Hello Tony,
Most of our encryption assertions want the public key in the x509 certificate format. I believe you can convert your RSA public key with the following command:
openssl req -x509 -in rsa_public.pem -nodes -days 365 -newkey rsa:2048 -out x509_public.pem
The x509_public.pem certificate will be a RSA public key with the same modulus as the rsa_public.pem, so I believe they should be interchangeable for encryption, though I may be wrong.
Regards,
Ben