Under the hood
The idea behind Themis is simple: wrap complicated multi-step procedures with shared data (like end-to-end perfect forward secrecy messaging, with session control) into simple objects.
All cryptographic decisions are being made by library's authors, so there's barely a chance to get anything wrong.
Interoperability
Same generation versions of Themis across different platforms are guaranteed to be compatible. For different versions, check build flags in makefile. Although, different platforms may have different implementations of cryptographic primitives, our pluggable cryptographic stack will take care of all differences.