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CentOS/RedHat 6.5+ Full-Disk Encryption With a USB Key Device

This page is only applicable to CentOS/RHEL 6.5 and later. Equivalent instructions are available for CentOS/RHEL 5.0 to 5.3 and 5.4 to 5.10.

Introduction

The CentOS default installation now allows one to use encrypted disks. If you have encrypted devices, you will be prompted at boot time for your LUKS passphrase.

This is perfect for laptop users, but crypto is sometimes desired in other situations as well where you might not necessarily want to be typing in a passphrase. This page documents a method by which you can place a second LUKS key on a USB stick to boot your machine. Note that if an antagonist gets both your USB stick and disk, your data is compromised.

This mechanism is designed to fail safe: If you don't have the USB stick, or if the patched files get overwritten (and not noticed), or if you have to boot from CD/DVD, then you will be prompted for the original LUKS key, which is a whole lot better than saying "oops ... too bad". (In this way, the current mechanism is more robust than the pre-CentOS-5.4 method.)

Related Resources

I would like to thank Gary Myers who provided the dracut-related information on which these instructions are based.

The crytpsetup wiki is an excellent source of material, and the FAQ in particular addresses various technical details and usage concerns.


Technical Overview

Note that this method has been tested only for dracut-004-336.el6_5.2.noarch. While it may work for newer versions of dracut, it will not work for older versions. In particular, if you have the version of dracut that comes with a stock non-updated CentOS 6.4 installation, the patch sequence below will not work. In order to get that one to work, you will need to patch /usr/share/dracut/modules.d/50plymouth/cryptroot-ask.sh, however once you upgrade CentOS that file will no longer exist and your changes will be lost. Therefore it is best to ensure that your CentOS 6.x system is completely updated before proceeding.

This procedure assumes that you have already performed a fresh install of CentOS 6.x where:

  1. /boot is unencrypted. If you are using a single disk, it may be the first partition /dev/sda1. If you are using a RAID mirror, then it might be /dev/md0.
  2. The remaining filesystems (/, /usr, /var, swap and so forth) are backed by LVM logical volumes in a single volume group. For our purposes, we assume that the name of this volume group is vg1.
  3. The volume group vg1 is on a encrypted LUKS device. If this is so, the pvs command will display the volume group's device as something like /dev/mapper/luks-*. If you are using a single disk, this LVM encrypted physical volume may be backed by a single partition such as /dev/sda2. If you're using a RAID mirror, it may be backed by /dev/md1.
    Figure 1
  4. The OS is currently prompting for the LUKS passphrase during boot.
  5. You will be using a USB device for your LUKS key. The smallest one that you can find is just fine. Unlike the 5.x method, this method will not use a filesystem on the USB device.

(If you have more than one encrypted container, Gary Myers' reference gives some information about how they can be handled, but I won't go into them here.)

The procedure is outlined as follows:

  1. Preparation
  2. Initialize the USB Device
  3. Register the Key
  4. Patch and Configure dracut
  5. Build New Initial Ramdisk
  6. Test the result
  7. Monitoring Patched Files
  8. Troubleshooting


Preparation

You will be modifying your boot procedure. While this should be benign, you should be paranoid, especially since your data is already encrypted. Take some time to be paranoid.

  1. Unless you have a virgin system containing no useful information (yet), back up your system before you start.

  2. If you have automated system updates enabled, disable them during this procedure. (It is far more common to be notified when updates are available, but for the system to not install them automatically. If your system behaves in this manner, you need not disable anything.)

  3. Next, you should configure a copy of your existing (working) initrd. This will allow you, during the testing phase, a boot mechanism should something go wrong. (You can delete the copy later.)

    If you're using the Grub bootloader, then edit your /etc/grub.conf file. Duplicate one of the entries, changing the entry title and the name of the initrd image. Make sure create a copy of the initrd image to match the modified name.

