Great :) @navara, good name :)

Well.... not perse difficult, but more time consuming.... 

for one, I needed to implement In-App Purchase for IOS and In-App Billing for Android on the app's side. 

Then it needed a backend RPC for validating these so-called "in app receipts" the phone sends on a successful purchase. 

Then there is the account creation part which is actually quite straight  forward thanks to Beempy by @holger80, it only needs the account name and the public key parts. 

Then there is Apple who can take a variable amount of time for reviewing your app :P 

<sub>Ps. I am happy to share my backend code for account creation and in-app validation through a private repo (for security) with other app projects. </sub>