the fact or impression of being final and irreversible

Finality

When is a transaction irreversible?

Let's look at
block mining

Trial and Error

Millions of times per second

=

Hash Rate

Current BTC Hash Rate:

Current BTC Hash Rate:

50 000 000 000 000 000 000

For every new block, a lot of work is put into the chain.

What if...

The chain with the most work wins

Tutorial:
Attacking Bitcoin

To revert Block #3, we need to:

- Create a new Block #3b

- Send it to everyone, before someone else creates Block #4

Not easy, but doable

(happens all the time, by accident)

To revert Block #2, we need to:

- Create a new Block #2b

- Create a new Block #3b as well

- Send them to everyone, before someone else creates Block #4 is created

Exponentially harder, might still be doable

To revert Block #2, we need to:

- Create a new Block #1b

- Create a new Block #2b

- Create a new Block #3b

- Send them to everyone, before someone else creates Block #4 is created
 

Probably 99% unlikely, or something

You need more power than the rest of network.

As in: You need at least 51%

Each new block helps confirm all previous blocks

Finality:
6 confirmations in BTC*
20-30 confirmations in ETH*