To convert a PEM-encoded public key to a TXT record, run

    $ SELECTOR="8f00fb94ec6c37aacb48bd43e073f9b7"
    $ DOMAIN="fripost.org"
    $ printf "%s._domainkey%s    IN TXT    (\n" "$SELECTOR" "${DOMAIN:+.$DOMAIN.}"; \
      { printf "v=DKIM1; k=rsa; t=s; s=email; p=";
        sed '/^--.*--$/d' <"./certs/dkim/$SELECTOR.pub" | tr -d '\n';
      } | fold -w64 | sed 's/.*/    "&"/; $s/$/ )\n/'

Remove the "t=s" tag if subdomaining or third-party signature (hosted
domain) is required, cf. RFC 6376 sec. 3.6.1.