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Disk Encryption HOWTODavid BraunCopyright © 2003 David Braun 2003-12-18
1. IntroductionI've got a shiny new tablet computer running Linux and I don't want to worry about someone reading the personal information it contains, in case it gets lost or stolen. My log on password may stop someone from booting it, but it won't prevent an attacker from removing the hard disk and reading its data. I need stronger protection. Fortunately, it's relatively easy to use encryption so the hard disk data would be unreadable if it were to fall into the wrong hands. Encryption's not only useful for portable computers like laptops—it can be used to protect any computer with personal information. I protect my computer's files with encryption for the same reason I lock my filing cabinet at home. For further motivation, you may be interested in reading Michael Crawford's Why You Should Use Encryption. I could encrypt only certain files, such as those in my home directory. This would protect the files but then I'd have to worry about information leaking out of them to other, unencrypted places on the disk. Instead I encrypt the whole disk so I don't have to manage this problem. There are many encryption algorithms to choose from. I chose AES because it has been approved by the US government's National Institute of Standards and Technology and is well regarded by the cryptography community. I want my use of it to be resistant to dictionary attacks, so I use a long, randomly generated key. There's no way I'm going to memorize such a key so I keep it in a form I can carry with me easily: on a USB flash drive on my keychain. I encrypt the key with a passphrase so my data is protected in two ways: by a) what I have (the USB flash drive) and b) what I know (the passphrase). I can even give a friend access to my computer without giving away my passphrase—she uses her own USB flash drive and her own passphrase. The operating system keeps the data encrypted on the disk at all times and decrypts it in RAM only as it's used. This way if the computer loses power suddenly the data will remain protected. The decryption key is loaded into RAM at boot time and kept there while the computer is on, so I don't need to keep the USB flash drive plugged in after starting the computer. The procedure outlined in this HOWTO is written for version 2.4 of the Linux kernel. It will become less complicated with the release of Linux 2.6, which will have built-in support for encryption and do a better job of managing partitions within loopback devices. This document assumes the reader has a moderate level of experience with Linux (you should be comfortable patching and compiling kernels as well as partitioning, mounting, and unmounting disks). 1.1. Technical SummaryThe encryption is implemented through a special kind of loopback device. A loopback device doesn't store any data itself; instead it takes all the data storage and retrieval requests it receives and passes them along to a real storage device, such as a disk or a file. As the data passes through, it can be filtered, and in our case the filter used is encryption. When the system is deployed, a removable medium (USB flash drive) boots using GRUB, a kernel, and an initrd. Both the key and the kernel are selected from the GRUB menu, allowing a single removable medium to be used with multiple computers. The initrd contains just enough tools to ask for a passphrase, set up an encrypted loopback device, and mount it. After mounting, pivot_root is used to resume the boot process from the encrypted device. Loopback device offsets are used, instead of partitions, to access separate swap and root file system spaces within the encrypted loopback device because the 2.4 kernel doesn't provide access to partitions within loopback devices. The offset method does not generalize to multiple partitions (unfortunately) because the maximum offset understood by losetup is 2GB. 1.2. Copyright and LicensePermission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in Appendix A. Linux is a registered trademark of Linus Torvalds.® 1.3. DisclaimerNo liability for the contents of this document can be accepted. Use the concepts, examples and information at your own risk. There may be errors and inaccuracies that could be damaging to your system and you may lose important data. Proceed with caution, and although this is highly unlikely, the author does not take any responsibility. All copyrights are held by their by their respective owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark. Naming of particular products or brands should not be seen as endorsements. I know you hate reading directions and want to skip to the meaty bit right away, but I advise you to read the whole document first before touching anything. I know all the HOWTOs say that, but I really mean it for this one. It's worth it; trust me. You may also want to run through the procedure first on a test system before tackling a production system. 1.4. AcknowledgmentsThanks to Linus Torvalds, Jari Ruusu, and all the developers who contributed to their software, without which this HOWTO would have been impossible. Thanks to the National Institute of Standards and Technology for carefully selecting a strong, open encryption algorithm. Thanks to Mark Garboden and others on the linux-crypto mailing list and The Linux Documentation Project mailing lists who took the time to critique my writing and offer suggestions. 1.5. FeedbackFeedback is solicited for this document. Please send additions, comments, and criticisms to the author. 1.6. ApproachesThere are three different approaches we can take to encrypt the disk: encrypt the whole thing, a single partition, or a single file. I strongly recommend the first approach for best security. The first two approaches assume you'll be booting from removable media, such as a USB flash drive or a business card size CD-ROM. If you don't want to do this, you may modify the method to boot from the disk instead by making a small, unencrypted boot partition. If you want to use a USB flash drive to boot your computer, be sure your motherboard can do it first. At the time of this writing many cannot. To avoid having to enumerate all three approaches everywhere I'm going to refer to what you're protecting as the asset. I will refer to the removable medium used to store the key as the keychain. I call it the keychain instead of the key because we can store lots of keys, each for different computers, on the same medium. 1.6.1. Whole DiskA problem with keeping data secret with encryption is that the data likes to move around. Imagine the encryption is like a fence around your data. While the data's inside the fence, it's safe. To be most useful, however, data likes to be transmitted on networks, put on removable disks like CD-ROMs, and shared with friends. Any time your data leaves the fenced area it's unprotected. We can't put an encryption fence around all possible locations where our data might play but we do want to make the fence as large as practical. By putting the encryption fence around your whole hard disk, you won't have to worry about data becoming unprotected if it jumps to another part of the disk.
