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      "body": "Congratulations @birdgang! You received a personal award!\n\n<table><tr><td>https://steemitimages.com/70x70/http://steemitboard.com/@birdgang/birthday2.png</td><td>Happy Birthday! - You are on the Steem blockchain for 2 years!</td></tr></table>\n\n<sub>_You can view [your badges on your Steem Board](https://steemitboard.com/@birdgang) and compare to others on the [Steem Ranking](https://steemitboard.com/ranking/index.php?name=birdgang)_</sub>\n\n\n###### [Vote for @Steemitboard as a witness](https://v2.steemconnect.com/sign/account-witness-vote?witness=steemitboard&approve=1) to get one more award and increased upvotes!",
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      "body": "Congratulations @birdgang! You received a personal award!\n\n<table><tr><td>https://steemitimages.com/70x70/http://steemitboard.com/@birdgang/birthday1.png</td><td>1 Year on Steemit</td></tr></table>\n\n<sub>_[Click here to view your Board](https://steemitboard.com/@birdgang)_</sub>\n\n\n**Do not miss the last post from @steemitboard:**\n<table><tr><td><a href=\"https://steemit.com/christmas/@steemitboard/christmas-challenge-send-a-gift-to-to-your-friends-the-party-continues\"><img src=\"https://steemitimages.com/64x128/http://i.cubeupload.com/kf4SJb.png\"></a></td><td><a href=\"https://steemit.com/christmas/@steemitboard/christmas-challenge-send-a-gift-to-to-your-friends-the-party-continues\">Christmas Challenge - The party continues</a></td></tr></table>\n\n> Support [SteemitBoard's project](https://steemit.com/@steemitboard)! **[Vote for its witness](https://v2.steemconnect.com/sign/account-witness-vote?witness=steemitboard&approve=1)** and **get one more award**!",
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      "body": "들러주셔서 감사합니다!\n자주뵈어요!ㅎㅎ",
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      "body": "관련 소식 잘 보고 갑니다. ^^",
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      "body": "\n[Decentralized Application Security Project (DASP)] (http://www.dasp.co/index.html)\n\n# 1. Reentrancy (recursive call vulnerability call to the unknown)  \n\n~~~\nfunction withdraw(uint _amount) {\n\trequire(balances[msg.sender] >= _amount);\n\tmsg.sender.call.value(_amount)();\n\tbalances[msg.sender] -= _amount;\n}\n~~~\n\n* msg.sender 가 smart contract 였을때 해당 smart contract가 실행이 되는데 이때 Fallback 함수에서 다시 withdraw 를 실행하게 되면 무한으로 계속 인출 되게 된다.  (다오 사건)\n\n-----\n# 2. Access Control \n\n~~~\nfunction initContract() public {\n\towner = msg.sender;\n}\n~~~\n* public으로 외부에서 아무나 접근할 수 있어서 문제가 발생.\n \n----\n\n~~~\nfunction () payable {\n   if (msg.value > 0)\n\tDeposit(msg.sender, msg.value);\n   else if (msg.data.length > 0)\n\t_walletLibary.delegatecall(msg.data);\n}\ncontract WalletLibary is WalletEvents {\n\tfunction initMultiOwned(address[] _owners, uint _required) {\n\t}\n\tfunction initWallet(address[] _owners, uint _required, uint _daylimit) {\n\t\tinitMultiOwned(_owners, _required);\n\t}\n}\n~~~\n* delegate call 을 통해서 라이브러리에 있는 함수를 호출해서 문제 발생. (Parity Multi-sig Wallet)\n\n-----\n\n# 3. Arithmetic Issues\n\n~~~\nfunction withdraw(uint _amount) {\n\trequire(balances[msg.sender] - _amount > 0);\n\tmsg.sender.transfer(_amount);\n\tbalances[msg.sender] -= _amount;\n}\n~~~\n\n* 위코드는 실제로 내 잔고 보다 _amount 양이 많다면 결과는 마이너스 값을 가지게 된다.  