    Reboot your system with your backup bootloader configuration to make sure it works.


Initialize the USB Device

You should now set up your USB device that will hold the LUKS key. This method uses data from your entropy pool. In a base configuration, a workstation will typically have more entropy available to it than a server (due to more mouse movement, keystrokes, and other human interaction), so you may find it better to initialize the USB device on a workstation and then move it over to your server.

  1. Attach the USB device to your machine. Your dmesg output and /var/log/messages should show the device attach, and you should be able to determine the device name. You should see something like the following:

        SCSI device sdc: 3940479 512-byte hdwr sectors (2018 MB)
        sdc: Write Protect is off
        sdc: Mode Sense: 45 00 00 08
        sdc: assuming drive cache: write through
        
    In this case, the device is /dev/sdc.

  2. Because the sd device names can get dynamically renamed under some circumstances, you need to be able to identify the USB device in some other fashion. In CentOS/RHEL 6, the best way to do this is by using the identifier under /dev/disk/by-id. Look for it with the following command:

        ls -l /dev/disk/by-id | grep usb
        
    for example, you might see
        /dev/disk/by-id/usb-LaCie_iamaKey_60f2f4441dc104-0:0 -> ../../sdc
        
    Verify that the sdX name in the previous command matches what you saw in the previous dmesg output.

    Take note of the /dev/disk/by-id/* name; you will use this later in the configuration step, below.

  3. Fill the memory stick with random data via the following command:

        dd if=/dev/urandom of=/dev/sdc bs=1
        

  4. [Optional] If you're feeling really paranoid, you can use the high-grade random device, /dev/random, to overwrite the portion of the USB key that you're actually going to use. Note that in this case, initializing the USB stick on a workstation where you have mouse and keyboard interactions is definitely better than doing it on a headless server.

    Note also in this case that reading from /dev/random will in general return partial reads for block sizes greater than 1, so it is important to use different input and output block sizes. (We keep the output block size of 512 bytes because of the I/O properties of the USB key.)

        dd if=/dev/random of=/dev/sdc ibs=1 obs=512 count=512
        
    Make sure you record the above parameters; you'll need them later, including any optional seek value that you may decide to give to dd.

    In this case, ibs must be 1 to avoid short reads, obs should be the native block size of your USB device, and count should be the number of ibs blocks (that is, the number of bytes) that are needed for the key, provided that it is a whole multiple of obs.

    Don't go crazy with the number of bits. The command cryptsetup --help will tell you the types of ciphers that can be used and their respective key sizes. Expanding the number of high-quality random bits past the key sizes (and past the subsequent block size of your USB device) won't buy you anything; the key will be truncated on use. In the stock CentOS 6.x install, the key size for "LUKS1" and "plain" is 256, which means that you only need 256 bits + padding = 512 bits of high quality random data.


Register the Key

Now that you have a secret key on the USB device, you need to register it with your boot device. If you generated the secret key on a different machine such as your workstation, you now have to be back on the server for which the USB key is destined.

  1. Issue the following command to see what LUKS key slots are currently in use. We assume that your encrypted device is /dev/md1, so adjust it as appropriate:

        cryptsetup luksDump /dev/md1
        
    You will likely see Key Slot 0: ENABLED and that all other slots are disabled.

  2. Pick a free key slot. We will assume for this example that you are using key slot 1.

  3. In the next step we both extract the secret key from your USB device and write it to a temporary file. We need to do this because cryptsetup will not accept a binary key on stdin. In doing this, we don't want to leave the secret key sitting around after the fact. Although we will attempt to do a secure erasure of the temporary file, if you write it to a cleartext disk (especially a cleartext SSD disk), there is no gaurantee that the secure erasure will work. Therefore pick the location of the temporary file with some forethought. A location that is on one of your encrypted filesystems is not a bad idea.

