Annotation of wikisrc/guide/cgd.mdwn, revision 1.2

1.1       jdf         1: # The cryptographic device driver (CGD)
                      3: The [cgd(4)]( driver 
                      4: provides functionality which allows you to use disks or partitions for encrypted 
                      5: storage. After providing the appropriate key, the encrypted partition is 
                      6: accessible using `cgd` pseudo-devices.
                      8: ## Overview
                     10: People often store sensitive information on their hard disks and are concerned 
                     11: about this information falling into the wrong hands. This is particularly 
                     12: relevant to users of laptops and other portable devices, or portable media, 
                     13: which might be stolen or accidentally misplaced.
                     15: ### Why use disk encryption?
                     17: File-oriented encryption tools like GnuPG are great for encrypting individual 
                     18: files, which can then be sent across untrusted networks as well as stored 
                     19: encrypted on disk. But sometimes they can be inconvenient, because the file must 
                     20: be decrypted each time it is to be used; this is especially cumbersome when you 
                     21: have a large collection of files to protect. Any time a security tool is 
                     22: cumbersome to use, there's a chance you'll forget to use it properly, leaving 
                     23: the files unprotected for the sake of convenience.
                     25: Worse, readable copies of the encrypted contents might still exist on the hard 
                     26: disk. Even if you overwrite these files (using `rm -P`) before unlinking them, 
                     27: your application software might make temporary copies you don't know about, or 
                     28: have been paged to swapspace - and even your hard disk might have silently 
                     29: remapped failing sectors with data still in them.
                     31: The solution is to simply never write the information unencrypted to the hard 
                     32: disk. Rather than taking a file-oriented approach to encryption, consider a 
                     33: block-oriented approach - a virtual hard disk, that looks just like a normal 
                     34: hard disk with normal filesystems, but which encrypts and decrypts each block on 
                     35: the way to and from the real disk.
                     37: ### Logical Disk Drivers
                     39: The `cgd` device looks and behaves to the rest of the operating system like any 
                     40: other disk driver. Rather than driving real hardware directly, it provides a 
                     41: logical function layered on top of another block device. It has a special 
                     42: configuration program, 
                     43: [cgdconfig(8)](, 
                     44: to create and configure a `cgd` device and point it at the underlying disk 
                     45: device that will hold the encrypted data.
                     47: NetBSD includes several other similar logical block devices, each of which 
                     48: provides some other function where `cgd` provides encryption. You can stack 
                     49: several of these logical block devices together: you can make an encrypted 
                     50: `raid` to protect your encrypted data against hard disk failure as well.
                     52: Once you have created a `cgd` disk, you can use 
                     53: [disklabel(8)]( 
                     54: to divide it up into partitions, 
                     55: [swapctl(8)]( to 
                     56: enable swapping to those partitions or 
                     57: [newfs(8)]( to make 
                     58: filesystems, then `mount` and use those filesystems, just like any other new 
                     59: disk.
                     61: ## Components of the Crypto-Graphic Disk system
                     63: A number of components and tools work together to make the `cgd` system 
                     64: effective.
                     66: ### Kernel driver pseudo-device
                     68: To use `cgd` you need a kernel with support for the `cgd` pseudo-device. Make 
                     69: sure the following line is in the kernel configuration file:
                     71:     pseudo-device   cgd     4       # cryptographic disk driver
                     73: The number specifies how many `cgd` devices may be configured at the same time. 
                     74: After configuring the `cgd` pseudo-device you can recompile the kernel and boot 
                     75: it to enable `cgd` support.
                     77: ### Ciphers
                     79: The `cgd` driver provides the following encryption algorithms:
                     81:  * `aes-cbc` -- AES (Rijndael). AES uses a 128 bit blocksize and accepts 128, 
                     82:    192 or 256 bit keys.
                     84:  * `blowfish-cbc` -- Blowfish uses a 64 bit blocksize and accepts 128 bit keys
                     86:  * `3des-cbc` -- Triple DES uses a 64 bit blocksize and accepts 192 bit keys 
                     87:    (only 168 bits are actually used for encryption)
                     89: All three ciphers are used in [CBC (Cipher Block 
                     90: Chaining)](
                     91: mode. This means each block is XORed with the previous encrypted block before 
                     92: encryption. This reduces the risk that a pattern can be found, which can be used 
                     93: to break the encryption.
