Utility for public key

Hi, new to GnuTLS

Using GnuTLS 3.6.5 (built on CentOS 7.6)

What I am looking for is a simple utility program to generate public key 
from private key *without* needing to generate a x.509 certificate.

Use case is OpenDKIM on mail servers

I figured out easy enough how to use certtool to generate ed25519 
private key, but I need to generate public key to stick in the DKIM DNS 
record.

I am guessing there is probably simple C code out there somewhere that 
can do it, but I didn't see public key export options in certtool man 
page to do it.

RSA private to public would be nice too, I can do that conversion w/ 
openssl but my openssl is too old for ed25519.

I know it probably is too early to use ed25519 dkim signing in the wild, 
but I want to set up signing from a web host that only ever e-mails me 
so that I can test validation and make sure my OpenDKIM (built against 
GnuTLS) validates it properly.

Thank you for any suggestions.