Strongswan 5.5.0 RSA sigkeys

I'm feeling a bit dumb because I'm sure I'm missing something simple, but I'm trying to set up an IPsec coection 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 caot figure out how to provide the private key for the "left side" participant. The documentation doesn't mention a way to do so, and this being asymmetric cryptography, of course that left side would need its private key provided somehow (confirmed by the message in the debug log: "no RSA private key found for..."). I've scoured the Strongswan documentation and website and the Internet in general, and I just can't find an answer to this (or even another mention of the "issue"). I'm hoping someone has already figured this out (or sees whatever I'm missing or misunderstanding) so that I won't have to go combing through the source code to try to figure it out...