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Is it possible to generate a key pair that has already been generated? Wouldn't it collide with an existing value?

I assume it is mathematically very unlikely, but odds higher than 0 are still possible.

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marked as duplicate by Murch May 19 '14 at 8:03

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up vote 4 down vote accepted

Yes, it is possible to create an already generated and in use address. The chances of that is indeed larger then 0, but minimalistic. But really small. And if I say really small, I mean extremely small. And if I say extremely small, I mean very extremely ultra low super duper close to zero. It is definitely NOT something to be considered as a weakness, never ever (because it will cost more energy before you have 50% to hit a duplicate address then the whole sun provides in its whole life time then). source

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You are assuming a perfect implementation with a flawless random number generator. However, random number generation is not as easy as it seems at first glance and can easily get screwed up. There are numerous cases of cryptographic systems losing security in practice because of weak RNGs. When two users use an implementation with the same flawed RNG, it could be more likely to happen than expected that they generate the same keypair. – Philipp May 18 '14 at 10:51
Yes, you are completely right. But afaik people have been testing this by running programs that generate and check addresses for balance, and had so far no success. But yes, if the RNG sucks, multiple addresses can be generated, but it is not expected at the moment – Mathias711 May 18 '14 at 15:51
Yes. But don't underestimate it. In the link it says that there are 10^38 addresses for each person in the world. The RNG must really really suck if is going to generate double addresses. Furthermore, it doesn't matter if it hits an address that once was used for a transaction and now obsolete. Only when there is a substantial amount of Bitcoin on the address it matters. It is not worth the cpu power. The chance that you are robbed or hit by a car (or the dinosaurs are revived and you get eaten) are MUCH higher. – Mathias711 May 19 '14 at 5:14
So to conclude, there is NO defense against double generated addresses, but the real life consequences are neglectable – Mathias711 May 19 '14 at 5:16
The unlikelyhood IS the defense. Otherwise they wouldn't have made those numbers that large. – Jannes May 22 '14 at 14:34

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