Bitcoin Forum

I was talking to my friend and he has told me that while using bitcoin core, he generated a new address and found almost 0.2 already in it. It has received two payments in 7 days that weren't made by him, one 0.004 and another one 0.01.

Has he stumbled over the first private key collision?

Probably not.

No lol, you're friend is lying.


Someone hacked my account, disregard this; was not actually me. PM me if you have questions/issues.

It's likely that his friend is lying, however these things are random. Just like you can mine a block with your laptop within a couple of seconds of joining the mining network. Just highly unlikely.

It could happen. OP, does your friend have any proof?

Extraordinary claim requires extraordinary proof.

There are possibilities that are MUCH MUCH more likely than a collision of properly generated private keys.

The most likely is that your friend is mistaken or lying.

Another likely possibility is that your friend used poorly written software to generate the address.  If the software isn't doing a good job of picking a random number, and more than one person uses that poorly written software, then it is possible that the software picks the same private key for both users.  As an example, I could write wallet software that picks a random number between 2¹⁰⁰ and 2¹⁰⁰ + 20 and hashes the result with SHA256 to generate a private key.  The first few times you use it, it will appear to be picking random numbers and getting random addresses, but before 20 addresses are generated a collision is likely to occur.

There have been several wallets in the past that have had bugs that resulted in too small of a set being used for the private keys.  Perhaps your friend is using one of those wallets.

I don't believe it until your friend gives you some sort of proof and posts it here. The chances of this happening are for basically impossible. Yes mathematically its possible but in practice it's impossible, so post proof.

Well, there is unlikely, and then there is so incredibly unlikely that we might as well just call it impossible. An address collision falls in the second category.

Fixed that for you.

There have been MANY address collisions using poorly written software, and weak "brainwallets".

Such as poor PRNGs for Android (and many others):
https://bitcoin.org/en/alert/2013-08-11-android

And brain wallets like:
e.g.  "correct horse battery staple":
https://blockexplorer.com/address/1JwSSubhmg6iPtRjtyqhUYYH7bZg3Lfy1T

 and "satoshi nakamoto":
https://blockexplorer.com/address/1JryTePceSiWVpoNBU8SbwiT7J4ghzijzW

Speaking of which, what happens when (if?) computers become so powerful that they can brute force into random wallets with balance? The scenario doesn't seem too plausible in the future. :P

if this will happen i guess we have some more problems..

But in bitcoin, then we can change the key algorithm and all "old btc" can send to a new address which is unbreakable then.
But this will have a big impact for sure..

If quantum computers become common, we would possibly have to make bitcoin addresses the length of a PGP key or more to be safe.

Unlikely and the fact you offer no proof is suspect.  even if he did the likelihood of it ever happening again is off the scale.

There have been a ton of discussions about this here and all over the internet.  No need to hijack this thread.

e.g.
https://bitcointalk.org/index.php?topic=1305887.msg13377953#msg13377953