Some weeks back, we were forced to reboot one of our server machines because it stopped responding. When the machine came back up, we were greeted with a password prompt to decrypt the partition. No problem, since we always used a password combination (ok, permutation) that consisted of a few words, something along the lines of “john”, “doe”, “1954”, and the server’s serial number. Except that it didn’t work, and we forgot the permutation rules AND whether we used “john” “doe” or “jack” “daniels”.

All the search results for bruteforcing LUKS are largely the same — “use cryptsetup luksOpen --test-passphrase“. In my case, the physical server is in the server room, and I don’t want to stand in front of the rack trying to figure all this out. My question is, can I do this offline on another machine? None of those blog entries were helpful in this regard.

The LUKS Header

To answer this question, I took a look at the LUKS header. This header is what provides multiple “key slots”, allowing you to specify up to 8 passwords or key files that can decrypt the volume. cryptsetup is the standard userspace tool (and library) to manipulate and mount LUKS volumes. Since LUKS was designed based on TKS1, the TKS1 document referenced by the cryptsetup project was very helpful. After consulting the documentation & code, I came up with the following diagram that describes the LUKS key verification process:

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