Information describing the creation of DKIM keys and the configuration of DNS to enable DKIM signing is found at the DKIM website.  Please see for the official DKIM documentation.

12.1.1 Creating a DKIM Key Pair

It is quite simple to create a DKIM key pair.  There are websites where you can enter the basic information (selector and domain name) and the website will generate the key pair for you.  However, it is questionable whether such sites will actually guarantee the confidentiality of the public keys they generate, so this may or may not be the best route for your site.

Otherwise, the simplest way to generate a DKIM key pair is to log into a unix machine that has OpenSSL installed, and issue the following commands in a terminal window:

$ openssl genrsa -out rsa.private 1024

$ openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM

This should result in two files being created:  rsa.private and rsa.public.  The rsa.private file contains your private key, which will be used below to create the DKIM file for LISTSERV; the rsa.public file contains the corresponding public key, which will be used to create the DNS TXT record you need for DKIM.