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It seems like the two main LUKS encryption options are:

  1. USB keyfile (see http://superuser.com/questions/149578/how-can-i-use-a-keyfile-on-a-removable-usb-drive-for-my-encrypted-root-in-debian)
  2. Typing in a password

These both have weaknesses. The first can be overcome by stealing the USB drive. In the second case, a keylogger could be used, or I could be forced to give over the password.

Would it be possible to create an encryption scheme that required both of the options? Then, if I was given an injuction to provide the password, I could destroy the USB drive (e.g. burn it on the stovetop); conversely, if someone stole my USB drive, they would need to get my password as well.

It seems the existing LUKS key slot idea is that providing a key for any key slot will unlock the device, not that one has to provide a key for all, or say "2 of 3" key slots (iirc the latter can be accomplished in a cryptographically secure manner using a Galois field).

(Also, I happen to be on an OpenSuSE system, so please keep answers distro-agnostic or something that can work on SuSE.)

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IANAL, but destroying the key after receiving a subpoena to produce it is likely to get you into serious trouble. –  cjm May 11 '12 at 9:05
    
@cjm, that would be hard to prove, however. I could have "lost" it earlier. –  gatoatigrado May 13 '12 at 19:40
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2 Answers

I don't think something like this exists right out of the box, but it should be possible. I'll put together some references for you.

First there's /etc/crypttab - typically you specify a key file or password in the third slot, but some distros allow you to specify an option in the fourth field called keyscript (debian and opensuse support this: http://linux.frank4dd.com/en/man5/crypttab.htm). This script (which must be in "/lib/cryptsetup/scripts" iirc) can use the third field (what would normally be just the keyfile or password) as an argument. The keyscript's job is to send the actual decryption key to stdout, which cryptsetup then uses as the luks key in the normal fashion.

Second is that you will likely need to write the keyscript. netkeyscript (in github) is an example of a keyscript that listens on a UDP multicast socket for the password; here is another more detailed example of a custom key script, complete with an implementation in sh: http://wejn.org/how-to-make-passwordless-cryptsetup.html. How you code the keyscript (which I'll just call multifactor) I leave as an exercise to you ;)

Third is the secret splitting. you can use an open source tool called "ssss" (google for "ssss-split") to split the luks password in the manner you describe. let's say you split it into 3 pieces (a,b,c) and require any 2 of them. store "a" on the usb drive, "b" in a vault, and "c" you store with the computer (unencrypted partition), or whatever. Of course you don't want "c" to be exposed, so encrypt it with a scheme of your choice using a passphrase you can remember, and call that passphrase "c0", and the encrypted "c" on your disk is "c'". Now your crypttab will look something like

root  /dev/disk/<blah blah>  /dev/<usb>:/path/to/a+$/dev/<blah>:/path/to/c'  keyscript=multifactor

what you actually will have for the third field will of course depend on what your keyscript is willing to accept

Ideally the keyscript should be able to determine that it needs your input to decrypt one or more of the keys (e.g.: c' -> c in this case), but you can use the third field to specify that as desired. In my example above, I used a leading $ to indicate that.

I'm not sure if it's possible to make the keyscript prompt for input, possibly on stderr, since any output on stdout is taken as the luks password. It would be nice if there was a sensible fallback.

Anyway, good luck and happy hacking :)

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Short answer: No.

You could have multiple keys in different slots , but that's not related to your question.

For security , I maintain a checksum for ramdisks and cryptsetup is statically linked , and I check them automatically everytime I boot.

UPDATE

The checksum is stored on encrypted FS , you can't modify it until you mount that partition.

That is , unless you're remotely exploited or your binary programs , e.g ramdisk / cryptsetup on rootfs is being modified, that's secure. (BTW: My rootfs is encrypted , only kernel / ramdisk is naked there , that's why I check them only , but prepare for the slow loading speed with a slow CPU/HDD)

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"I maintain a checksum for ramdisks ... and I check them automatically everytime I boot." What's to prevent someone from replacing the "expected" checksum value, if they have already gone to the trouble of stealing a thumb drive or making you (inadvertantly or not) reveal the passphrase? –  Michael Kjörling May 11 '12 at 7:47
    
@MichaelKjörling updated , please check –  warl0ck May 11 '12 at 7:50
    
I don't see how that addresses what seems to be the OP's concern, namely in the face of involuntary disclosure of key material to an attacker. If you use a passphrase and they have installed a keylogger, they have your passphrase after the next reboot and will have no more trouble than you do using it to access the encrypted file system. –  Michael Kjörling May 11 '12 at 7:56
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