Additional tests and results: Test 1. /etc/crypttab: cryptswap1 UUID=ba7eaa11-bfcf-4d28-917d-f9b4e2a48830 /dev/urandom swap,offset=1024,cipher=aes-xts-plain64 ctmp /dev/vdb6 /dev/urandom tmp /etc/fstab: /dev/mapper/cryptswap1 none swap sw 0 0 /dev/mapper/ctmp /tmp ext4 defaults,noatime,nodiratime,nosuid,nofail 0 2 The above works, both encrypted swap and /tmp are mounted at boot time. Also: # # cryptsetup status ctmp /dev/mapper/ctmp is active and is in use. type: PLAIN cipher: aes-cbc-essiv:sha256 keysize: 256 bits device: /dev/vdb6 offset: 0 sectors size: 258048 sectors mode: read/write Test 2. /etc/crypttab: cryptswap1 UUID=ba7eaa11-bfcf-4d28-917d-f9b4e2a48830 /dev/urandom swap,offset=1024,cipher=aes-xts-plain64 ctmp /dev/vdb6 /dev/urandom tmp=ext2 /etc/fstab: /dev/mapper/cryptswap1 none swap sw 0 0 /dev/mapper/ctmp /tmp ext2 defaults,noatime,nodiratime,nosuid,nofail 0 2 Encrypted swap is mounted at boot time, /tmp doesn't. Also: # grep ctmp /var/log/syslog Oct 15 09:00:23 ubuntu-1604-home systemd[1]: Starting Cryptography Setup for ctmp... Oct 15 09:00:23 ubuntu-1604-home systemd[1]: