What is meant by Blockhash Minus-256 Problem?
I am reading an article at:
It talks about Blockhash Minus-256 problem but does not explain much about it. As a result I can’t understand this problem.
Please guide me.
It is talking about only the last 256 blocks being available to the contract, see https://solidity.readthedocs.io/en/v0.6.8/units-and-global-variables.html#block-and-transaction-properties under
There is a whole section about it in your article:
As mentioned earlier, the blockhash function is only defined for the previous 256 blocks. (In the non-immediate future, EIP-210 aims to change this.) Therefore, if the second step of the above protocol is performed too late (>256 blocks later) or too early (in the same transaction as the first step), the result (zero) of blockhash will be known to an attacker.
This was for example the case in the SmartBillions hack: https://www.reddit.com/r/ethereum/comments/74d3dc/smartbillions_lottery_contract_just_got_hacked/