than just 0 or 1, which is the limit of classical computing systems.

The capacity to compute using qubits renders quantum computers many orders of magnitude faster than classical computers. Google showed a D-Wave quantum annealing computer could be 100 million times faster than classical computers at certain specialized tasks. And Google and IBM are working on their own quantum computers. Further, although there are but a handful of quantum computing algorithms, one of the most famous ones, Shor’s algorithm, allows for the quick factoring of large primes. Therefore, a working quantum computer could, in theory, break today’s public key cryptography.

Quantum computers capable of speedy number factoring are not here yet. However, if quantum computing continues to progress, it will get there eventually. And when it does, this advance will pose an existential threat to public key cryptography, and the blockchain technology that relies on it, including Bitcoin, will be vulnerable to hacking.

**So, does that mean security of blockchain will be lost in the dark underworld?**

Will the advent of quantum computing render blockchain technology obsolete? Maybe, but not if we can develop a solution first. Cryptographers are working like mad-hatters to construct cryptographic systems that are quantum resistant. As it happens, few blockchain projects are on their way of implementing quantum resistant cryptography.

**That’s fine, but what is ‘quantum resistant’?**

When private keys are generated from public keys in ways that are much more mathematically complex than traditional prime factorization. The Quantum Resistant Ledger team worked to implement hash-based cryptography, a form of post-quantum cryptography. In hash-based cryptography, private keys are generated from public keys using complex hash-based cryptographic structures, rather than prime number factorization. The connection between the public and private key pair is therefore much more complex than in traditional public key cryptography and would be invulnerable to Shor's algorithm, as it utilizes factorisation, which is not a part of their signature scheme. Their blockchain utilises this and launched in June.

These post-quantum cryptographic schemes do not need to run on quantum computers. The Quantum Resistant Ledger is a blockchain project already working to implement post-quantum cryptography. It remains to be seen how successful the effort and others like it will prove when full-scale quantum computing becomes a practical reality.

To put it concisely, quantum computing is not just a roadblock for the potential future of blockchain, but it is a major threat looming, for all computer security systems. Post-quantum cryptography will ensure the data security of all existing systems.

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