The the recovery key added as part of the Ubiquity update for Debian sounds similar to what's described here if it's one that is made during initial installation of the OS. If it is, in fact, similar or the same then the recovery key merely acted as another backup password taking up one of the other key slots and can thus be treated as a secondary password to add a new password.
But, if your goal was just to reset the password to access the data, you could just use the recovery key to access the data instead of using the recovery key to make a new one.
Back up
Make a backup of the LUKS header before making any changes so that if issues arise, it can easily be restored.
cryptsetup luksHeaderBackup /path/to/ddcomputer.img.dd --header-backup-file=/path/to/backupheader.luks
# the filename and file type for 'backupheader.luks' can be anything you wish
# and doesn't need to end in .luks
Since it's unclear if the recovery key you have is one that you type in one you would type in through the GUI interface that's described or if it's a key file instructions for both are provided separately.
Accessing the Data
If the recovery key is created similarly to how it is in Ubiquity use cryptsetup
to unlock the image and then mount it for access
sudo cryptsetup luksOpen /path/to/ddofcomputer.img.dd decrypt_dd
Enter a passphrase for /path/to/ddofcomputer.img.dd: [Enter the recovery key]
# The 'decrypt_dd' is just the name used for mapping and can be changed
# Once decrypted, the image will be available at /dev/mapper/decrypt_dd
Cryptsetup will ask you for a passphrase whereby you can enter the recovery password to unlock it, then mount it and access it like normal
"Resetting" the password
However, if you still prefer to add a new password, you can do so with cryptsetup by adding a new key to a keyslot
sudo cryptsetup luksAddKey /path/to/ddcomputer.img
Enter any existing passphrase: [enter recovery key]
Enter new passphrase for key slot: [enter desired password]
Verify passphrase: [enter desired passsword again]
This will add a new password to the next available key slot which means the old password can still be used if needed. If you wanted to replace the previously existing password, you would need to specify the default key slot zero, using adding --key-slot=0
to the above command.
If the Recovery Key is a File
However if the recovery key is a file that is saved and is not just a text file containing the recovery password insidethen you can just designate the keyfile when using cryptsetup
sudo cryptsetup luksAddkey /path/to/ddcomputer.img --key-file=/path/to/recoverykey.file
Enter any existing passph...
If you run into issues because you only needed to mount the root partition or data partition then it's possible the system didn't recognize the image file as having LVM members.
Mounting an image.dd with LVM
To access the data it may be necessary to mount the system using the previously provided answer as described by forest, but a simpler and easier method would be to utilize kpartx
which saves you from having to find the offset and all that.
# Installing kpartx
sudo apt-get install kpartx
# Integrate the volumes after using cryptsetup luksOpen in image file
sudo kpartx -a -v /path/to/ddcomputer.img.dd
# Checking the volumes appear
ls -l /dev/mapper
# Fetch the mapped named for mounting
sudo lvscan
# Mount using found name of in lvscan (replacing decrypt_dd) at mountpoint /mnt
sudo mount /dev/mapper/decrypt_dd/root /mnt