You can add your own certificate from the device42 appliance manager. Step by step instructions for same are in the appliance manager section in device42 documentation.

You can use any external/internal certificate provider to generate certificate for device42. Following is a brief guide on how to create a self-signed certificate with openssl.

1. Create key for CA.

  openssl genrsa -des3 -out ca.key 4096
2. Create certificate file.
  openssl req -new -x509 -days 3650 -key ca.key -out ca_file
3. Create key for server certificate.
  openssl genrsa -des3 -out server.key 2048
4.Create certificate request.
  openssl req -new -key server.key -out server.csr
5. Sign and create the server certificate.
  openssl x509 -req -days 365 -in server.csr -CA ca_file -CAkey ca.key -set_serial 01 -out cert_file
6. Export the key file in correct format.
openssl rsa -in server.key -out key_file

If you wish you can append ca_file to cert_file(cat ca_file >> cert_file) to complete the certificate chain.

Now you can use key_file and cert_file for the appliance(as mentioned in the documentation link above).