Configuring IPsec with ipsecctl(8)

ipsecctl can also be used to configure static flows or to manipulate
the security policy database directly.

flow esp from 192.168.0.1 to 192.168.0.2 type require
esp from 192.168.0.1 to 192.168.0.2 spi 0xdeadbeef:0xbeefdead \ 
    authkey file "auth14:auth12" enckey file "enc14:enc12"