# Tag Info

## Hot answers tagged quantum-cryptography

15

Well, cryptographers have been contemplating a post-quantum world for some time now. Quantum computing, although in its infancy as far as real-life computers go, has been studied in a theoretical sense for a quite a while. Shor's algorithm was published 19 years ago; Grover's, 17 years ago. These are the two most-famous quantum algorithms, I think, but the ...

5

With Grover's algorithm, quantum computers can brute-force a block cipher with $n$-bit keys using $2^{n/2}$ steps, which is much smaller than the regular effort ($2^n$). This means, for example, that AES-128 could be broken with $2^{64}$ steps, and that AES-256 would offer the same security that AES-128 offers currently. In short, key sizes would need to be ...

5

QKD aims at exchanging key material to be used with encryption based on OTPs between two parties and thus to achieve perfect secrecy for transmitted messages. There are, however, several drawbacks for practical use in a wired setting of QKD (required hardware and their vulnerability to hacks, limited distance which does not support end-to-end ...

4

I'm not going to exactly answer your question, because I have no idea. I simply do not know how fast the quantum computer is that NSA is building in secret. However I could explain why people recommend 256-bit security in the face of quantum computing using some numbers. If you feel that $2^{128}$ is a comfortable security against bruteforcing, remember ...

3

In fact, the basic idea of Shor's algorithm for the discrete logarithm problem is reasonably simple. Assume (as in Section 4 Discrete Log: the easy case of Shor's paper) that you have an efficient quantum algorithm for the Fourier transform. Then, applying this Fourier transform twice (once for $a$ and once for $b$) on a quantum superposition of values ...

2

What you are describing is known as the Photon Number Splitting Attack (PNS), described for the first time (I think) by Brassard, Lütkenhaus, Mor ans Sanders in this 1999 paper. Several countermeasures have been invented since (single photon sources, robust protocols, decoy states), but detailing them would stray away from of your question. If one sends 2 ...

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D-Wave does quantum annealing. It's not general-purpose quantum computing; in fact, the CEO claims that the gate-model for quantum computers is the worst thing that ever happened to the field. I have worked on quantum research as recently as 2012 and the gate-model is still the main focus for funded research. Shor's algorithm for factorization (which runs ...

1

Adding more qubits does not increase the computation speed. A quantum computer with 4 qubits does not factorize faster than one with 2. The qubits are the "memory" of the quantum computer. More qubits mean you can factor bigger numbers. If I remember correctly, you need a superposition of $\Theta(N^2)$ terms, which means $\Theta(\log(N^2))$ qubits to factor ...

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If the school has graduate courses that interest you and you think you can do well in (i.e. for comprehensive exams), and there is a strong crypto research group there, I would recommend any school that satisfied these criteria. As you move through academics, it becomes more and more clear that the quality of your research is the most important, and it is ...

1

Quantum computers are not yet at the stage where they can be deployed to brute-forcing public RSA moduli. There is no evidence of a quantum computer using more than 7 qubits. The company D-Wave has made several bold claims, but offered little evidence. source: http://www.technologyreview.com/view/426586/worlds-largest-quantum-computation-uses-84-qubits/

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