I've been doing freelance to create an altcoin and my client wants this new coin to be private which means only authorized users can mine and use the coin. What I've been doing so far is to make a new altcoin, keep it in a private repo, implement mobile wallet apps with authentication gate that requires user to enter his credential info (I have a server to contain this kind of info), etc. Everything seems to work well 'til yesterday when I see a strange IP in the output of getpeerinfo command. So I was wondering how could someone get my code and whether he was able to mine my altcoin.

So my question is: Has anyone of you had experiences in such situation of creating private coin? And do you have any ideas or suggestions on how I can stop someone from mining my altcoin or only allow certain machines from certain IP addresses can connect to my network and mine even if they have my altcoin source code?

Thank you in advance and have a nice day!