step-ca supports the Automated Certificate Management Environment (ACME) protocol. You can get X.509 certificates from your own certificate authority (CA) using popular ACME clients and libraries, or via the step command's built-in ACME client.
Run your own private CA & ACME Server using step-ca
Configure popular ACME clients to use a private CA with the ACME protocol
ACME Registration Authority for Smallstep Certificate Manager