Running sudo file -s /dev/sdc
returns:
/dev/sdc: x86 boot sector
Making it very probable that the LUKS header, which was raw on disk, was overwritten by a bootloader. The default for Ubuntu without LVM seems to be to install a LUKS volume directly to the disk.
The system is actually dual-booting Windows 10 and Ubuntu (should have mentioned this, sorry!). To find out the culprit (hint: It's always Microsoft): we parse the strings in the first sector of the disk:
sudo dd bs=512 count=1 if=/dev/sdc 2>/dev/null | strings
Which shows for the backup disk with the lost LUKS header:
Invalid partition table
Error loading operating system
Missing operating system
The default boot drive has Grub 2 and for the same command returns:
GRUB
Geom
Hard Disk
Read
Error
The Windows 10 disk with the Windows 10 boatloader:
Invalid partition table
Error loading operating system
Missing operating system
Bingo! Now I know that I manually installed the Windows bootloader only to the Windows 10 disk, as I had accidentally overwritten it myself with a Grub bootloader and I know the LUKS volume was still there afterwards. However, I have run several automatic Windows 10 repair utilities, as my Windows 10 install keeps freezing/crashing. So it could be possible that one of them decided to appropriate the backup disk for an extra bootloader.
Be sure dm-crypt kernel module is loaded.
modprobe dm-crypt
Then I would suggest:
cryptsetup open --type luks /path/to/dump desired-name
This should create a device /dev/mapper/desired-name which you can then mount as used to.
mount /dev/mapper/desired-name /mnt
I'm not sure if this works for a dump. But it's quite possible.
Best Answer
Yes, it is secure. Ubuntu uses AES-256 to encrypt the disk volume and has a cypher feedback to help protect it from frequency attacks and others attacks that target statically encrypted data.
As an algorithm, AES is secure and this has been proved by crypt-analysis testing. The weakness actually lies within the cypher and the software to pass it the keys. Specifically lies in the keystore (which is stored in the header of the volume), the keystore is secured by a passphrase. Passphrases are of course open to some attacks such as dictionary/brute force (if this was successful, it would decrypt the keystore). Using long "complex" non-word passwords would reduce the chance of this happening.
The only other possibility to decrypt is using recording devices or social engineering to determine your passphrase.
In short, your computer is reasonably safe unless you are subject to serious organised cyber crime or Government investigation!