[Developers] Building a wallet for your site without private keys?

Hi guys,

Ok, so site developers will know that there is sometimes a need to have a mixed wallet on your server because:

* As well as deposits and withdrawals (which are obviously on chain) you also have off chain fees, such as trades, or micro transaction purchases

This off chain requirement forces you to store private keys somehow, be that encoded, procedural, or whatever because:

* Alls the TXOs get mixed (by necessity of the off chain part), so you can't maintain a 1 - 1 relationship between customer and address or key.

Now, the only way around this that I can see is to have everything on chain. This also means you don't need to store any private keys on the server because there will then be a 1-1 relationship between customer and key.

But with everything on chain:

* You can't do cost effective microtransactions
* Blockchain becomes even more bloated.

Is there a way around this?

Cheers, Paul.