                     95: ### Verification Methods
                     97: Another aspect of `cgd` that needs some attention are the verification methods 
                     98: `cgdconfig` provides. These verification methods are used to verify the 
                     99: passphrase is correct. The following verification methods are available:
                    101:  * `none` -- no verification is performed. This can be dangerous, because the 
                    102:    key is not verified at all. When a wrong key is entered, `cgdconfig` 
                    103:    configures the `cgd` device as normal, but data which was available on the 
                    104:    volume will be destroyed (decrypting blocks with a wrong key will result in 
                    105:    random data, which will result in a regeneration of the disklabel with the 
                    106:    current key).
                    108:  * `disklabel` -- `cgdconfig` scans for a valid disklabel. If a valid disklabel 
                    109:    is found with the key that is provided authentication will succeed.
                    111:  * `ffs` -- `cgdconfig` scans for a valid FFS file system. If a valid FFS file 
                    112:    system is found with the key that is provided authentication will succeed.
                    114: ## Example: encrypting your disk
                    116: This section works through a step-by-step example of converting an existing 
                    117: system to use `cgd`, performing the following actions:
                    119:  1. Preparing the disk and partitions
                    120:  2. Scrub off all data
                    121:  3. Create the cgd
                    122:  4. Adjust config-files
                    123:  5. Restoring your backed-up files to the encrypted disk
                    125: ### Preparing the disk
                    127: First, decide which filesystems you want to move to an encrypted device. You're 
                    128: going to need to leave at least the small root (`/`) filesystem unencrypted, in 
                    129: order to load the kernel and run `init`, `cgdconfig` and the `rc.d` scripts that 
                    130: configure your `cgd`. In this example, we'll encrypt everything except the root 
                    131: (`/`) filesystem.
                    133: We are going to delete and re-make partitions and filesystems, and will require 
                    134: a backup to restore the data. So make sure you have a current, reliable backup 
                    135: stored on a different disk or machine. Do your backup in single-user mode, with 
                    136: the filesystems unmounted, to ensure you get a clean 
                    137: [dump(8)]( Make sure you 
                    138: back up the disklabel of your hard disk as well, so you have a record of the 
                    139: partition layout before you started.
                    141: With the system at single user, `/` mounted read-write and everything else 
                    142: unmounted, use 
                    143: [disklabel(8)](
                    144: to delete all the data partitions you want to move into `cgd`.
                    146: Then make a single new partition in all the space you just freed up, say, 
                    147: `wd0e`. Set the partition type for this partition to `cgd` Though it doesn't 
                    148: really matter what it is, it will help remind you that it's not a normal 
                    149: filesystem later. When finished, label the disk to save the new partition table.
                    151: ### Scrubbing the disk
                    153: We have removed the partition table information, but the existing filesystems 
                    154: and data are still on disk. Even after we make a `cgd` device, create 
                    155: filesystems, and restore our data, some of these disk blocks might not yet be 
                    156: overwritten and still contain our data in plaintext. This is especially likely 
                    157: if the filesystems are mostly empty. We want to scrub the disk before we go 
                    158: further.
                    160: We could use 
                    161: [dd(1)](
                    162: to copy `/dev/zero` over the new `wd0e` partition, but this will leave our disk 
                    163: full of zeros, except where we've written encrypted data later. We might not 
                    164: want to give an attacker any clues about which blocks contain real data, and 
                    165: which are free space, so we want to write "noise" into all the disk blocks. So 
                    166: we'll create a temporary `cgd`, configured with a random, unknown key.
