The reason it's impossible to fake an entry in the block chain and place the fake entry more that a very short time in the past is it's a competition. Sure, someone with a reasonably powerful computer could start with the block chain for Dec. 30, 2017, add a fake entry on the 31st, and then re-add all the entries from then to today. But while the bad guy is doing that, the competition, with lots of computing power of their own, are adding more entries to the end of the chain. The competitors are only worrying about the last 2000 or so transactions (about 10 minutes worth) so they have a much easier task. The bad guy has no chance of catching and passing the competitors. |