Enabling encrypted swap has changed in FreeBSD stable/10. I was interested in running AES-XTS with a 256 bit random key and a simulated blocksize of 4096 bytes.
I had to change the line in
/etc/fstab for the swap partition from
/dev/ada0s4b none swap sw 0 0
/dev/ada0s4b.eli none swap sw,keylen=256,sectorsize=4096 0 0
The kernel could then happily report:
GEOM_ELI: Device ada0s4b.eli created. GEOM_ELI: Encryption: AES-XTS 256 GEOM_ELI: Crypto: software