for the dApp on Ethereum (later will be on BitLattice), will probably use this,
entropy ^= uint(blockhash(entropy % block.number));
uint randomNumber = entropy % counter;
entropy always based on previous entropy, recursive. if you attack the social consensus (PoW/PoS/PoP), you can choose what order to process transactions in, occasionally (low probability based on percentage of attackers to honest people) you get a random number that pairs you with another attacker. the gain/cost ratio is very low, take into account pair sorting being continuous, shuffled over time. gas cost low as well.
in Constantinople network upgrade you can access all blocks, not just latest 256