Do you really need to use a Java class, can’t you use the built-in Erlang crypto module for encryption and decryption or one of the many Elixir libraries?
Anyway, I am not an expert but it looks like you will need to write a NIF to call your java class, by leveraging the BEAM interoperability capabilities:
This section informs on interoperability, that is, information exchange, between Erlang and other programming languages. The included examples mainly treat interoperability between Erlang and C.
