I'm setting up an IPSec connection between two Strongswan clients using RSA "sigkeys." The documentation says that leftsigkey and rightsigkey are used to provide the public RSA keys for the two participants but I don't know how to provide the private key for the "left side" participant. The documentation doesn't mention how to do this. Being asymmetric cryptography, the left side needs its private key provided somehow (which is confirmed by the message in the debug log: no RSA private key found for...).
I've scoured the Strongswan documentation, website and the Internet in general and can't find an answer to this or even another mention of this "issue".
How can I provide the private key for the "left side" participant?