I managed to get the swap encrypted and functional:
$ swapon -s
Filename Type Size Used Priority
/dev/mapper/cryptswap1 partition 8388604 384 -1
However, I am not quite sure how! Something I did was to *only* have that
line in /etc/crypttab:
cryptswap1 /dev/sda1 /dev/urandom swap,cipher=aes-cbc-essiv:sha256
/etc/crypttab was containing several lines (probably one per attempt to run
'sudo ecryptfs-setup-swap'), where the swap partition was specified by
UUID... with a different UUID on each line. As you can see above, I specified
the device (/dev/sda1; to change if 'lsblk' indicates another device) rather
than its UUID. I then rebooted. But I guess there is a way to enable the
encrypted swap without rebooting.
