We do not ask for your private key - that is for the miners. However, we do advise that you use a seperate account and send a small amount of ETH to this for mining. Of course, you could also use metamask and manually sign - but of course you know that Metamask also requires your private key, as do the wallets you use etc. The mining software you can use is all opensource, so you can check that all is as it should be. I trust you verified the various code of all other software that you use with ethereum as well