Does ikev1 or ikev2 support a no-authentication option? If so, how can I enable that in strongswan?
For IKEv2, there is an extension defined in RFC 7619 that allows establishing IKE SAs in which only one side (similar to TLS) or neither side is authenticated. strongSwan currently does not support this. But you could try Libreswan, where the null authentication method enables it.