Recently, I started to play around with ext4 transparent encryption. Following basic instructions from Arch Linux wiki, it was really easy to get it up and running. However, when using it with two ext4 filesystems, things get a little more complicated.
How to get the ext4 filesystem salt?
Each encryption enabled ext4 filesystem has a randomised salt. Salt is added to your key to thwart rainbow table attacks, especially with weak passwords. The salt for an ext4 filesystem is stored in the superblock and can be obtained with the dumpe2fs command:
# sudo dumpe2fs /dev/sdb2 | grep Salt
Encryption PW Salt: d24c7f08-5092-4b3a-9180-6ed8244513e8
Which key descriptor corresponds with which filesystem?
When using e4crypt add_key, you will get a separate descriptor for each mounted ext4 encryption enabled filesystem for the same password, due to different filesystems having different salt. Unfortunately it doesn’t say which descriptor was generated from which FS. However, you can determine this by providing the salt (obtained in the previous step) manually to the e4crypt command:
$ /usr/sbin/e4crypt add_key -S d24c7f08-5092-4b3a-9180-6ed8244513e8
Enter passphrase (echo disabled):
Added key with descriptor [9b9cf8b934f6d7bc]
It is important to know which key descriptor corresponds with which filesystem as if you used a descriptor with salt from filesystem 1 on filesystem 2, the descriptor will only ever be there if you add_key while filesystem 1 is mounted. Worse yet, if you reformat filesystem 1 and lose the salt, your filesystem 2 data will be gone forever.
To be safe, when you have multiple mounted filesystems with encryption, I would recommend always providing the salt when add_key.