Write to DVD-RAM

Hi, I'm trying to encrypt a DVD-RAM before putting some files onto it on my OpenBSD 6.5 desktop. But neither dd nor disklabel seems able to work on the drive. Did I miss something?

$ dmesg | grep cd
cd0 at scsibus3 targ 1 lun 0: <HL-DT-ST, BD-RE BU40N, FR07> ATAPI 5/cdrom removable serial.13fd3940302020202020
cd0 at scsibus3 targ 1 lun 0: <HL-DT-ST, BD-RE BU40N, FR07> ATAPI 5/cdrom removable serial.13fd3940302020202020

$ doas dd if=/dev/urandom of=/dev/rcd0c bs=1k
dd: /dev/rcd0c: Invalid argument
1+0 records in
0+0 records out
0 bytes transferred in 0.000 secs (0 bytes/sec)

$ doas disklabel -E cd0
cd0> a
partition: [a]
offset: [0]
size: [2236704]
FS type: [4.2BSD]
cd0> w
cd0> p
OpenBSD area: 0-2236704; size: 2236704; free: 0
# size offset fstype [fsize bsize cpg]
a: 2236704 0 4.2BSD 2048 16384 1
c: 2236704 0 unused
cd0> q
No label changes.

The same drive can be formatted and used on Mac OS X.

Thanks and best regards,
Siegfried