Question: I'm currently reviewing Xamarin as an option for developing a tablet app (right now aim for iOS and Android but in the future think of porting it to Win8 tablets)

Data to be stored is really sensitive so security is my main concern.

So far I'm thinking of two options:

a) SQLite + SQLCipher: Assuming that Win8 tablets will have SQLite support and that SQLCipher will also work on Win8.

b) Isolated Storage + DPAPI: Assuming that this DPAPI approach will work on Mono platforms and that it will still be available on Win8 tablets.

My concern here is that to securely store the key I guess I'll still need to have separated implementations for each platform (Data Protection on iOS and I'm still reviewing options for Android and Win8 tablets).

Is there any cross platform solution that will work to securely store the encryption key or another completely cross platform way to securely store data? If there is no completely cross platform solution then any recommended options to use on each platform?

Answer: 
I have written a plugin for to address secure storage. It works cross platform and secures the data natively on each platform. It is open source.

Please check it out.
<https://www.nuget.org/packages/sameerIOTApps.Plugin.SecureStorage/>

Blog:
<https://sameer.blog/2018/01/19/whats-new-in-secure-storage-2-0/>

Let me know, should you have any questions.
We use SQLite + SQLCipher in our solution, it works really well on a cross-platform environment.
If I am not mistaken there is no problem with using SQLCipher because it uses AES. I would suggest you to check it anyway but I don't think it's a concern. SQLCipher itself doesn't make the data impossible to read, maybe you have a problem if you allow the option to use Passwords with SQLCipher, but still. The process to get a permission is really simple for AES and can be completed in a few days.