[Swan] authentication method: IKEv2_AUTH_ECDSA_P384 not supported in I2 Auth Payload

[Manfred]

Hi all,

I'm trying to configure a connection to use IKEv2 + ECDSA certificates, 
but pluto barks the message above. I'm running libreswan 3.29.
I see that it should support ECDSA since 3.26, and the only conf item I 
could find is authby=ecdsa (or possibly authby=ecdsa-sha2_384), both of 
which are accepted but not described in the man page.

Any pointers to where to find info about this configuration, or hints on 
what am I missing?

TIA
Manfred