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opensc_luks

How to use a PKCS#15-compliant smartcard to unlock a LUKS encrypted /root

This has been tested on Ubuntu 18.04 LTS with a smartcard and a generic USB smartcard reader. The Ubuntu instance has an unencrypted /boot partition and an encrypted /root partition. Essential info for unlocking the partition was found here: https://blog.g3rt.nl/luks-smartcard-or-token.html. After many VM restarts I was able to put together the patches for the initramfs hook and script and am successfully using this on several laptops.

For smartcard, i'm inspired of the great works of https://github.com/ramann/smartcard-luks/. For USBKey, i'm inspired of the great works of https://www.oxygenimpaired.com/debian-lenny-luks-encrypted-root-hidden-usb-keyfile.1

The general steps:

  • Erase and initialize card
  • Create public/private key pair on smartcard
  • Create key file and add it to a LUKS key slot
  • Encrypt key file using public key from smartcard
  • Modify initramfs to use smartcard to decrypt the encrypted keyfile
  • Modify decrypt_opensc script to swicth between usbkey, smartcard or luks password

The details:

  1. Install smartcard middleware

    sudo apt-get install pcscd opensc

  2. Erase smartcard

    pkcs15-init -E

  3. Initialize smartcard

    pkcs15-init --create-pkcs15 -p pkcs15+onepin --pin 1234 --puk 4321

  4. Create public/private key pair on smartcard

    pkcs15-init -G rsa/2048 -i 01 -a 01 -u decrypt --pin 1234

  5. Create a random key file and add it to a LUKS key slot

    sudo touch /root/rootkey

    sudo chmod 600 /root/rootkey

    sudo dd if=/dev/random of=/root/rootkey bs=1 count=245 #change to urandom if you can't wait

    sudo cryptsetup luksAddKey /dev/sda2 /root/rootkey

  6. Export the public key from smartcard

    pkcs15-tool --read-public-key 01 -o public_key_rsa2048.pem

  7. Encrypt key file using public key

    sudo openssl rsautl -encrypt -pubin -inkey public_key_rsa2048.pem -in /root/rootkey -out /root/rootkey.enc

    sudo rm /root/rootkey

  8. Edit crypttab. This change sends the encrypted key file as a param to the keyscript

    This should be of the form:

    mapped_device_name source_block_device key_file luks,keyscript=decrypt_opensc

    For example:

    sda2_crypt UUID=d332ecc5-ce8b-4900-a04a-a79abd029d6d /root/rootkey.enc luks,keyscript=decrypt_opensc

  9. Plug USB Key and Extract 4096 bits of random data off the usb stick that will become the new keyfile from in between

    dd if=/dev/sdx of=/root/luks-secret.key bs=512 skip=4 count=8

  10. Add the key to your LUKS encrypted partition in new key slot. You’ll be prompted for you current password when running this.

    cryptsetup luksAddKey /dev/sda2 /root/luks-secret.key

    Your usb stick is now ready to go.

  11. If you don’t want to keep a backup copy of this key on your filesystem then use shred.

    shred --remove --zero /root/luks-secret.key

  12. Do actions in "Creating a udev rule" of https://www.oxygenimpaired.com/debian-lenny-luks-encrypted-root-hidden-usb-keyfile.

    You MUST use most specific informations about your USB Key and you must apply 0600 on file /etc/udev/rules.d/99-unlock-lukcs.rules

  13. Apply patch to cryptopensc hook and regenerate initramfs

    sudo patch /lib/cryptsetup/script/decrypt_cryptopensc < decrypt_opensc.patch

    sudo patch /etc/initramfs-tools/modules < etc_initramfs-tools_modules.patch

    sudo update-initramfs -u

  14. Test with USB Key

  15. Test smartcard (without USB Key)

  16. Test LUKS Password (without USB KEY and Smartcard)