/** *Submitted for verification at Etherscan.io on 2018-02-06 */ pragma solidity ^0.4.18; // ---------------------------------------------------------------------------- // '0xBitcoin Token' contract // Mineable ERC20 Token using Proof Of Work // // Symbol : 0xBTC // Name : 0xBitcoin Token // Total supply: 21,000,000.00 // Decimals : 8 // // ---------------------------------------------------------------------------- // ---------------------------------------------------------------------------- // Safe maths // ---------------------------------------------------------------------------- library SafeMath { function add(uint a, uint b) internal pure returns (uint c) { c = a + b; require(c >= a); } function sub(uint a, uint b) internal pure returns (uint c) { require(b <= a); c = a - b; } function mul(uint a, uint b) internal pure returns (uint c) { c = a * b; require(a == 0 || c / a == b); } function div(uint a, uint b) internal pure returns (uint c) { require(b > 0); c = a / b; } } library ExtendedMath { //return the smaller of the two inputs (a or b) function limitLessThan(uint a, uint b) internal pure returns (uint c) { if(a > b) return b; return a; } } // ---------------------------------------------------------------------------- // ERC Token Standard #20 Interface // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md // ---------------------------------------------------------------------------- contract ERC20Interface { function totalSupply() public constant returns (uint); function balanceOf(address tokenOwner) public constant returns (uint balance); function allowance(address tokenOwner, address spender) public constant returns (uint remaining); function transfer(address to, uint tokens) public returns (bool success); function approve(address spender, uint tokens) public returns (bool success); function transferFrom(address from, address to, uint tokens) public returns (bool success); event Transfer(address indexed from, address indexed to, uint tokens); event Approval(address indexed tokenOwner, address indexed spender, uint tokens); } // ---------------------------------------------------------------------------- // Contract function to receive approval and execute function in one call // // Borrowed from MiniMeToken // ---------------------------------------------------------------------------- contract ApproveAndCallFallBack { function receiveApproval(address from, uint256 tokens, address token, bytes data) public; } // ---------------------------------------------------------------------------- // Owned contract // ---------------------------------------------------------------------------- contract Owned { address public owner; address public newOwner; event OwnershipTransferred(address indexed _from, address indexed _to); function Owned() public { owner = msg.sender; } modifier onlyOwner { require(msg.sender == owner); _; } function transferOwnership(address _newOwner) public onlyOwner { newOwner = _newOwner; } function acceptOwnership() public { require(msg.sender == newOwner); OwnershipTransferred(owner, newOwner); owner = newOwner; newOwner = address(0); } } // ---------------------------------------------------------------------------- // ERC20 Token, with the addition of symbol, name and decimals and an // initial fixed supply // ---------------------------------------------------------------------------- contract _0xBitcoinToken is ERC20Interface, Owned { using SafeMath for uint; using ExtendedMath for uint; string public symbol; string public name; uint8 public decimals; uint public _totalSupply; uint public latestDifficultyPeriodStarted; uint public epochCount;//number of 'blocks' mined uint public _BLOCKS_PER_READJUSTMENT = 1024; //a little number uint public _MINIMUM_TARGET = 2**16; //a big number is easier ; just find a solution that is smaller //uint public _MAXIMUM_TARGET = 2**224; bitcoin uses 224 uint public _MAXIMUM_TARGET = 2**234; uint public miningTarget; bytes32 public challengeNumber; //generate a new one when a new reward is minted uint public rewardEra; uint public maxSupplyForEra; address public lastRewardTo; uint public lastRewardAmount; uint public lastRewardEthBlockNumber; bool locked = false; mapping(bytes32 => bytes32) solutionForChallenge; uint public tokensMinted; mapping(address => uint) balances; mapping(address => mapping(address => uint)) allowed; event Mint(address indexed from, uint reward_amount, uint epochCount, bytes32 newChallengeNumber); // ------------------------------------------------------------------------ // Constructor // ------------------------------------------------------------------------ function _0xBitcoinToken() public onlyOwner{ symbol = "0xBTC"; name = "0xBitcoin Token"; decimals = 8; _totalSupply = 21000000 * 10**uint(decimals); if(locked) revert(); locked = true; tokensMinted = 0; rewardEra = 0; maxSupplyForEra = _totalSupply.div(2); miningTarget = _MAXIMUM_TARGET; latestDifficultyPeriodStarted = block.number; _startNewMiningEpoch(); //The owner gets nothing! You must mine this ERC20 token //balances[owner] = _totalSupply; //Transfer(address(0), owner, _totalSupply); } function mint(uint256 nonce, bytes32 challenge_digest) public returns (bool success) { //the PoW must contain work that includes a recent ethereum block hash (challenge number) and the msg.sender's address to prevent MITM attacks bytes32 digest = keccak256(challengeNumber, msg.sender, nonce ); //the challenge digest must match the expected if (digest != challenge_digest) revert(); //the digest must be smaller than the target if(uint256(digest) > miningTarget) revert(); //only allow one reward for each challenge bytes32 solution = solutionForChallenge[challengeNumber]; solutionForChallenge[challengeNumber] = digest; if(solution != 0x0) revert(); //prevent the same answer from awarding twice uint reward_amount = getMiningReward(); balances[msg.sender] = balances[msg.sender].add(reward_amount); tokensMinted = tokensMinted.add(reward_amount); //Cannot mint more tokens than there are assert(tokensMinted <= maxSupplyForEra); //set readonly diagnostics data lastRewardTo = msg.sender; lastRewardAmount = reward_amount; lastRewardEthBlockNumber = block.number; _startNewMiningEpoch(); Mint(msg.sender, reward_amount, epochCount, challengeNumber ); return true; } //a new 'block' to be mined function _startNewMiningEpoch() internal { //if max supply for the era will be exceeded next reward round then enter the new era before that happens //40 is the final reward era, almost all tokens minted //once the final era is reached, more tokens will not be given out because the assert function if( tokensMinted.add(getMiningReward()) > maxSupplyForEra && rewardEra < 39) { rewardEra = rewardEra + 1; } //set the next minted supply at which the era will change // total supply is 2100000000000000 because of 8 decimal places maxSupplyForEra = _totalSupply - _totalSupply.div( 2**(rewardEra + 1)); epochCount = epochCount.add(1); //every so often, readjust difficulty. Dont readjust when deploying if(epochCount % _BLOCKS_PER_READJUSTMENT == 0) { _reAdjustDifficulty(); } //make the latest ethereum block hash a part of the next challenge for PoW to prevent pre-mining future blocks //do this last since this is a protection mechanism in the mint() function challengeNumber = block.blockhash(block.number - 1); } //https://en.bitcoin.it/wiki/Difficulty#What_is_the_formula_for_difficulty.3F //as of 2017 the bitcoin difficulty was up to 17 zeroes, it was only 8 in the early days //readjust the target by 5 percent function _reAdjustDifficulty() internal { uint ethBlocksSinceLastDifficultyPeriod = block.number - latestDifficultyPeriodStarted; //assume 360 ethereum blocks per hour //we want miners to spend 10 minutes to mine each 'block', about 60 ethereum blocks = one 0xbitcoin epoch uint epochsMined = _BLOCKS_PER_READJUSTMENT; //256 uint targetEthBlocksPerDiffPeriod = epochsMined * 60; //should be 60 times slower than ethereum //if there were less eth blocks passed in time than expected if( ethBlocksSinceLastDifficultyPeriod < targetEthBlocksPerDiffPeriod ) { uint excess_block_pct = (targetEthBlocksPerDiffPeriod.mul(100)).div( ethBlocksSinceLastDifficultyPeriod ); uint excess_block_pct_extra = excess_block_pct.sub(100).limitLessThan(1000); // If there were 5% more blocks mined than expected then this is 5. If there were 100% more blocks mined than expected then this is 100. //make it harder miningTarget = miningTarget.sub(miningTarget.div(2000).mul(excess_block_pct_extra)); //by up to 50 % }else{ uint shortage_block_pct = (ethBlocksSinceLastDifficultyPeriod.mul(100)).div( targetEthBlocksPerDiffPeriod ); uint shortage_block_pct_extra = shortage_block_pct.sub(100).limitLessThan(1000); //always between 0 and 1000 //make it easier miningTarget = miningTarget.add(miningTarget.div(2000).mul(shortage_block_pct_extra)); //by up to 50 % } latestDifficultyPeriodStarted = block.number; if(miningTarget < _MINIMUM_TARGET) //very difficult { miningTarget = _MINIMUM_TARGET; } if(miningTarget > _MAXIMUM_TARGET) //very easy { miningTarget = _MAXIMUM_TARGET; } } //this is a recent ethereum block hash, used to prevent pre-mining future blocks function getChallengeNumber() public constant returns (bytes32) { return challengeNumber; } //the number of zeroes the digest of the PoW solution requires. Auto adjusts function getMiningDifficulty() public constant returns (uint) { return _MAXIMUM_TARGET.div(miningTarget); } function getMiningTarget() public constant returns (uint) { return miningTarget; } //21m coins total //reward begins at 50 and is cut in half every reward era (as tokens are mined) function getMiningReward() public constant returns (uint) { //once