하지만 _amount 타입이 unsigned integer 이므로  마이너스 를 표현하기 위해서 어마무시한 큰 숫자로 변하게 된다. \n* openzeppelin safeMath lib 사용 하여 방지. (https://github.com/OpenZeppelin/openzeppelin-solidity)\n\n-----\n\n# 4. Unchecked Return Values For Low Level Calls\n\n~~~\nfunction withdraw(uint256 _amount) public {\n\trequire(balances[msg.sender] >= _amount);\n\tbalances[msg.sender] -= _amount;\n\tetherLeft -= _amount;\n\tmsg.sender.send(_amount);\n}\n~~~\n\n* msg.sender.send()를 할때 문제가 발생할때에 대한 예외가 필요하다.\n* msg.sender.transfer 를 사용하거나 msg.sender.send 의 반환값을 확인하여 성공 유무에 따라서 별도의 예외처리가 필요하다.\n\n\n----\n# 5. Denial of Service\n\n~~~\nfunction becomePresident() payable {\n    require(msg.value >= price); // must pay the price to become president\n    president.transfer(price);   // we pay the previous president\n    president = msg.sender;      // we crown the new president\n    price = price * 2;           // we double the price to become president\n}\nfunction selectNextWinners(uint256 _largestWinner) {\n\tfor (uint256 i = 0; i < largestWinner, i++) {\n\t\t// heavy code\n\t}\n\tlargestWinner = _largestWinner;\n}\n~~~\n\n* Gas Limit에 도달했거나 예상치 못한 throw 를 실행시켰을 때 사용할 수 없는 계약이 되어버리는 취약점.\n* 악성 사용자가 Gas를 소모하게 되는 트랜잭션에 특정 조건을 증가시키게 되면 필요한 Gas 양이 Gas Limit 에 초과가 되면며 실행이 되지 못한게된다.\n\n----\n\n# 6. Bad Randomness\n\n~~~\nuint256 private seed;\nfunction play() public payable {\n\trequire(msg.value >= 1 ether);\n\titeration++;\n\tuint randomNumber = uint(keccak256(seed + iteration));\n\tif (randomNumber % 2 == 0) {\n\t\tmsg.sender.transfer(this.balance);\n\t}\n}\nfunction play() public payable {\n\trequire(msg.value >= 1 ether);\n\tif (block.blockhash(blockNumber) % 2 == 0) {\n\t\tmsg.sender.transfer(this.balance);\n\t}\n}\n\n~~~\n\n* Contract 내 생성된 랜던값은 모든 노드들이 같은 결과물을 실행해야 하기 때문에 의 랜덤의 결과값이 같아야 하므로 하드웨어적인 랜던값을 사용하지 못하고 TimeStamp, Seed, Block을 기준으로 랜던값을 생성해 내게 된다.\n* 위 코드는 짝수일 경우 Ethereum 을 보내게 되는데 악의적인 Smart Contract가 짝수일때만 요청을 보내게 되면 항상 이기게 된다.\n\n\n----\n\n# 7. Front-Running\n\n~~~\n1. Alice는 일정량의 가스를 포함한 트랜잭션을 수행한다.(해결책 포함)\n2. 네트워크에있는 누군가는 Alice의 트랜잭션(해결책 포함)을 보고 더 높은 가스 가격으로 제출하고 최종 마이닝 되기를 기다린다.\n3. 마이너들은 더 높은 수수료를 받기위해서 두번째 트랜잭션을 채택하게 되고, 결국 공격자의 트랜잭션이 선택되어 지게된다.\n~~~\n\n* 이더리움 마이너 들은 외부 소유 주소 (EOA)를 대신하여 코드 실행에 대한 가스 요금을 통해 항상 보상을받으며, 사용자는 거래를보다 신속하게 채굴 할 수 있도록 더 높은 수수료를 지정할 수 있다. \nEthereum 블록 체인은 공개되어 있으므로 누구나 다른 사람들의 보류중인 거래 내용을 볼 수 있다. 즉, 특정 사용자가 퍼즐이나 다른 중요한 비밀에 대한 솔루션을 공개하는 경우 악의적 인 사용자가 솔루션을 훔쳐 원래 거래를 선점하기 위해 더 높은 수수료로 거래를 복사 할 수 있게된다.\n\n----\n\n# 8. Time manipulation\n~~~\nfunction play() public {\n\trequire(now > 1521763200 && neverPlayed == true);\n\tneverPlayed = false;\n\tmsg.sender.transfer(1500 ether);\n}\n~~~\n\n* 특정시간에 선착순 한명한테 보상을 준다고 하였을때 시간을 조작해서 우선순위를 조작할 수 있다.\n\n----\n\n# 9 Short Address Attack\n\n* Ethereum Address 는 20byte로 인자값 32byte는 아래와 같이 채워지게 된다.\n> (함수의해쉬값[4byte], Zero-padding[12byte] + Address[29byte] , 함수의 인자2 [32byte] ) \n\n문제는 Address 주소가 19byte 뿐이라면 데이타구조가 (함수의해쉬값 [4byte], Zero-padding[12byte] + Address[19byte] , 함수의 인자2 [33byte] ) 가 되게되므로 amount 양이 8bit왼쪽으로 이동하게되어 원래 보내려고 하던 양보다 256배 큰수가 된다. 따라서 web3 library 에서 check address 로 주소가 20byte 인지 확인을 해서 개발을 해야한다. https://web3js.readthedocs.io/en/1.0/web3-utils.html\n\n----\n",
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