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A system is installed with an encrypted VG. How can I find out if they used dm_crypt or luks? It's running Fedora 14, so I think it's one of those. Or are they the same thing?

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I am creating a new answer because the currently accepted answer is incorrect in calling them the same.

LUKS adds key management to dm-crypt. It's Linux Unified Key Setup. Without LUKS, you can only have a single master password. LUKS allows you to have multiple keys that can decrypt the single master key that the disk is encrypted with. This allows you to rotate passwords or provide multiple administrators a key that can be revoked later if needed. Also, passwords are better protected against dictionary attacks through the use of PBKDF2 making LUKS + dm-crypt much stronger to crack. @stribika is correct about using cryptsetup luksDump /dev/sdXX to correctly detect the presense of the LUKS header. I believe dm-crypt has it's own header to record the type of encryption used, but I am not 100% sure on that issue.

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You can find out using cryptsetup luksDump /dev/sdXX. As far as I know dm-crypt is a kernel module for disk encryption while LUKS is a specific format. I am not aware of any other formats you can use with dm-crypt so they are basically the same.

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dm-crypt has no header, it's just encrypted, so in theory it's impossible to detect. –  Teddy May 19 '11 at 7:28

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