<p>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 <strong>NOT</strong> 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). <a href="https://en.bitcoin.it/wiki/Weaknesses#Generating_tons_of_addresses" rel="nofollow">source </a> </p>

CC BY-SA 3.0
2014-05-18T10:51:16.160Z
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
CC BY-SA 3.0
2014-05-18T15:51:56.110Z
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
CC BY-SA 3.0
2014-05-18T21:49:52.417Z
So, we can conclude bitcoin has no defense at all apart from the mathematical improbability of key pair generation? No code at all to handle this seemingly improbable case.
JustAnotherStackOverflowUser
CC BY-SA 3.0
2014-05-19T05:14:30.780Z
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
CC BY-SA 3.0
2014-05-19T05:16:02.357Z
So to conclude, there is NO defense against double generated addresses, but the real life consequences are neglectable
Mathias711
CC BY-SA 3.0
2014-05-22T14:34:49.460Z
The unlikelyhood IS the defense. Otherwise they wouldn't have made those numbers that large.
Jannes
CC BY-SA 3.0
2014-05-18T07:30:36.747Z
2014-05-18T07:30:36.747Z
Mathias711
<p>Is it possible to generate a key pair that has already been generated? Wouldn't it collide with an existing value? </p>

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

CC BY-SA 3.0
2017-11-22T06:50:13.027Z
If you are worried someone will find your key, spread your wealth among several addresses.
zundi
<address><address-generation>
How does the bitcoin network disallow people generating a key pair already generated?
JustAnotherStackOverflowUser
# How does the bitcoin network disallow people generating a key pair already generated?

(Asked by: username_0 on 2014-05-18)

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.

**Tags:** address, address-generation

> **Comments:**
> **username_1**: If you are worried someone will find your key, spread your wealth among several addresses.  

---

## 1 answers

### Answer by username_2 on 2014-05-18. Score: 4 | Accepted answer

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](https://en.bitcoin.it/wiki/Weaknesses#Generating_tons_of_addresses)

> **Comments:**
> **username_3**: 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.  
> **username_2**: 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  
> **username_0**: So, we can conclude bitcoin has no defense at all apart from the mathematical improbability of key pair generation? No code at all to handle this seemingly improbable case.  
> **username_2**: 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.  
> **username_2**: So to conclude, there is NO defense against double generated addresses, but the real life consequences are neglectable  
> **username_4**: The unlikelyhood IS the defense. Otherwise they wouldn't have made those numbers that large.  

---

