1) Create smart contract with some use case. For example a secure Chat message system 2) Make the register function of your smart contract (solidity in this cas

1) Create smart contract with some use case. For example a secure Chat message system 2) Make the register function of your smart contract (solidity in this case) payable. Meaning that the user can register with his public/private key pair only when he's sending eth with the transaction. 3) make his pair valid for example 1 month to enable some kind of subscription. --- Thats only one idea, another would be make the smart contract and charge for your android/ios app accessing the data in the blockchain. In the second scenario users could bypass the pay wall by using the contract directly, but most wouldn't want to bother with that.