Comment by thebigshane | Hacker News Reader

Comment by thebigshane | Hacker News Reader

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Ok, off topic, sorry, but... Are you saying that if/when quantum computing comes around, existing encryption systems would be suddenly ineffective? So if the NSA gets their hands on a quantum computer, they'd be able to brute force a 1024bit RSA encrypted file in far quicker time than their other super computers?

I found some info here, but I didn't get enough out of it to answer my question directly. I think the answer is "Well theoretically, but it's complicated":

http://stackoverflow.com/questions/2768807/quantum-computing...

http://en.wikipedia.org/wiki/Quantum_computer#Potential

http://en.wikipedia.org/wiki/Shor%27s_algorithm

http://en.wikipedia.org/wiki/Post-quantum_cryptography

  [...] most currently popular public-key cryptosystems 
  rely on the integer factorization problem or discrete 
  logarithm problem, both of which would be easily solvable 
  on large enough quantum computers using Shor's algorithm. 
  Even though current publicly known experimental quantum 
  computing is nowhere near powerful enough to attack real 
  cryptosystems, many cryptographers are researching new 
  algorithms, in case quantum computing becomes a threat in 
  the future.

Yes. The naive algorithm for factorization runs in O(2^N) (where N is the number of bits in the number you want to factorize), which is impractical for any sufficiently large N (2^64 > 10^19, 2^1024 > 10^328 which is BIG). There are better known algorithms, but none of them are polynomial, so a bruteforce is highly impractical.

Shor's algorithm, on the other hand, runs in O((logN)^3). For an idea of the difference, log(64)^3 = 216, log(1024)^3 = 1000.

So Shor's algorithm could be used to very quickly factorize large numbers and would effectively break RSA. We just need a quantum computer, which as far as we know nobody has yet.

Ok, thanks. But when I start hearing conspiracy theories that the NSA (or anyone) has already developed one, I should basically look for an encryption algorithm that isn't based on prime factorization?

Encryption algorithms exist and are being developed which are not based on prime factorization and for which no known quantum algorithm exists which can break them in polynomial time.

There will probably be significant warning before quantum computers exist which can break RSA. In that time it will probably become the norm to no longer use algorithms which are rendered obsolete by quantum computers.

> Are you saying that if/when quantum computing comes around, existing encryption systems would be suddenly ineffective?

Yes, but it's a big "if". There are many obstacles that stand in the way of quantum computing on a large scale, in fact some believe it will never be possible to maintain the delicate conditions required to perform quantum computation on a large scale.

The reason quantum computing could outperform conventional computation is because a quantum computer would, in a manner of speaking, process all possible problem statements at once, in parallel, and arrive at the best solution just as though only one problem statement had been processed. I know that may be difficult to imagine, but so is quantum. About quantum theory, John Wheeler said, "If you are not completely confused by quantum mechanics, you do not understand it."

Suffice it to say that quantum computing on a scale sufficient to pose a threat to public-key cryptosystems is not just around the corner.

I think that's the wrong attitude to take. If you look at the mathematics there is really nothing so weird or magical about quantum mechanics or using it to perform factorization; you just take a set of qbits initially in an equal superposition of all possible factors, manipulate them through quantum operations such that the amplitudes of all the factors that don't divide the number go to zero, and then measure the state to force a collapse/entangle yourself with it, and the number you measure will necessarily be a factor. QM in general is really pretty simple and even obvious if you ignore the silly mythology and just follow the math.

As for the specifics, I believe we have working 4-qbit quantum computers, and shor's algorithm has been successfully applied to calculate that 15=3x5. But current approaches do indeed seem unlikely to scale up to 1024 qbits.

> If you look at the mathematics there is really nothing so weird or magical about quantum mechanics or using it to perform factorization ...

Yes, if you examine the mathematics that's true, but if you examine the physics it's not true. The problem doesn't lie with algorithm design, the problem lies with putting them into practice in actual physical computers.

> QM in general is really pretty simple and even obvious if you ignore the silly mythology and just follow the math.

On the contrary, not at all simple if one must try create an actual physical realization.

This is not to suggest that the problems won't be overcome, only that they're more formidable than describing how easy it is in principle.

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