1.6.2. Partition (for multiboot systems)Encrypting the whole disk is fine if Linux is the only operating system on it, but this won't work for people who have set up their computer to boot multiple operating systems, e.g., Linux, NetBSD, and Darwin. In this case we can encrypt just the Linux partition and leave the others alone. Since we're booting from a removable medium, we won't even need to include the Linux partition in the multiboot menu with the others. To see why this isn't as secure as encrypting the whole disk, read Table 1. 1.6.3. File (for home directories)You may want to encrypt only a file on a file system. Once you've encrypted it you can put into it whatever you want, including other file systems. You might want to use this approach to encrypt only your home directory, for example. This is the least secure of the three approaches and not recommended. If you choose this approach you will notice instructions below to skip whole sections. This is because I'm assuming you've already booted an operating system and have your swap issues handled, so those sections don't apply to you. This HOWTO may be overkill for your needs and you can probably get away with just reading the fine README that comes with loop-AES. If you do, be sure to read Section 1.7 before you finish here. 1.7. Threat ModelIn order to protect our asset well, we must first understand what we're protecting it against. The general idea is that you've got a laptop which is vulnerable to being stolen or lost, and have a USB flash drive on your keychain that isn't, so this system is designed to handle the case that your laptop is stolen. I'm guessing your keychain won't be as easily stolen because it's in your pocket, and because an attacker won't know that it's important. If you pull your USB flash drive out of your pocket and someone non-technical exclaims, "What's that?", tell them it's a Pez dispenser.
Table 1. Attack Tree
1.8. Caveats
1.9. Requirements
1.9.1. A Digression about USB Flash DrivesThere are many choices on the market. When I bought mine, I found one which fit the following requirements:
You might be tempted to get one with a fingerprint reader. I strongly encourage you not to. It might initially seem like a good idea, because by adding the biometric, your security protection expands to:
However, suppose something goes wrong. If you are now asking yourself, "What could go wrong?", then why are you reading this HOWTO? If something goes wrong, you make a change (see Corrective Reactions):
Stop and ponder that last line for a while. 1.10. Looking to the FutureThe method described here is specific to the 2.4 kernel for two reasons: it doesn't have loopback encryption support built in and there's no way to mount partitions within loopback devices. Both of these will change in 2.6 so watch for changes to this HOWTO when it's released. From what I've read you'll probably be able to use disks encrypted with today's method without requiring any significant changes. I chose the loop-AES patch to perform the encryption because it's the recommended solution for loopback encryption today. 2. ProcedureThis method is designed to erase the contents of the asset before encrypting it. If you already have data on the disk you intend to encrypt, you should copy it somewhere else temporarily and then move it back once the encryption is set up. It is possible to encrypt data in place, but for now I consider such magic too advanced for this HOWTO. See loop-AES's README for more details if you're interested in that method. To do the following operations you will need to be running a system which has a loop-AES capable kernel. If you don't have one already, I recommend using KNOPPIX. It boots off a CD-ROM and doesn't need to be installed, so it's very little hassle. For simplicity these instructions assume you'll be preparing the keychain and the asset on the same computer, but this needn't be the case. Adapt the instructions to whatever's convenient for you. 2.1. Prepare the KeychainIf you're taking the approach of encrypting only a file instead of a disk or a partition, you may skip this section and proceed directly to Section 2.2. In the ideal setup you will use a bootable keychain device, such as a USB flash drive or a business card size CD-ROM. This is because we want to expose as little of your disk as possible, but we're going to have to expose a minimal boot process or the computer will never start. Since the boot process will be necessarily unencrypted, it's better to have it away from your computer (on your keychain). If you can't or don't want to use a bootable keychain for some reason, then follow these instructions anyway but instead apply them to a small boot partition on your disk instead of the keychain. In the following example the keychain shows up as the first SCSI drive /dev/sda. Replace /dev/sda with the device for your drive as appropriate. The first step—zeroing out the keychain—is technically unnecessary, but it will make the keychain backup smaller if you back it up as an image as I suggest in Section 2.4.
