I am in the bounty campaign and was looking around the ICO site to contribute. I have added my Public ETH Address for the bounty and to connect it for the ICO, but there is now a spot to put my Private Key? Why are you asking for this? Seems a little sketchy?
We have 3 ways of purchasing tokens:
1. Direct transaction of ETH to smart contract address.
2. Cryptocurrency (not ETH) transaction to shown addresses in account.
3. Transaction from your ETH wallet on our website.
Only using 3rd way you need to type your private key. It is voluntarily. The key is needed to sign your transaction and we don't save or watch your key. These method is MyEtherWallet website analogue. It works the following way: There is a cryptocurrency node for Etherium. So the node is the distribution programm packages which are provided by ETH developers. Any website including Midex refers to that node by API to transfer transaction which afterwards sends to blockchain. On the moment of sending the transaction from website to the node it is necessary to sign up the transaction by your private key. Transaction is signed and send. During this process your private key is not sending anywhere and the transaction is sending already encrypted.
This way of payment is made for your convenience and Midex account don't save your private key as well as any other keys. The account is only the tool for displaying of wallets ballances and a guide for token purchasing.