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Author Topic: Stopping a 51% attack  (Read 298 times)
Vincent Fabro
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April 02, 2013, 04:30:49 AM
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It would require clients to forbid rollback of blockchain (if the current block number is 1000,
an attacker with 90% hash power cannot restart from block 950 -or 999- and regrow a new blockchain),
solving double spending problem. It would also force blocks from "good" miners into the blockchain.

This poses the problem of blocks commited at almost the same time, or generaly different clients
not seing two candidate blocks arriving in the same order.
Clients would have to rely on a referee for these cases. The referee could not trick anyone,
or marginaly so (if 2 blocks arrive 0.01 seconds appart, it can favor one, but not if
they come 20 seconds appart), without being spotted.
Clients don't have to trust the referee, they can compare with their own result or with another referee.
If the referee gets corrupted or disabled, the clients will have to find a new one through consensus.

Problems:
 - agreeing that there's a problem with the current referee, and finding a new referee:
   thanks to the winner-takes-all effect (Huh - not so sure), a general agreement would emerge through observation
   of other clients (do they agree with current referee/choice of new referee?) and discussion
   over the internet.
 - time taken to reach these general agreements (during which there may be a fork):
   can be mitigated by use of "secure decentralized networks allowing instant, off-chain payments"
   (https://en.bitcoin.it/wiki/User:Aakselrod/Draft - from blueadept). If you commit 10BTC beforehand
   to this network for 1 week e.g., you can securely pay as much as 10BTC, as long as you can have
   access to a valid (non forked) blockchain within a week.

Seeing other atempts to solve the "51% attack" problem,
I'm not sure it's so simple, though.
Anyone poking holes in my arguments is welcome!

Vincent
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April 02, 2013, 05:54:16 AM
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Make the total network hash rate so large that obtaining 51% is not cost effective.  This is a side effect of the ASIC units hitting the market.  The Total hash rate will likely be around 1 PH (1,000 TH) by years end.  Buying BFL 1.5 TH units it would take about $20,000,000 to match the network. 

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Vincent Fabro
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April 02, 2013, 12:01:16 PM
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Thanks for the answer mjc.
Although I'm well aware that getting 51% of hash power is not easy, I'm not trying to answer
that question, but rather: what to do if it happens.
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