Currently there are two big differences between MRI and JRuby authenticators:

1. They have different class names. There is no reason for this since they can't be both loaded at the same time, since one is defined as a C extension and the other as a JRuby extension.
2. They have different parameter APIs - the MRI one takes and returns BSON::Binary instances and JRuby one takes and returns strings. The payload size that is in these binary instances is miniscule (< 100 bytes) hence strings should work just fine.