                    168: First, we configure a `cgd` to use a random key:
                    170:     # cgdconfig -s cgd0 /dev/wd0e aes-cbc 128 < /dev/urandom 
                    172: Now we can write zeros into the raw partition of our `cgd` (`/dev/rcgd0d` on 
                    173: NetBSD/i386, `/dev/rcgd0c` on most other platforms):
                    175:     # dd if=/dev/zero of=/dev/rcgd0d bs=32k
                    177: The encrypted zeros will look like random data on disk. This might take a while 
                    178: if you have a large disk. Once finished, unconfigure the random-key `cgd`:
                    180:     # cgdconfig -u cgd0
                    182: ### Creating the `cgd`
                    184: The 
                    185: [cgdconfig(8)]( 
                    186: program, which manipulates `cgd` devices, uses parameters files to store such 
                    187: information as the encryption type, key length, and a random password salt for 
                    188: each `cgd`. These files are very important, and need to be kept safe - without 
                    189: them, you will not be able to decrypt the data!
                    191: We'll generate a parameters file and write it into the default location (make 
                    192: sure the directory `/etc/cgd` exists and is mode 700):
                    194:     # cgdconfig -g -V disklabel -o /etc/cgd/wd0e aes-cbc 256
                    196: This creates a parameters file `/etc/cgd/wd0e` describing a `cgd` using the 
                    197: `aes-cbc` cipher method, a key verification method of `disklabel`, and a key 
                    198: length of `256` bits. It will look something like this:
                    200:     algorithm aes-cbc;
                    201:     iv-method encblkno;
                    202:     keylength 256;
                    203:     verify_method disklabel;
                    204:     keygen pkcs5_pbkdf2/sha1 {
                    205:             iterations 6275;
                    206:             salt AAAAgHTg/jKCd2ZJiOSGrgnadGw=;
                    207:     };
                    209: *Note*: Remember, you'll want to save this file somewhere safe later.
                    211: *Tip*: When creating the parameters file, `cgdconfig` reads from `/dev/random` 
                    212: to create the password salt. This read may block if there is not enough 
                    213: collected entropy in the random pool. This is unlikely, especially if you just 
                    214: finished overwriting the disk as in the previous step, but if it happens you can 
                    215: press keys on the console and/or move your mouse until the `rnd` device gathers 
                    216: enough entropy.
                    218: Now it's time to create our `cgd`, for which we'll need a passphrase. This 
                    219: passphrase needs to be entered every time the `cgd` is opened, which is usually 
                    220: at each reboot. The encryption key is derived from this passphrase and the salt. 
                    221: Make sure you choose something you won't forget, and others won't guess.
                    223: The first time we configure the `cgd`, there is no valid disklabel on the 
                    224: logical device, so the validation mechanism we want to use later won't work. We 
                    225: override it this one time:
                    227:     # cgdconfig -V re-enter cgd0 /dev/wd0e
                    229: This will prompt twice for a matching passphrase, just in case you make a typo, 
                    230: which would otherwise leave you with a `cgd` encrypted with a passphrase that's 
                    231: different to what you expected.
                    233: Now that we have a new `cgd`, we need to partition it and create filesystems. 
                    234: Recreate your previous partitions with all the same sizes, with the same letter 
                    235: names.
                    237: *Tip*: Remember to use the `disklabel -I` argument, because you're creating an 
                    238: initial label for a new disk.
                    240: *Note*: Although you want the sizes of your new partitions to be the same as the 
                    241: old, unencrypted ones, the offsets will be different because they're starting at 
                    242: the beginning of this virtual disk.
                    244: Then, use 
                    245: [newfs(8)]( to 
                    246: create filesystems on all the relevant partitions. This time your partitions 
                    247: will reflect the `cgd` disk names, for example:
                    249:     # newfs /dev/rcgd0h
                    251: ### Modifying configuration files
                    253: We've moved several filesystems to another (logical) disk, and we need to update 
                    254: `/etc/fstab` accordingly. Each partition will have the same letter (in this 
                    255: example), but will be on `cgd0` rather than `wd0`. So you'll have `/etc/fstab` 
                    256: entries something like this:
                    258:     /dev/wd0a   /     ffs     rw    1 1
                    259:     /dev/cgd0b  none  swap    sw            0 0
                    260:     /dev/cgd0b  /tmp  mfs     rw,-s=132m    0 0
                    261:     /dev/cgd0e  /var  ffs     rw            1 2
                    262:     /dev/cgd0f  /usr  ffs     rw            1 2
                    263:     /dev/cgd0h  /home ffs     rw            1 2
                    265: *Note*: `/tmp` should be a separate filesystem, either `mfs` or `ffs`, inside 
                    266: the `cgd`, so that your temporary files are not stored in plain text in the `/` 
                    267: filesystem.
