JUCE’s RSAKey class is very stripped down, and only handles RSA encryption/decryption. It does not do RSA signatures & verification, which is a better option in my opinion for the XML license file schemes being discussed here.
It also only handles one format of RSA key, making its interoperability with other RSA code a bit tricky (like if you’re generating license files on a server for customers). The base64-encoded PEM format is a common RSA key format, and JUCE cannot read this in. (I did find this post, however, where @matkatmusic posted some code to help with that process.)