Converting between Fiat (USD) to ether in solidity

Question

I have a situation where I send 1$ worth of ether to my smart contract..

Now, how do I validate the same on solidity? How do I make sure that the user is sending 1$ worth of ether at any given time.

I mean to say, how can I make sure msg.value in solidity is actually equal to 1$ worth of fiat from solidity.

Edit: I do know, an oracle can be used as a data source. But is there an oracle that provides an api like poloinex.

Step 1: I call the https://poloniex.com/support/api/ and obtain ether - fiat value

Step 2: I send a transaction to a contract using the previously obtained value: contract.method.aPayableMethod().sendTransaction({ value: toWei('value from poloinex') })

Step 3: In my payable function inside solidity, how do I make sure, that that the msg.value is always equal to 1$.

Answer 1

Please take a look at the FiatContract. It is a donation driven contract that updates fiat values every hour.

Answer 2

You'd need to use something like an oracle which could provide that information on-chain, but you need to trust that the oracle will return valid data at all times.