From OpenSCADAWiki
The simplest configuration of the certificate is self-subscription certificate and private key. The following describes how to create them using the tool openssl:
# Generation the secret key
openssl genrsa -out ./key.pem -des3 -rand /var/log/messages 2048
# Generation of self-subscription certificate
openssl req -x509 -new -key ./key.pem -out ./selfcert.pem -days 365