Installing RbThemis # RbThemis is available on RubyGems and can be installed with the usual gem install. Additionally, Themis Core library has to be installed in your system as well. Themis Core is available as a system package for Linux and macOS. Usually you want to install the stable package to benefit from automatic dependency management and security updates. However, you can also build and install the latest RbThemis from the source code.