Someone has obtained a contract address and roughly knows what the contract does, but doesn't have the exact JSON interface. Would he in that case be able to interact with the contract (for instance by decompiling the contract opcode)?
Note that this is a hypothetical question.
You do not need any permission to interact with a contract. If you have the address you can send messages to it.
If you decompile the contract you may be able to interpret which calls are supported and what are the required parameters for each call.
A contract can implement extra safeguards like only allowing the processing of messages for predetermined set of addresses, for example onlyOwner modifier.
