Quantum bit vs bitcoin

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This independence is one reason Bitcoin has become so popular, causing its value to rise steeply. A crucial feature of Bitcoin is its security. Bitcoins have two important security features that prevent them from being stolen or copied. Both are based on cryptographic protocols that are hard to crack. In other words, they exploit mathematical functions, like factorization, that are easy in one direction but hard in the other—at least for an ordinary classical computer.

But there is a problem on the horizon. Quantum computers can solve these problems easily. And the first quantum computers are currently under development. That raises an urgent question: Today, we get an answer thanks to the work of Divesh Aggarwal at the National University of Singapore and a few pals. These guys have studied the threat to Bitcoin posed by quantum computers and say that the danger is real and imminent.

Bitcoin transactions are stored in a distributed ledger that collates all the deals carried out in a specific time period, usually about 10 minutes. This collection, called a block, also contains a cryptographic hash of the previous block, which contains a cryptographic hash of the one before that, and so on in a chain. Hence the term blockchain. A hash is a mathematical function that turns a set of data of any length into a set of specific length. The new block must also contain a number called a nonce that has a special property.

When this nonce is hashed, or combined mathematically, with the content of the block, the result must be less than some specific target value. Given the nonce and the block content, this is easy to show, which allows anybody to verify the block. But generating the nonce is time consuming, since the only way to do it is by brute force—to try numbers one after the other until a nonce is found. This process of finding a nonce, called mining, is rewarded with Bitcoins. Mining is so computationally intensive that the task is usually divided among many computers that share the reward.

The block is then placed on the distributed ledger and, once validated, incorporated into the blockchain. The miners then start work on the next block. Occasionally, two mining groups find different nonces and declare two different blocks.

The Bitcoin protocol states that in this case, the block that has been worked on more will be incorporated into the chain and the other discarded. In that case, it effectively controls the ledger. In their paper , which landed at arXiv in late October, Divesh Aggarwal and his collaborators say ASIC-based mining rigs are fast compared to even optimistic theoretical quantum computer clock speeds. The extreme speed of current specialized ASIC hardware for performing the hashcash PoW, coupled with much slower projected gate speeds for current quantum architectures, essentially negates this quadratic speedup, at the current difficulty level, giving quantum computers no advantage.

Future improvements to quantum technology allowing gate speeds up to GHz could allow quantum computers to solve the PoW about times faster than current technology. As far as defeating hashcash goes, the numbers are daunting for quantum computer designers: Shor's algorithm, a quantum algorithm for factoring integers that's how it would attack cryptography , is a better path, they write.

Deploying a quantum computer against the secpk1 elliptic curve Bitcoin uses is much more dangerous: As with cracking the proof-of-work, the researchers assume quantum computers get big and fast relatively quickly, and even so, they fall slightly short: The Register - Independent news and views for the tech community.

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