Based on
A successful DOUBLE SPEND US$10000 against OKPAY this morningTwo factors.
1. Transition to ASICs
Network hashrate is low now, and interested parties can get >50% of hashrate power due to new asic technology.
With that they can create financially effective 51% double spend attack:
- Create a bitcoin transaction $tx1 to own address $a2 from own address $a1
- Wait 1-3 blocks
- Turn on hash power, start mining from 1-3 earlier blocks telling your miner to ignore your transaction $tx1. Enough hash power will create chain fork where $tx1 is not confirmed.
- Create a double-spended bitcoin transaction $tx2 from own address $a1 to some financial service/marketplace address $a3
- As long as your hashpower is up and it ignores $tx1, service/marketplace thinks that $tx2 is valid and confirmed.
- Make clearing to fiat that $tx2
- Configure miner to start mining back from parallel chain fork where $tx1 is confirmed
- Service/marketplace looses fiat.
2. maxblocksize
Obviously this shit should be changed soon or later. With following hard fork and another "A successful DOUBLE SPEND" topics.