Are mineable tokens possible?

Question

I'm a bit puzzled about the ethereum.org/token example which states further down in the advanced examples that it can be coupled to proof-of-work. I really like the idea of mining a token. The example looks something like this:

function giveBlockReward() {
  balanceOf[block.coinbase] += 1;
}

Now my question is, this looks like a honeypot for malicious inflation. Isn't it possible to execute giveBlockReward() multiple times per block? Would a simple mechanism to make sanity checks on blocknumber be sufficient to make this possible?

Answer 1

You should keep track of the last block to "mine" its reward to prevent abuse:

uint lastBlockRewarded;
function giveBlockReward() {
  if (lastBlockRewarded >= block.number) 
    throw;
  lastBlockRewarded = block.number;
  balanceOf[block.coinbase] += 1;
}

Answer 2

Take a look at EIP 918: Mineable Token Standard. Proposed interface:

contract ERC918  {

   function mint(uint256 nonce) public returns (bool success);

   function getAdjustmentInterval() public view returns (uint);

   function getChallengeNumber() public view returns (bytes32);

   function getMiningDifficulty() public view returns (uint);

   function getMiningTarget() public view returns (uint);

   function getMiningReward() public view returns (uint);

   function hash(uint256 _nonce, address _minter) public returns (bytes32 digest);

   function _reward(address _minter) internal returns (uint);

   function _epoch() internal returns (uint);

   function _adjustDifficulty() internal returns (uint);

   event Mint(address indexed from, uint rewardAmount, uint epochCount, bytes32 newChallengeNumber);
}