Next, partition the keychain as you would any bootable disk. See the Linux Partition HOWTO if you need help with partitioning.
Put a file system on the first partition.
Mount the keychain.
2.1.1. Build the KernelIf you want to use the keychain with multiple computers, you may want to build a different kernel for each one. You probably need to build a custom kernel for your keychain so you can ensure two things:
You can load device drivers as modules, since we're using an initrd, but I chose to compile them into the kernel in order to keep the boot disk as simple as possible. Feel free to do differently. For help building a custom kernel read The Linux Kernel HOWTO. Be sure to configure the kernel to include RAM disk support so it can boot using an initrd. Follow the directions that come with loop-AES to patch the kernel and configure it to provide AES encrypted loop device support. Also follow the directions to rebuild the util-linux tools, some of which we'll copy to the keychain later. Once you've built the kernel, copy it to the keychain.
Install GRUB or your favorite boot loader.
Here is a sample menu.lst for GRUB. It has entries for two computers named laptop and desktop.
2.1.2. Make the initrdWe boot the keychain using an initrd so we can remove it after the boot process starts (who wants a USB flash drive hanging out of their laptop while trying to look cool in a café?). To gain access to the asset we create a loopback device attached to the initrd's /dev/loop0. Putting the device file on the initrd means the initrd will have to stay mounted while the asset is mounted (not a big deal). To learn all about making initial RAM disks you're welcome to read The Linux Bootdisk HOWTO and Linux's Documentation/initrd.txt, or don't bother and just follow along. We start by choosing 4MB for the size of the initial RAM disk, all of which we won't need, but it's the conventional maximum size (and it won't hurt) so that's one less decision to make.
Mount the initrd so we can work on it.
Create the minimal directory structure we'll need.
Create the minimal set of devices we'll need. Note that tty is necessary for the password prompt. This command assumes your asset is the drive /dev/hda. Change it as appropriate.
We'll copy the six programs we'll need.
Copy the programs:
Use ldd to find out which shared libraries are used by each program:
Copy the libraries. On my system I copied these libraries (yours may be different):
2.2. Prepare the AssetIt's possible to repeat these steps as many times as you want to handle multiple computers using the same keychain. Each computer will have its own key and probably its own kernel. The instructions here assume the computer's name is laptop; substitute the name of the computer you're working with each time you repeat the steps. First, back up your data. See the Linux Complete Backup and Recovery HOWTO. No, stop, listen to me. Back up your data. Really. It's no fun to have an encrypted hard disk if you can't decrypt it because of some mistake you made. These tools are powerful magic; if you blow it you can't just call up Computer Gurus Are Us and expect them to get your data back for you. That's the whole point of this exercise. If you are encrypting your whole disk (recommended), replace /dev/hda with the device for your disk.
If you are encrypting a partition (multiboot case), replace /dev/hda3 with the device for your partition.
If you are encrypting a file only, replace ~/encrypted with the name of the file and create a link named /tmp/keychain that points to where you decide to store your key file (an already prepared removable medium, e.g., /mnt/cf).
Initialize the asset with random data. This will make it less obvious to the attacker which parts are free space.
Here we create an encrypted file system to hold the keys. More encryption, you say? Yes, in case your keychain is stolen (see Table 1), you don't want your keys to be exposed. I chose one megabyte as the size of the file system because it's a round number. There's no way we're going to need that much space for keys so feel free to chose a smaller size if you like (each key file will be 61 bytes long). Again, initialize with random data.
To make the passphrase resistant to dictionary attacks we'll generate a seed. Whenever you see the symbol <seed> be sure to replace it with the one you generated. The following command will display a random seed on the screen.
Set up the loopback device using the seed. This is where you choose your passphrase, which must be at least 20 characters in length. Choose one with care that you know you won't forget. You may want to use the Diceware method for choosing a secure passphrase.
Format and mount the keys file system (the decrypt.sh script assumes you use the ext2 file system here).
Now for the actual asset key, 45 bytes as random as your computer can make them. Try a dictionary attack against that, attacker! Ha! We name the key after the computer with which it will be used (laptop). Substitute the name of your computer instead.
Set up a loopback device with the key for encrypted access to the asset.
