Quote from: matthewio on February 23, 2018, 11:05:51 AM

In addition to the aforementioned limitations of blockchain as enumerated by some contributors here on this forum,i would like to say that there are fears that that future development in cryptography may leave the blockchain vulnerable,ie hackable.That is why there is so much fear about the development of quantum computers,which may become commercially available 10-20 years time frame.Part of the solution to this future vulnerability is to use a bitcoin address only ONCE.Thanks to experts who already see the future.By the way,the world will have bigger problems to deal with when quantum computers become mainstay-sensitive military installations for example.

(Boldface is here added to question-begging. Smart, well-informed people dont have so much fear about quantum computers; thus, there is no why. I have never beaten my wife; therefore, I have never stopped beating my wife.)

Quote from: Xynerise on February 23, 2018, 09:56:22 PM

Quote from: jhean_arcane on February 23, 2018, 06:25:30 PM

This is new info to me. Forgive my ignorance about quantum computers, but how will it be a huge threat to blockchain? i mean, i just can't see the point why blockchain will be vulnerable. I thought people are aiming to improve it?

There are a lot of threads on Bitcointalk about the threat of quantum computers on bitcoin.
http://lmgtfy.com/?q=quantum+site%3Abitcointalk.org

Yes, so very many threads. Behold the power of Quantum FUD®, superposition of which is entangled across the forum!

Quote from: Xynerise on February 23, 2018, 09:56:22 PM

The gist of the matter is that quantum computers can break ECDSA which bitcoin uses for digital signatures and will be theoretically able to calculate private keys from public keys.
This is prevented by not reusing addresses because addresses are not public keys but a hash of them and quantum computing can't do anything with them. (An address's public keys are exposed whenever a transaction is made with that address)

Xynerise, you seem generally well-informed; but here, I think you are sorely mistaken in picking up an unfortunately popular meme. Quoting one of my better Jr. Member posts from way back when, in the context of a thread about Bitcoin and quantum computers (bracketed clarification added):

Quote from: nullius on December 24, 2017, 06:39:37 PM

As a general point, I will worry about disclosing Bitcoin public keys at the same time I start to worry about disclosing my long-term PGP public key. (For those in the peanut gallery: The latter would be entirely useless without public disclosure.)

There are excellent reasons to avoid address reuse; but this [QC resistance] is not one of them. I say this as a paranoid security nut: The security of publicly disclosed public keys is just fine. That is why they are called public keys. The only exception I would here make is if you have coins which you intend to potentially leave in cold storage for decades. Then, yes, you will want the extra security margin of the key being unpublished. Thats not only a concern about quantum computers: Unexpected cryptanalytic techniques could develop over the course of many years. For cryptography which really needs to stand the test of time, reducing your security requirements to a hash is simply good security hygiene.

Also apropos, in a distinct context (whereas FUD can only be distinct without difference):

Quote from: nullius on January 04, 2018, 04:47:23 PM

The security of exposed Bitcoin public keys is just fine for general usage. They cannot be hacked. [...] But there is a different, unrelated reason to avoid address reuse: Privacy. Avoiding address reuse gives you a modicum of privacy. That at least makes Chainalysis work for their pay. Re-using addresses makes transaction linkage trivial, childs play.

A public key is called a public key, because it is secure when exposed in public. I publish my PGP public keys (and if I didnt, PGP would be useless). I am not worried about that. Each and every time you connect to an https website secured by TLS, the servers public key is exposed to youand your symmetric session key is derived from a key-agreement process based on the hardness of the same DLP as is the fundamental basis of most widely-used public-key cryptography other than RSA. I am not worried about that, either! Likewise, I am not worried about the security of my Bitcoin public keys.

