Looks like this will handle RIJNDAEL_256 cyphers: elixir-mcrypt/lib/mcrypt.ex at master · system76/elixir-mcrypt · GitHub, Its 6 years old though, probably only built to handle some forward porting of old code. It may still work it may not.
Poking around it seems that MCRYPT_RIJNDAEL_256 is a non-standard crypto block size/method, I would not try to implement it in newer code.
If you’re working with an existing dataset, porting that to an Elixir application, I would consider decrypting it, then re-encrypting in Elixir with something in the elixir/erlang crypto package (:crypto or maybe GitHub - ntrepid8/ex_crypto: Wrapper around the Erlang crypto module for Elixir. · GitHub (not used myself)). This depends on how sensitive your data is though, transport method, etc etc, you will know if this matters or not.
If you’re just starting a new project and want to encrypted things then I would look for Elixir specific tutorials (Maybe this? Building an Elixir Encryption Engine with Erlang's Crypto Module).
