https://bh.cosmico.net/2012/08/encrypted-partition-on-openbsd
# make two partitions on two disks, type RAID
disklabel -E sd0
disklabel -E sd1
# will result on this
h: 398460195 79762725 RAID
# now make a softraid device with RAID 1 (mirror)
bioctl -c 1 -l /dev/sd0h,/dev/sd1h softraid0
# will get you this
sd2 at scsibus3 targ 0 lun 0: <OPENBSD, SR RAID 1, 004> SCSI2 0/direct fixed
sd2: 194560MB, 512 bytes/sec, 398459605 sec total
# using this new device, define a RAID partition too
disklabel -E sd2
# will result on this
a: 398459605 0 RAID
# another softraid device now with cryto on top of RAID 1 (will ask for passphrase)
bioctl -c C -l /dev/sd2a softraid0
# finally you have your device
sd3 at scsibus4 targ 0 lun 0: <OPENBSD, SR CRYPTO, 004> SCSI2 0/direct fixed
sd3: 194560MB, 512 bytes/sec, 398459078 sec total
# make a normal partition on it (sd3) with disklabel
disklabel -E sd3
# will result on this:
a: 398459078 0 4.2BSD 4096 32768 1
# newfs it as usual and mount it
newfs /dev/rsd3a
mount /dev/sd3a/ /crypto
# next boot bring up the crypto volume (will ask for passphrase)
bioctl -c C -l /dev/sd2a softraid0
# mount it as usual
mount /dev/sd3a/ /crypto
# softraid status
bioctl softraid0
Volume Status Size Device
softraid0 0 Online 204011317248 sd2 RAID1
0 Online 204011349504 0:0.0 noencl <sd0h>
1 Online 204011349504 0:1.0 noencl <sd1h>
# rebuild drive 1 (sd1h) from the sd2 softraid volume
bioctl -R /dev/sd1h sd2