if I got it right smart contracts do not have a private key, so they can not sign transactions. The first transaction is signed buy the user and if a contract calls another contract and so on, those transactions are also signed buy the user. So, what if we have two ERC20 contracts A and B and B holds some A tokens.
contract A{
....
//balance of contract B
balanceOf[0xE4e5a16C8fx207a07f7df98e3a85e2067feacB9w]=500;
function transfer(address _to, uint256 _value) public {
_transfer(msg.sender, _to, _value);
}
....
}
contract B{
//address this=0xE4e5a16C8fx207a07f7df98e3a85e2067feacB9w
}
What if some user pretend to to be a contract B calling contract A?
I mean he will sign the sequence of transactions where the last one would not come from the contract B, but contract A will think so.
It will look like this:
{
data: "0xa9059cbb000000000000000000000000cf2ee9c0dccd39aac2fd44b744270f50f8af13b00000000000000000000000000000000000000000000000000000000000000064",
from: "0xE4e5a16C8fx207a07f7df98e3a85e2067feacB9w ",//address B
gas: 210000,
gasPrice: 1,
nonce: "24",
to: "0xa6d90569018967c5esc7d056f74eg4hc3j8ae93" //address A
}
If he do so, it is possible for him, using function transfer in contract A and passing in it his own address to steal tokens from contract B balance in contract A.
So am I right and this is really possible or I made a mistake somewhere? And if it is possible, how in this case a contract can own tokens of other contracts?
Yes, contracts can own tokens.
The transaction you specified won't work; you can't just pick a from address. A transaction is sent from an externally owned account (EOA), meaning an account that has a private key. And only the person with that private key can sign such a transaction. See if this blog post helps: https://programtheblockchain.com/posts/2017/12/29/how-ethereum-transactions-work/.
I think I found an answer to my question.
A first person, who signs a sequence of messages can not cheat and substitute the second message, because when nodes will validate his actions they will all run this sequence of messages on their EVM and if the second message is not valid(i.e it is not coming from the contract) the state will not change.
Related
I'm creating a smart contract where users can create NFT raffles. I will be using Chainlink VRF for getting provably fair results. For this, the user who creates the raffle needs to supply the contract with LINK tokens. I'm attempting to transfer these tokens using an allowance.
function initRaffle(address _tokenContract, uint256 _tokenId, uint256 _ticketPrice) external {
require(_ticketPrice > 0, "Ticket price must be bigger than 0");
require(LINKToken.balanceOf(msg.sender) >= ChainlinkFee, "Insufficient LINK supplied");
require(LINKToken.allowance(msg.sender, address(this)) >= ChainlinkFee, "Allowance failed");
Running initRaffle results in Allowance failed. I've checked and the LINKToken.balanceOf(msg.sender) is bigger than the fee, so that shouldn't be the problem. The LINKToken.balanceOf(address(this)) is 0.
What's going wrong? And how do I create a working function for having the user transfer (fee amount) link tokens to the contract.
The user needs to invoke approve(yourContractAddress, feeAmount) directly on the LINKToken contract (not through your contract). This sets the allowance.
You can then use transferFrom() to pull tokens (up to the allowed amount) from the user's wallet.
bool success = LINKToken.transferFrom(msg.sender, address(this), ChainlinkFee);
I'm studying UniswapV2Pair.sol https://github.com/Uniswap/v2-core/blob/master/contracts/UniswapV2Pair.sol and I have some question about the mint and burn function.
What I understand:
When user deposit the token pair, mint function mints the new liquidity token and send to the user
When user withdraw the token pair, burn function burns the new liquidity token and sends the deposited token pair back to user .
What I'm confused about:
I'm confused about the bold part of burn function I mentioned above. I think that mint and burn function is like mirror(opposite) function, but mint function doesn't include the feature which token pair are send to the exchange contract. However, burn function uses _safeTransfer which sends the token pair back to user.
I'm confused why did they designed assymetrically.
The mint() function calculates the amount of minted LP tokens from the difference of
recorded reserves of the underlying tokens (_reserve0 and _reserve1)
and the actual underlying token balance owned by the pair contract (balance0 and balance1)
So theoretically, if Alice just sent the underlying tokens to the pair contract without invoking the mint() function, that would make the accounting difference described above. And Bob would be able to invoke the mint() function and mint the LP tokens for himself profiting off Alice.
But that's not the usual process flow. Usually, the liquidity provider (Alice), invokes the addLiquidity() function of the router contract that performs both actions at once:
transfers the (approved) tokens from the user to the pair contract
invokes the mint() function on the pair contract calculating the difference created in this transaction
Which removes the possibility for Bob to intercept the Alice's minting process.
And having the mint() function executable by itself also allows anyone to claim unclaimed tokens that were sent to the pair contract by mistake for example.
However, if you want to transfer the underlying tokens out of the pair contract (i.e. burn() the LP tokens), there needs to be check already in the burn() function so that you can't claim more of the underlying tokens than you're eligible to.
No matter if you're invoking the pair burn() function directly or from the router removeLiquidity() (that's normally invoked from the Uniswap UI).
but mint function doesn't include the feature which token pair are
send to the exchange contract.
