<p>Sure, he can try. But the algorithms Bitcoin uses were specifically chosen to make this attack completely impractical. It would be trillions of times easier to just buy the Bitcoins.</p>

CC BY-SA 3.0
2013-12-24T00:40:19.030Z
And suppose he could come up with such a block (he can't, but for arguments sake). How is he going to give that block to others? Noone is going to replace a block that's been on their harddisks for years with a new one. He could only pollute the  blockchain of some random new user downloading a fresh copy. Surely an awkward situation for a few individuals, but hardly a significant fork.
Jannes
CC BY-SA 3.0
2013-12-23T18:35:03.830Z
2013-12-23T18:35:03.830Z
David Schwartz
<p>Since multiple values can result in the same hash, couldn’t an attacker try to find a way of increasing his balance in a way that the checksum of the changed block remains the same (so that any proofs-of-work are still valid) and then propagate the change into the network without peers noticing it?</p>

<security>
Can an attacker change an old block without changing the checksum?
Lenar Hoyt
# Can an attacker change an old block without changing the checksum?

(Asked by: username_0 on 2013-12-23)

Since multiple values can result in the same hash, couldn’t an attacker try to find a way of increasing his balance in a way that the checksum of the changed block remains the same (so that any proofs-of-work are still valid) and then propagate the change into the network without peers noticing it?

**Tags:** security


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## 1 answers

### Answer by username_1 on 2013-12-23. Score: 3 | Accepted answer

Sure, he can try. But the algorithms Bitcoin uses were specifically chosen to make this attack completely impractical. It would be trillions of times easier to just buy the Bitcoins.

> **Comments:**
> **username_2**: And suppose he could come up with such a block (he can't, but for arguments sake). How is he going to give that block to others? Noone is going to replace a block that's been on their harddisks for years with a new one. He could only pollute the  blockchain of some random new user downloading a fresh copy. Surely an awkward situation for a few individuals, but hardly a significant fork.  

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