How to have users sign transactions on an ethereum based platform without surrendering their Private Keys to the platform?
Need the process to be automated, while users not giving up their private keys to the platform.
What your describing can be achieved with signing transactions offline (or at least outside the platform).
The process works like this:
1) A raw transaction is prepared by someone (the platform). This includes all the information required for the transaction except the private key part.
2) The raw transaction is given to the private key holder (user)
3) The user signs the given transaction with his private key and the transaction is ready. It can either be given back to the platform to be published or it can be published by some other means.