You showed the UniswapV2Pair.sol. mint function is used when we add liquidity which is defined in UniswapV2Router02.sol
function addLiquidity(
address tokenA,
address tokenB,
uint amountADesired,
uint amountBDesired,
uint amountAMin,
uint amountBMin,
address to,
uint deadline
) external virtual override ensure(deadline) returns (uint amountA, uint amountB, uint liquidity) {
(amountA, amountB) = _addLiquidity(tokenA, tokenB, amountADesired, amountBDesired, amountAMin, amountBMin);
address pair = UniswapV2Library.pairFor(factory, tokenA, tokenB);
TransferHelper.safeTransferFrom(tokenA, msg.sender, pair, amountA);
TransferHelper.safeTransferFrom(tokenB, msg.sender, pair, amountB);
liquidity = IUniswapV2Pair(pair).mint(to);
}
we are passing the amount of tokens that we want to add liquidity, since we cannot add an arbitrary amount, adding liquidity should not affect on the price, so the function itself is calculating the proper amounts and then those amounts are transferred to the UniswapV2Router02 contract. After those amounts are transferred, we are minting a new LP token and sending it to the to address
IUniswapV2Pair(pair).mint(to)
Inside mint function how much token will be minted is calculated and then _mint function called
_mint(to, liquidity);
Hi I'm creating a voting smart contract for a DAO and I have a security question. The voting system works like this:
You send your tokens to the smart contract then the smart contract registers how much tokens you have and assignes you "Power" which you use when you vote. Then the smart contract sends the funds back immidiately.
My question is if there is more secure way to do this. Without funds leaving usere's wallet.
Here is the code I have so far.
function getPower() payable public {
require(msg.value > 0, "The amount can't be 0");
require(election_state == ELECTION_STATE.OPEN);
require(votingPeriod > block.timestamp);
uint amountSent = msg.value;
// This function will take their money and assign power to the voter
// The power is equal to their deposit in eth * 10 so for each eth they get 10 power
voters[msg.sender].power = msg.value * 10;
payable(msg.sender).transfer(amountSent);
}
Thanks in advance.
Based on the provided code and question, I'm assuming you want to calculate the voting power based on the users' ETH balance - not based on their balance of the DAO token.
You can get the current ETH balance of an address, using the .balance member of an address type. So you could simplify your function as this:
function getPower() public {
require(election_state == ELECTION_STATE.OPEN);
require(votingPeriod > block.timestamp);
voters[msg.sender].power = msg.sender.balance * 10;
}
After performing the validations, it assigns the value based on the msg.sender ETH balance at the moment of the getPower() function being invoked. Without them needing to send ETH to the contract.
Note that this approach is not common and can be misused for example by users loaning a large amount of ETH just before executing the getPower() function. I'd recommend you to use a more common pattern of calculating the voting power based on their current holdings of the token representing their stake in the DAO.
We can transfer funds from address(this) to recipient. But is there any way to transfer funds directly msg.sender wallet to recipient? I can not set msg.value at the time of invoking payoutBonus call. Because I can get the amount only inside payoutBonus method.
function payoutBonus(address recipient) public payable returns (bool) {
// bonus = calculateBonus();
//transfer this bonus to recipient from msg.sender;
return true;
}
msg.value is a read-only property reflecting value of the incoming transaction. If you want to send an ETH amount, you can do it in one of two ways.
Notes:
These examples work on Solidity 0.8. Some previous versions also allow the send() method that is now deprecated, don't require the payable type, and have slightly different syntax for the call() method.
bonus is amount of wei (1 ETH is 10^18 wei)
The transfer() method
uint256 bonus = calculateBonus();
payable(msg.sender).transfer(bonus);
The transfer() method only allows consuming 2300 gas, which is enough for non-contract recipients. If the recipient is a contract requiring more than 2300 gas to receive the ETH (for instance it sets some local variable), the transaction reverts.
Low-level call()
uint256 bonus = calculateBonus();
msg.sender.call{value: bonus}("");
With the low-level call() method, you can also specify the gas limit, function to call and it's arguments. Use this method only if you know what you're doing.
I am writing a contract where I want to transfer money (present in the contract owners account and not the contract) to an account address passed to a function in the contract.
for some reason this code won't work
function payBill(uint value, address account) payable public {
account.transfer(value);
transactionCount += 1;
transactionAmount += value;
}
Your problem can be related to any frontend code, not only this.
You've got two options.
get that other user address from the contract and then run a direct transaction between two accounts (example here https://web3js.readthedocs.io/en/v1.2.6/web3-eth.html#id80)
send a value (see here https://web3js.readthedocs.io/en/v1.2.6/web3-eth-contract.html#id33) when you are calling the payBill method. If you don't do it, the default value is zero, and you don't see any transfer.
Also, please consider having a look at this https://diligence.consensys.net/blog/2019/09/stop-using-soliditys-transfer-now/
EDIT:
what you are trying to do here, is also not doable. This requires the contract to have the number of funds that you want to transfer. That is because, with the payable method, you are sending funds to the contract, but the funds will only be in the contract, once the transaction is accepted. On the other hand, you are trying to send that number of funds to another user, without having them on the contract. That is why you never see any balance.