Usage of "friendly Captcha" on CB Registration and Forgot password

We want to use "friendly captcha", because we cannot use re-captcha on our sites. Is there a (simple) way to implement this into the registration and forgot password forms of community builder?

CB AntiSpam includes support for all versions of reCaptcha and multiple internal captcha modes. There is no support for "friendly captcha". I recommend using one of the internal captcha modes.

I'm from Germany. We have quite strict rules when it comes to data protection and our lawyer strongly recommended to not use re-captcha or any other non-european service for spam prevention. Bots bypass the image captcha and our customer does not want the questions. So CB Antispam is useless for me.
Is there a way to replace the community builder registration form with a different form? We use RSForm as form generator which includes a hashcash field for spam prevention, which seems to work quite well and I'd use this

CB AntiSpam captcha is completely customizable. Adjust its settings as needed away from the defaults to improve your chances of beating bots. Be sure you also have the honeypot field enabled and consider renaming it. It is not possible to replace CBs registration unless your 3rd party extension uses CB API to create CB users. Additional captcha modes are planned for a future CB AntiSpam release.

HashCash is just rate limiting. That's built into CB AntiSpam as well. So for example if someone fails captcha 5 times temporarily block them. This can be configured within CB AntiSpam parameters. You can also prevent duplicate registrations from same ip address, which should help if a single bot is spamming accounts.

OK, so it is possible to create own registration forms when I use the API? Where can I find a comprehensive documentation of the CB api?

That is going to be a difficult task and I really don't recommend doing that, but the API can be found below.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18362-registering-a-user-through-cb-api

The simpler solution would be worst case to make a custom CB plugin and implement whatever captcha you want or use one of the internal captcha modes while waiting for new captcha modes to be implement as well as using the other anti-spam features in CB AntiSpam.