Unmount the keys file system.
2.2.1. Swap PartitionSkip this section if you're encrypting only a file. It's critical to give mkswap a size parameter here because we're not handing it a dedicated partition. Choose whatever size you want; I chose 2GB.
2.2.2. Root File SystemIf you're encrypting only a file, format it with a file system like this and skip to Section 2.3.
We'll create the root "partition" after the swap space. I put the word 'partition' in quotes because it's not a real partition. We're faking it using the offset argument of losetup. Notice how mkswap told us the actual size of the swapspace, which is not necessarily the size requested. Use the actual size (which was 2147471360 in the above example) when specifying the offset to begin the root file system.
If the asset is the whole disk or the last partition on the disk, then we needn't worry about specifying a size for the file system. If this applies to you, do the following and skip to Section 2.2.2.1.
Since the asset isn't the last partition on the disk, we must give mkfs a size limitation or it will write all over whatever partitions are between this one and the end of the disk. I repeat, if you don't give mkfs the correct size parameter here, you may lose data. mkfs is actually just a front end, so to be as careful as possible we'll choose an actual file system maker, in this case mke2fs. It's possible to limit the size of the file system by specifying its size in blocks, but mke2fs chooses the block size based on the size of the file system. A classic Catch-22! We can ask it to do a dry run on the rest of the disk (more than we want) to see what block size it would chose.
In this case it chose 4096. Whatever it chooses is probably close enough for our file system. Calculate the correct size in blocks.
Suppose the size of the partition is 10GB and the size of the swap is 2GB. The correct size for mke2fs is (10 − 2) × 230 ÷ 4096 = 2097152. Don't get this wrong! Make backups! Measure twice, cut once!
2.2.2.1. initrd Mount PointMount the new root file system and create the initrd mount point. This is necessary for the linuxrc script's call to pivot_root.
2.3. ScriptsWe have enough information to create the decryption script. Change the variables at the beginning to reflect your setup (including the seed you generated earlier). If you're encrypting the whole disk or a partition, set ROOT_OFFSET to the size you got from mkswap. Put the script in /tmp/initrd and name it decrypt.sh. If you're encrypting only a file then this script can live anywhere. In this case be sure to set ROOT_OFFSET to zero and set MOUNT to a convenient mount point (probably not /mnt/new-root). Figure 1. /tmp/initrd/decrypt.sh
If you're encrypting only a file, skip to Section 2.4. Otherwise, save the following boot script as linuxrc and place it in /tmp/initrd. Figure 2. /tmp/initrd/linuxrc
Okay, the keychain and asset are now ready. Unmount everything.
You now have an empty, encrypted file system. Hurray! 2.4. Testing and BackupTest your system by booting the keychain or executing the decrypt.sh script as appropriate (give it the name of the key you want to use as a parameter). After booting there may be a complaint about a nonexistent /sbin/init but that's okay for now. Check to make sure your root file system mounted successfully. When you're confident everything is working, back up your keychain. In fact, make lots of backups. You might ask, "But isn't it insecure to have a copy of my keychain somewhere?" The answer is yes, it is, but not as insecure as losing your only keychain, if you define security as also meaning "securing access to my data". Because my keychain is small I decided to back up the whole image so it's easy to restore:
If you're encrypting only a file, you can pat yourself on your back at this point because you've finished. 2.5. Rescue DiskRescue disks are useful when a system isn't behaving properly and/or refuses to boot. Check to make sure your rescue disk has loop-AES support in its kernel and has the correctly patched util-linux tools such as losetup and mount, otherwise it will be worthless with your newly encrypted asset. In the future, all rescue disks will include this support because it will come standard with the 2.6 kernel. In the meantime, KNOPPIX (for example) already has all the necessary support and can be used as a rescue disk. After booting an appropriate rescue disk, mount your keychain and execute the decrypt.sh script.
You can now access your asset through the mount point you specified in decrypt.sh. 2.6. Installing LinuxYour final task is to install Linux to your new encrypted file system. As you do this make sure the entries in your /etc/fstab for the root and swap look like those below:
If you already have an installation elsewhere, read the Hard Disk Upgrade Mini How-To to learn how to copy it over. The procedure for a fresh installation of Linux is different for each distribution. Please send me instructions for distributions not listed below and I will include them here. 2.6.1. Debian
2.6.2. Gentoo
2.6.3. Idle LogoutOnce your system is up and running, consider configuring it to log out automatically after a period of inactivity. This will lessen (but not eliminate) the risk of exposing your asset if the laptop is stolen while on (see Table 1). 3. More Information
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