When using spm command to create workspace in SDF, the command can fail with message certificate signed by unknown authority. The reason is that the certificate is signed by an internal company authority or self-signed certificate that cannot be confirmed by the command.
In order to avoid this problem please add "options insecure" to the first line of the .signals.cfg file under /root or under the <user>/home directory configuring SDF.
File will look like
options insecure
default https://inventa.mycompany.com super password