we get half way thru the coins, only get 25 per block //every reward era, the reward amount halves. return (50 * 10**uint(decimals) ).div( 2**rewardEra ) ; } //help debug mining software function getMintDigest(uint256 nonce, bytes32 challenge_digest, bytes32 challenge_number) public view returns (bytes32 digesttest) { bytes32 digest = keccak256(challenge_number,msg.sender,nonce); return digest; } //help debug mining software function checkMintSolution(uint256 nonce, bytes32 challenge_digest, bytes32 challenge_number, uint testTarget) public view returns (bool success) { bytes32 digest = keccak256(challenge_number,msg.sender,nonce); if(uint256(digest) > testTarget) revert(); return (digest == challenge_digest); } // ------------------------------------------------------------------------ // Total supply // ------------------------------------------------------------------------ function totalSupply() public constant returns (uint) { return _totalSupply - balances[address(0)]; } // ------------------------------------------------------------------------ // Get the token balance for account `tokenOwner` // ------------------------------------------------------------------------ function balanceOf(address tokenOwner) public constant returns (uint balance) { return balances[tokenOwner]; } // ------------------------------------------------------------------------ // Transfer the balance from token owner's account to `to` account // - Owner's account must have sufficient balance to transfer // - 0 value transfers are allowed // ------------------------------------------------------------------------ function transfer(address to, uint tokens) public returns (bool success) { balances[msg.sender] = balances[msg.sender].sub(tokens); balances[to] = balances[to].add(tokens); Transfer(msg.sender, to, tokens); return true; } // ------------------------------------------------------------------------ // Token owner can approve for `spender` to transferFrom(...) `tokens` // from the token owner's account // // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20-token-standard.md // recommends that there are no checks for the approval double-spend attack // as this should be implemented in user interfaces // ------------------------------------------------------------------------ function approve(address spender, uint tokens) public returns (bool success) { allowed[msg.sender][spender] = tokens; Approval(msg.sender, spender, tokens); return true; } // ------------------------------------------------------------------------ // Transfer `tokens` from the `from` account to the `to` account // // The calling account must already have sufficient tokens approve(...)-d // for spending from the `from` account and // - From account must have sufficient balance to transfer // - Spender must have sufficient allowance to transfer // - 0 value transfers are allowed // ------------------------------------------------------------------------ function transferFrom(address from, address to, uint tokens) public returns (bool success) { balances[from] = balances[from].sub(tokens); allowed[from][msg.sender] = allowed[from][msg.sender].sub(tokens); balances[to] = balances[to].add(tokens); Transfer(from, to, tokens); return true; } // ------------------------------------------------------------------------ // Returns the amount of tokens approved by the owner that can be // transferred to the spender's account // ------------------------------------------------------------------------ function allowance(address tokenOwner, address spender) public constant returns (uint remaining) { return allowed[tokenOwner][spender]; } // ------------------------------------------------------------------------ // Token owner can approve for `spender` to transferFrom(...) `tokens` // from the token owner's account. The `spender` contract function // `receiveApproval(...)` is then executed // ------------------------------------------------------------------------ function approveAndCall(address spender, uint tokens, bytes data) public returns (bool success) { allowed[msg.sender][spender] = tokens; Approval(msg.sender, spender, tokens); ApproveAndCallFallBack(spender).receiveApproval(msg.sender, tokens, this, data); return true; } // ------------------------------------------------------------------------ // Don't accept ETH // ------------------------------------------------------------------------ function () public payable { revert(); } // ------------------------------------------------------------------------ // Owner can transfer out any accidentally sent ERC20 tokens // ------------------------------------------------------------------------ function transferAnyERC20Token(address tokenAddress, uint tokens) public onlyOwner returns (bool success) { return ERC20Interface(tokenAddress).transfer(owner, tokens); } }