                    269: Each time you reboot, you're going to need your `cgd` configured early, before 
                    270: [fsck(8)]( runs and 
                    271: filesystems are mounted.
                    273: Put the following line in `/etc/cgd/cgd.conf`:
                    275:     cgd0    /dev/wd0e
                    277: This will use `/etc/cgd/wd0e` as config file for `cgd0`.
                    279: To finally enable cgd on each boot, put the following line into `/etc/rc.conf`:
                    281:     cgd=YES
                    283: You should now be prompted for `/dev/cgd0`'s passphrase whenever `/etc/rc` 
                    284: starts.
                    286: ### Restoring data
                    288: Next, mount your new filesystems, and 
                    289: [restore(8)]( your 
                    290: data into them. It often helps to have `/tmp` mounted properly first, as 
                    291: `restore` can use a fair amount of temporary space when extracting a large 
                    292: dumpfile.
                    294: To test your changes to the boot configuration, umount the filesystems and 
                    295: unconfigure the `cgd`, so when you exit the single-user shell, *rc* will run 
                    296: like on a clean boot, prompting you for the passphrase and mounting your 
                    297: filesystems correctly. Now you can bring the system up to multi-user, and make 
                    298: sure everything works as before.
                    300: ## Example: encrypted CDs/DVDs
                    302: ### Introduction
                    304: This section explains how to create and use encrypted CDs/DVDs with NetBSD (all 
                    305: I say about CDs here does also apply to DVDs). I assume that you have basic 
                    306: knowledge of cgd(4), so I will not explain what cgd is or what's inside it in 
                    307: detail. The same applies to vnd(4). One can make use of encrypted CDs after 
                    308: reading this howto, but for more detailed information about different cgd 
                    309: configuration options, please read the previous parts or the manpages.
                    311: ### Creating an encrypted CD/DVD
                    313: cgd(4) provides highly secure encryption of whole partitions or disks. 
                    314: Unfortunately, creating "normal" CDs is not disklabeling something and running 
                    315: newfs on it. Neither can you just put a CDR into the drive, configure cgd and 
                    316: assume it to write encrypted data when syncing. Standard CDs contain at least an 
                    317: ISO-9660 filesystem created with mkisofs(8) from the 
                    318: [`sysutils/cdrtools`]( 
                    319: package. ISO images must *not* contain disklabels or cgd partitions.
                    321: But of course CD reader/writer hardware doesn't care about filesystems at all. 
                    322: You can write raw data to the CD if you like - or an encrypted FFS filesystem, 
                    323: which is what we'll do here. But be warned, there is NO way to read this CD with 
                    324: any OS except NetBSD - not even other BSDs due to the lack of cgd.
                    326: The basic steps when creating an encrypted CD are:
                    328:  * Create an (empty) imagefile
                    329:  * Register it as a virtual disk using vnd(4)
                    330:  * Configure cgd inside the vnd disk
                    331:  * Copy content to the cgd
                    332:  * Unconfigure all (flush!)
                    333:  * Write the image on a CD
                    335: The first step when creating an encrypted CD is to create a single image file 
                    336: with dd. The image may not grow, so make it large enough to allow all CD content 
                    337: to fit into. Note that the whole image gets written to the CD later, so creating 
                    338: a 700 MB image for 100 MB content will still require a 700 MB write operation to 
                    339: the CD. Some info on DVDs here: DVDs are only 4.7 GB in marketing language. 
                    340: 4.7GB = 4.7 x 1024 x 1024 x 1024 = 5046586573 bytes. In fact, a DVD can only 
                    341: approximately hold 4.7 x 1000 x 1000 x 1000 = 4700000000 bytes, which is about 
                    342: 4482 MB or about 4.37 GB. Keep this in mind when creating DVD images. Don't 
                    343: worry for CDs, they hold "real" 700 MB (734003200 Bytes).
