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Hashcash is a mechanism to prove that the sender of a message is really intending to send that message instead of just performing a denial of service attack. It makes denial of service attacks harder by requiring the sender to perform a mathematical puzzle: finding a prefix of a hash that results in a certain number of zeroes in the hash output.

GPUs, FPGAs and ASICs can make hashcash easier to solve but only by few orders of magnitude.

Do quantum computers break hashcash? If for example I need to calculate 2^24 hashes, does Grover's algorithm mean that a quantum computer could do the same in 2^12 steps?

juhist
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1 Answers1

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Do quantum computers break hashcash?

This question sounds to me as analogous to this question:

Would a spaceship capable of taking a man to Mars help me get to the grocery store faster?

These questions are analogous in several ways [1].

For one, they are both asking about technologies that don't currently exist, and for which we can foresee a number of technical challenges that must be overcome before they do exist.

For another, once these technologies do exist, they would be nontrivial to use. In the case of Quantum Computers, what they will need is aggressive isolation from the environment, be it by extreme cryogenic temperatures or working in a hard vacuum. In addition, they also need to integrate Quantum Error Correction into their computations, which means that quite often during the computation, measurements of some of the qubits will need to be taken, and the measurements fed into some classical circuitry, which would determine what actions to perform on the remaining Qubits.

Thirdly, both questions are asking about solving a problem that we can easily do with current technologies. In the hashcase case, an ASIC with a million hashers built on it could easily find the preimage in milliseconds - and if you don't want to go to the bother of building a custom ASIC, an FPGA farm wouldn't be much slower.

And so, if you ask whether a Quantum Computer would break hashcash, well, if the expense of that is on the table, hashcash is broken already...

[1]: To be fair, using a Real (CRQC) Quantum Computer to make hashcash go faster isn't quite as silly as using a spaceship to go to the grocery store - it's closer to using a jet to do so. However, I went with a manned Martian spaceship because, like CRQC's, we don't have them yet, and it appears to me that the technical challenges left are similar in scope.

poncho
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