I'm trying to create a mobile app that will
1) generate transactions
2) send them to my server as a string
3) and my server will relay that transaction to a Smart Contract on the Ethereum network.
I want to encrypt / sign these transactions on the client side (the mobile app) and then decrypt it on the Smart Contract side using a private / public key, so my plan is to hardcode the private key into the mobile app.
If the message is successfully decrypted by the Smart Contract, that should mean that it is a valid transaction coming from an authorized source. If the message decryption fails, that means it's a hacker and nothing will happen.
In this context, how would I generate a private / public key for just that one Smart Contract? Is there a way in Ethereum or can I just make up any rubbish I want? e.g. "abc123"