Evidently, I am not the only one to be of the opinion that hash public keys for quantum resistance is a regrettable meme. Quoting from discussion of Taproot on bitcoin-dev, 2018-01-23:

Quote from: Gregory Maxwell

Considering the considerable level of address reuse -- I recall prior stats that a majority of circulating funds are on addresses that had previously been used, on top of the general race limitations-- I am now dubious to the idea that hashing provides any kind of meaningful quantum resistance and somewhat regret introducing that meme to the space in the first place. If we considered quantum resistance a meaningful concern we should address that specifically. --- so I don't think that should be a factor that drives a decision here.

As a general point about the strength of Bitcoins cryptographic security, from an adapted self-quote, I hereby formally posit the Nullian Flying Fire Hydrant Rule:

Quote from: nullius on January 02, 2018, 08:44:15 PM

You had better be worried about being killed by a flying fire hydrant than about [the security] of Bitcoins cryptographic keys. It has happened at least once somewhere that a man was killed by a flying fire hydrant.

Please. You dont worry about being killed by a flying fire hydrant. Whereas to be killed by a flying fire hydrant is not only possible, but astronomically more probable than any cryptographic break of Bitcoin security.

At some point, after the size of 2¹²⁸ is explained for the 2¹²⁷th time, anti-Botcoin FUDsters realize that FUDding Bitcoin security in the present just makes them look absurd. So, they upgrade to Quantum FUD® technology: Point to an uncertain future, oversimplify complex technological questions much debated by experts, and then beg the question of why there is so much fear about the development of quantum computers.

Practical, usable quantum computers do not exist. Their present is fantasy, and their future is unknown. Some experts are of the opinion that a practical quantum computer capable of useful computation may be impossiblescientifically, physically, mathematically impossible.

E.g., quoting from one of my Newbie-rank posts, q.v.; see also the ensuing discussion between myself and haltingprobability:

Quote from: nullius on December 14, 2017, 04:59:00 AM

A quantum computer is not a sure thing!

I should preface this by saying, Im not endorsing the opinions of Scott Aaronson. Im only citing him as someone who is not a moron, and wrote a book on quantum computing (which I have not read). I seem to recall some wager on his blog over the (im)possibility of quantum computing, but I cant find it right now; anyway, D-Wave has a long history (2013) of drawing his ire (2017), to say the least.

See how he discusses skepticism of quantum computers:

Quote

What I did is to write out every skeptical argument against the possibility of quantum computing that I could think of. We'll just go through them, and make commentary along the way. Let me just start by saying that my point of view has always been rather simple: it's entirely conceivable that quantum computing is impossible for some fundamental reason. If so, then that's by far the most exciting thing that could happen for us. That would be much more interesting than if quantum computing were possible, because it changes our understanding of physics. To have a quantum computer capable of factoring 10000-digit integers is the relatively boring outcome -- the outcome that we'd expect based on the theories we already have.

[...]

As for myself, I account myself moderately skeptical of quantum computing; Ill believe it when I see it, but meanwhile I think its a good idea to move to PQ crypto. I would be more surprised if quantum cryptography can deliver on its promises. I dont like the hype around any of it, especially when its sometimes used to FUD Bitcoin.

On that note, in closing, I will repeat in this Quantum FUD® context what I said more generally about Bitcoins public-key security:

Quote from: nullius on January 31, 2018, 10:29:46 PM

I strongly recommend that anybody not deeply involved in developing Bitcoins long-term security should absolutely not worry about the strength of Bitcoins public-key security. Its worse than useless worry: It is a distraction from real problems. Worry instead about your computer security, your operational security, and your financial privacy. (Nobody can target you for theft or coercion if nobody knows you have anything significant to take.)

It is as if many people are keeping their coins in a safe with an unbreakable door (the cryptographyall of it) and walls made of tissue paper (the malware-infested PC, privacy leaks which may allow thieves to identify you and know what money you have, etc., etc.). Then, they obsessively worry about the security of the door! Dont do that.

I think Xynerise concludes correctly:

Quote from: Xynerise on February 23, 2018, 09:56:22 PM

However this is not a significant threat as quantum computing isn't that advanced yet, there are better easier targets than bitcoins (or other cryptocurrencies), and bitcoin can always move to a quantum resistant algorithm.