I'm working on a Stellar-based app. Part of our protocol requires us to submit transactions from users accounts. We'd prefer to have users enter their secret keys when their accounts need to submit the transactions. Then we wouldn't need to store the secret keys. What would be the best way to securely pass the users secret key between the user, us, and Stellar? Is caching the key on the users device considered secure?
Alternatively, would it be considered more secure to have users set up accounts on our application that are associated with Stellar accounts that we store the secret keys for?