                    345: Invoke all following commands as root!
                    347: For a CD:
                    349:     # dd if=/dev/zero of=image.img bs=1m count=700
                    351: or, for a DVD:
                    353:     # dd if=/dev/zero of=image.img bs=1m count=4482
                    355: Now configure a 
                    356: [vnd(4)]( 
                    357: disk with the image:
                    359:     # vnconfig vnd0 image.img
                    361: In order to use cgd, a so-called parameter file, describing encryption 
                    362: parameters and a containing "password salt" must be generated. We'll call it 
                    363: `/etc/cgd/image` here. You can use one parameter file for several encrypted 
                    364: partitions (I use one different file for each host and a shared file `image` for 
                    365: all removable media, but that's up to you).
                    367: I'll use AES-CBC with a keylength of 256 bits. Refer to 
                    368: [cgd(4)]( and 
                    369: [cgdconfig(8)]( 
                    370: for details and alternatives.
                    372: The following command will create the parameter file as `/etc/cgd/image`. *YOU 
                    373: DO NOT WANT TO INVOKE THE FOLLOWING COMMAND AGAIN* after you burnt any CD, since 
                    374: a recreated parameter file is a lost parameter file and you'll never access your 
                    375: encrypted CD again (the "salt" this file contains will differ among each call). 
                    376: Consider this file being *HOLY, BACKUP IT* and *BACKUP IT AGAIN!* Use switch -V 
                    377: to specify verification method "disklabel" for the CD (cgd cannot detect whether 
                    378: you entered a valid password for the CD later when mounting it otherwise).
                    380:     # cgdconfig -g -V disklabel aes-cbc 256 > /etc/cgd/image
                    382: Now it's time to configure a cgd for our vnd drive. (Replace slice `d` with `c` 
                    383: for all platforms that use `c` as the whole disk (where
                    384: `sysctl kern.rawpartition` prints `2`, not `3`); if you're on i386 or amd64, `d` 
                    385: is OK for you):
                    387:     # cgdconfig -V re-enter cgd1 /dev/vnd0d /etc/cgd/image
                    389: The `-V re-enter` option is necessary as long as the cgd doesn't have a 
                    390: disklabel yet so we can access and configure it. This switch asks for a password 
                    391: twice and uses it for encryption.
                    393: Now it's time to create a disklabel inside the cgd. The defaults of the label 
                    394: are ok, so invoking disklabel with
                    396:     # disklabel -e -I cgd1
                    398: and leaving vi with `:wq` immediately will do.
                    400: Let's create a filesystem on the cgd, and finally mount it somewhere:
                    402:     # newfs /dev/rcgd1a
                    403:     # mount /dev/cgd1a /mnt
                    405: The cgd is alive! Now fill `/mnt` with content. When finished, reverse the 
                    406: configuration process. The steps are:
                    408: 1.  Unmounting the cgd1a:
                    410:         # umount /mnt
                    412: 2.  Unconfiguring the cgd:
                    414:         # cgdconfig -u cgd1
                    416: 3.  Unconfiguring the vnd:
                    418:         # vnconfig -u vnd0
                    421: The following commands are examples to burn the images on CD or DVD. Please 
                    422: adjust the `dev=` for cdrecord or the `/dev/rcd0d` for growisofs. Note the 
                    423: `r` on the `rcd0d` *is* necessary with NetBSD. Growisofs is available in the 
                    424: [`sysutils/dvd+rw-tools`]( 
                    425: package. Again, use `c` instead of `d` if this is the raw partition on your 
                    426: platform.
                    428: Finally, write the image file to a CD:
                    430:     # cdrecord dev=/dev/rcd0d -v image.img
                    432: ...or to a DVD:
                    434:     # growisofs -dvd-compat -Z /dev/rcd0d=image.img
                    436: Congratulations! You've just created a really secure CD!