    Assuming that the location you picked was /root, extract the key using the following command:

        dd if=/dev/sdc of=/root/luks-secret.key bs=512 count=1
        
    Of course, adjust the various parameters as appropriate to your installation.

  4. Using the extracted key from the last step, register it with LUKS:

        cryptsetup luksAddKey /dev/md1 /root/luks-secret.key --key-slot 1
        
    You will be prompted for the current LUKS passphrase to complete the step.

  5. Delete the temporary file containing your secret from the filesystem:

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

  6. You should now see two keys in the LUKS header:

        cryptsetup luksDump /dev/md1
        


Patch and Configure Dracut

There are two patch files that you should download. They are:

Download them to an appropriate spot. For the purposes of this discussion, I will assume that they are in /tmp.

Next, apply the patches:

    cd /usr/share/dracut/modules.d/90crypt/
    patch -p1 < /tmp/cryptroot-ask.sh.patch
    patch -p1 < /tmp/install.patch

By themselves, the above patches won't have any effect. Before they can do anything of interest, you must provide a configuration file. Create the file /etc/usbcrypt.conf, and within it, place these lines:

    #! /bin/sh
    # This file is used as the configuration for unlocking encrypted
    # devices at boot time via a USB key.  It is used by a modified
    # version of /usr/share/dracut/modules.d/90crypt/cryptroot-ask.sh
    # For details, see:
    #   http://www.gno.org/~gdr/sysadmin/centos/6/usb-crypto-key.html
    #
    
    USBKEY=/dev/disk/by-id/usb-LaCie_iamaKey_60f2f4441dc104-0\:0
    USBKEY_BLOCK_SIZE=512
    USBKEY_BLOCK_COUNT=1
  
(A sample is also available.)

Ensure that you set the variables in the /etc/usbcrypt.conf file to be appropriate to your installation; they must correspond to the way that you extracted the secret key from your USB device in the instructions, above.

If for any reason you are using the skip option of dd, the variable USBKEY_BLOCK_SKIP can also be set. It defaults to zero.


Build New Initial Ramdisk

Before you do this step, you've already configured a backup bootloader entry, right?

After performing all the steps above, you're now ready to create your new initial ram disk. Assuming you're building it for the currently running kernel, issue the following command:

    /usr/libexec/plymouth/plymouth-update-initrd
There should not be any errors or other output. It may take a couple of minutes to complete.


Test the Result

Test by rebooting your machine a couple of times:

  1. First try booting when the USB device is not attached. You should be prompted for your passphrase (you can use the LUKS passphrase you used when doing the initial install; you do not need to memorize what is on the USB device).
  2. Next try booting with the USB device attached; it should boot without intervention. If your system reports an invalid file system, it is probably trying to boot from the USB device instead of hard disk; in this case, examine your BIOS settings with respect to prioriy of boot devices.

If you have problems, then boot using your backup bootloader entry and go splunking. If you have problems with the backup bootloader, you can boot the DVD/CD in rescue mode.

After you're finished testing, don't forget to reenable your automated updates, if you had previously disabled them.


Monitoring Patched Files for Changes

Of course, the danger to patching system files like this is that occasionally a system update will come along and clobber your patches. On the bright side, the boot process is still fail-safe, which means that you will be prompted for your LUKS passphrase. It is, however, inconvenient. (Otherwise you wouldn't be looking at the USB stick solution to begin with, would you?)

You can mitigate this problem by downloading the check-dracut-cryptusb script and placing it in cron-daily. It will complain bitterly to root if the patched files are changed (and give instructions about what should be done about it), so as long as a human is receiving (and reading and reacting to) root's email, you should be fine.

Note that the first time that this script runs, it will give an innocuous message about generating a hash file for the patched files.


Troubleshooting

Gary Myers' web page on this topic has a few suggestions for troubleshooting, including how to debug the boot process, dealing with missing programs in the initrd image, and dealing with additional encrypted volumes.


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Last Updated: 22 May 2014