but couldn't someone with 51% of the power rewrite a block at any point in time in the past, and change history.
In order to change history your miners would need to solve blocks much faster than the rest of the network consistently for multiple blocks. Then all the nodes on the bitcoin network would accept the false fork because it was longer.
Or even write bogus transactions to the blockchain ? This would seem much more serious than double-spend
You can't write a transaction without knowing the private key of the address you're transferring from. This is true whether there's a 51% attack or not. So you can't just write any old transaction to the blockchain. The rest of the network would reject the block with bad transactions in it.
The big problem is buying something with bitcoin and receiving the purchased good--this transaction goes in Fork A. Then the attacker would start the 51% attack and create another fork--Fork B--which competes with Fork A. In Fork B the attack writes another transaction in which she sends the coins back to a wallet she controls.
Then the attacker must continue to solve blocks at a faster rate than the rest of the network is solving it....AND before the rest of the bitcoin network notices what is going on. This is no small feat.
and not to be brushed off so casually
It's not brushed off casually. The 51% attack is brought up a lot in the cryptocurrency community. But it really isn't feasible on closer examination.
In bitcoin it hasn't been a big deal because everyone is aware of the potential and self-polices.
In order to change history your miners would need to solve blocks much faster than the rest of the network consistently for multiple blocks. Then all the nodes on the bitcoin network would accept the false fork because it was longer.
Or even write bogus transactions to the blockchain ? This would seem much more serious than double-spend
You can't write a transaction without knowing the private key of the address you're transferring from. This is true whether there's a 51% attack or not. So you can't just write any old transaction to the blockchain. The rest of the network would reject the block with bad transactions in it.
The big problem is buying something with bitcoin and receiving the purchased good--this transaction goes in Fork A. Then the attacker would start the 51% attack and create another fork--Fork B--which competes with Fork A. In Fork B the attack writes another transaction in which she sends the coins back to a wallet she controls.
Then the attacker must continue to solve blocks at a faster rate than the rest of the network is solving it....AND before the rest of the bitcoin network notices what is going on. This is no small feat.
and not to be brushed off so casually
It's not brushed off casually. The 51% attack is brought up a lot in the cryptocurrency community. But it really isn't feasible on closer examination.
In bitcoin it hasn't been a big deal because everyone is aware of the potential and self-polices.