                    438: ### Using an encrypted CD/DVD
                    440: After creating an encrypted CD as described above, we're not done yet - what 
                    441: about mounting it again? One might guess, configuring the cgd on `/dev/cd0d` is 
                    442: enough - no, it is not.
                    444: NetBSD cannot access FFS file systems on media that is not 512 bytes/sector 
                    445: format. It doesn't matter that the cgd on the CD is, since the CD's disklabel 
                    446: the cgd resides in has 2048 bytes/sector.
                    448: But the CD driver cd(4) is smart enough to grant write access to the 
                    449: (emulated) disklabel on the CD. So before configuring the cgd, let's have a look 
                    450: at the disklabel and modify it a bit:
                    452:     # disklabel -e cd0
                    453:     # /dev/rcd0d:
                    454:     type: ATAPI
                    455:     disk: mydisc
                    456:     label: fictitious
                    457:     flags: removable
                    458:     bytes/sector: 2048    # -- Change to 512 (= orig / 4)
                    459:     sectors/track: 100    # -- Change to 400 (= orig * 4)
                    460:     tracks/cylinder: 1
                    461:     sectors/cylinder: 100 # -- Change to 400 (= orig * 4)
                    462:     cylinders: 164
                    463:     total sectors: 16386  # -- Change to value of slice "d" (=65544)
                    464:     rpm: 300
                    465:     interleave: 1
                    466:     trackskew: 0
                    467:     cylinderskew: 0
                    468:     headswitch: 0           # microseconds
                    469:     track-to-track seek: 0  # microseconds
                    470:     drivedata: 0
                    472:     4 partitions:
                    473:     #     size  offset  fstype [fsize bsize cpg/sgs]
                    474:      a:   65544   0     4.2BSD  0     0     0  # (Cyl. 0 - 655+)
                    475:      d:   65544   0     ISO9660 0     0        # (Cyl. 0 - 655+)
                    477: Now as the disklabel is in 512 b/s format, accessing the CD is as easy as:
                    479:     # cgdconfig cgd1 /dev/cd0d /etc/cgd/image
                    480:     # mount -o ro /dev/cgd1a /mnt
                    482: Note that the cgd *MUST* be mounted read-only or you'll get illegal command 
                    483: errors from the cd(4) driver which can in some cases make even mounting a 
                    484: CD-based cgd impossible!
                    486: Now we're done! Enjoy your secure CD!
                    488:     # ls /mnt
                    490: Remember you have to reverse all steps to remove the CD:
                    492:     # umount /mnt
                    493:     # cgdconfig -u cgd1
                    494:     # eject cd0
                    496: ## Suggestions and Warnings
                    498: You now have your filesystems encrypted within a `cgd`. When your machine is 
                    499: shut down, the data is protected, and can't be decrypted without the passphrase. 
                    500: However, there are still some dangers you should be aware of, and more you can 
                    501: do with `cgd`. This section documents several further suggestions and warnings 
                    502:        that will help you use `cgd` effectively.
                    504:  * Use multiple `cgd`'s for different kinds of data, one mounted all the time 
                    505:    and others mounted only when needed.
                    507:  * Use a `cgd` configured on top of a `vnd` made from a file on a remote network 
                    508:    fileserver (NFS, SMBFS, CODA, etc) to safely store private data on a shared 
                    509:    system. This is similar to the procedure for using encrypted CDs and DVDs 
                    510:    described in [[Example: encrypted CDs/DVDs|guide/cgd#cryptocds]].
                    512: ### Using a random-key cgd for swap
                    514: You may want to use a dedicated random-key `cgd` for swap space, regenerating 
                    515: the key each reboot. The advantage of this is that once your machine is 
                    516: rebooted, any sensitive program memory contents that may have been paged out are 
                    517: permanently unrecoverable, because the decryption key is never known to you.
                    519: We created a temporary `cgd` with a random key when scrubbing the disk in the 
                    520: example above, using a shorthand `cgdconfig -s` invocation to avoid creating a 
                    521: parameters file.
                    523: The `cgdconfig` params file includes a *randomkey* keygen method. This is more 
                    524: appropriate for *permanent* random-key configurations, and facilitates the easy 
                    525: automatic configuration of these volumes at boot time.
                    527: For example, if you wanted to convert your existing `/dev/wd0b` partition to a 
                    528: dedicated random-key cgd1, use the following command to generate 
                    529: `/etc/cgd/wd0b`:
                    531:     # cgdconfig -g -o /etc/cgd/wd0b -V none -k randomkey blowfish-cbc
                    533: When using the randomkey keygen method, only verification method `none` can be 
                    534: used, because the contents of the new `cgd` are effectively random each time 
                    535: (the previous data decrypted with a random key). Likewise, the new disk will not 
                    536: have a valid label or partitions, and `swapctl` will complain about 
                    537: configuring swap devices not marked as such in a disklabel.
                    539: In order to automate the process of labeling the disk, prepare an appropriate 
                    540: disklabel and save it to a file, for example `/etc/cgd/wd0b.disklabel`. Please 
                    541: refer to 
                    542: [disklabel(8)]( 
                    543: for information about how to use `disklabel` to set up a swap partition.
                    545: On each reboot, to restore this saved label to the new `cgd`, create the 
                    546: `/etc/rc.conf.d/cgd` file as below:
                    548:     swap_device="cgd1"
                    549:     swap_disklabel="/etc/cgd/wd0b.disklabel"
                    550:     start_postcmd="cgd_swap"
                    552:     cgd_swap()
                    553:     {
                    554:         if [ -f $swap_disklabel ]; then
                    555:             disklabel -R -r $swap_device $swap_disklabel
                    556:         fi
                    557:     }
                    559: The same technique could be extended to encompass using `newfs` to re-create 
                    560: an `ffs` filesystem for `/tmp` if you didn't want to use `mfs`.
                    562: ### Warnings
                    564: Prevent cryptographic disasters by making sure you can always recover your 
                    565: passphrase and parameters file. Protect the parameters file from disclosure, 
                    566: perhaps by storing it on removable media as above, because the salt it contains 
                    567: helps protect against dictionary attacks on the passphrase.
                    569: Keeping the data encrypted on your disk is all very well, but what about other 
                    570: copies? You already have at least one other such copy (the backup we used during 
                    571: this setup), and it's not encrypted. Piping `dump` through file-based 
1.2     ! jdf       572: encryption tools like `gpg` can be one way of addressing this issue, but make 
1.1       jdf       573: sure you have all the keys and tools you need to decrypt it to `restore` after 
                    574: a disaster.
                    576: Like any form of software encryption, the `cgd` key stays in kernel memory while 
                    577: the device is configured, and may be accessible to privileged programs and 
                    578: users, such as `/dev/kmem` grovellers. Taking other system security steps, such 
                    579: as running with elevated securelevel, is highly recommended.
                    581: Once the `cgd` volumes are mounted as normal filesystems, their contents are 
                    582: accessible like any other file. Take care of file permissions and ensure your 
                    583: running system is protected against application and network security attack.
                    585: Avoid using suspend/resume, especially for laptops with a BIOS suspend-to-disk 
                    586: function. If an attacker can resume your laptop with the key still in memory, or 
                    587: read it from the suspend-to-disk memory image on the hard disk later, the whole 
                    588: point of using `cgd` is lost.
                    590: ## Further Reading
                    592: The following resources contain more information on CGD:
                    594: ### Bibliography
1.2     ! jdf       596:  * [smackie-cgd]: *[NetBSD CGD Setup](*. Stuart Mackie.
        !           597:  * [nycbug-cgd]: *[I want my cgd]( aka: I want an encrypted pseudo-device on my laptop*.
        !           598:  * [elric-cgd]: *The original paper on [The CryptoGraphic Disk Driver](*. Roland Dowdeswell and John Ioannidis.
        !           599:  * [biancuzzi-cgd]: *[Inside NetBSD's CGD]( - an interview with CGD creator Roland Dowdeswell*. Biancuzzi Federico.
        !           600:  * [hubertf-cgd]: *[CryptoGraphicFile (CGF)](, or how to keep sensitive data on your laptop*. Feyrer Hubert.
1.1       jdf       601: 

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