File:  [NetBSD Developer Wiki] / wikisrc / guide / cgd.mdwn
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Wed Mar 6 23:40:25 2013 UTC (8 years, 6 months ago) by jdf
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Cosmetics.

    1: # The cryptographic device driver (CGD)
    2: 
    3: The [cgd(4)](http://netbsd.gw.com/cgi-bin/man-cgi?cgd+4+NetBSD-current) 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.
    7: 
    8: ## Overview
    9: 
   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.
   14: 
   15: ### Why use disk encryption?
   16: 
   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.
   24: 
   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.
   30: 
   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.
   36: 
   37: ### Logical Disk Drivers
   38: 
   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)](http://netbsd.gw.com/cgi-bin/man-cgi?cgdconfig+8+NetBSD-current), 
   44: to create and configure a `cgd` device and point it at the underlying disk 
   45: device that will hold the encrypted data.
   46: 
   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.
   51: 
   52: Once you have created a `cgd` disk, you can use 
   53: [disklabel(8)](http://netbsd.gw.com/cgi-bin/man-cgi?disklabel+8+NetBSD-current) 
   54: to divide it up into partitions, 
   55: [swapctl(8)](http://netbsd.gw.com/cgi-bin/man-cgi?swapctl+8+NetBSD-current) to 
   56: enable swapping to those partitions or 
   57: [newfs(8)](http://netbsd.gw.com/cgi-bin/man-cgi?newfs+8+NetBSD-current) to make 
   58: filesystems, then `mount` and use those filesystems, just like any other new 
   59: disk.
   60: 
   61: ## Components of the Crypto-Graphic Disk system
   62: 
   63: A number of components and tools work together to make the `cgd` system 
   64: effective.
   65: 
   66: ### Kernel driver pseudo-device
   67: 
   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:
   70: 
   71:     pseudo-device   cgd     4       # cryptographic disk driver
   72: 
   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.
   76: 
   77: ### Ciphers
   78: 
   79: The `cgd` driver provides the following encryption algorithms:
   80: 
   81:  * `aes-cbc` -- AES (Rijndael). AES uses a 128 bit blocksize and accepts 128, 
   82:    192 or 256 bit keys.
   83: 
   84:  * `blowfish-cbc` -- Blowfish uses a 64 bit blocksize and accepts 128 bit keys
   85: 
   86:  * `3des-cbc` -- Triple DES uses a 64 bit blocksize and accepts 192 bit keys 
   87:    (only 168 bits are actually used for encryption)
   88: 
   89: All three ciphers are used in [CBC (Cipher Block 
   90: Chaining)](http://en.wikipedia.org/wiki/Cipher_block_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.
   94: 
   95: ### Verification Methods
   96: 
   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:
  100: 
  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).
  107: 
  108:  * `disklabel` -- `cgdconfig` scans for a valid disklabel. If a valid disklabel 
  109:    is found with the key that is provided authentication will succeed.
  110: 
  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.
  113: 
  114: ## Example: encrypting your disk
  115: 
  116: This section works through a step-by-step example of converting an existing 
  117: system to use `cgd`, performing the following actions:
  118: 
  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
  124: 
  125: ### Preparing the disk
  126: 
  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.
  132: 
  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)](http://netbsd.gw.com/cgi-bin/man-cgi?dump+8+NetBSD-current). 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.
  140: 
  141: With the system at single user, `/` mounted read-write and everything else 
  142: unmounted, use 
  143: [disklabel(8)](http://netbsd.gw.com/cgi-bin/man-cgi?disklabel+8+NetBSD-current)
  144: to delete all the data partitions you want to move into `cgd`.
  145: 
  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.
  150: 
  151: ### Scrubbing the disk
  152: 
  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.
  159: 
  160: We could use 
  161: [dd(1)](http://netbsd.gw.com/cgi-bin/man-cgi?dd+1+NetBSD-current)
  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.
  167: 
  168: First, we configure a `cgd` to use a random key:
  169: 
  170:     # cgdconfig -s cgd0 /dev/wd0e aes-cbc 128 < /dev/urandom 
  171: 
  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):
  174: 
  175:     # dd if=/dev/zero of=/dev/rcgd0d bs=32k
  176: 
  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`:
  179: 
  180:     # cgdconfig -u cgd0
  181: 
  182: ### Creating the `cgd`
  183: 
  184: The 
  185: [cgdconfig(8)](http://netbsd.gw.com/cgi-bin/man-cgi?cgdconfig+8+NetBSD-current) 
  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!
  190: 
  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):
  193: 
  194:     # cgdconfig -g -V disklabel -o /etc/cgd/wd0e aes-cbc 256
  195: 
  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:
  199: 
  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:     };
  208: 
  209: *Note*: Remember, you'll want to save this file somewhere safe later.
  210: 
  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.
  217: 
  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.
  222: 
  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:
  226: 
  227:     # cgdconfig -V re-enter cgd0 /dev/wd0e
  228: 
  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.
  232: 
  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.
  236: 
  237: *Tip*: Remember to use the `disklabel -I` argument, because you're creating an 
  238: initial label for a new disk.
  239: 
  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.
  243: 
  244: Then, use 
  245: [newfs(8)](http://netbsd.gw.com/cgi-bin/man-cgi?newfs+8+NetBSD-current) to 
  246: create filesystems on all the relevant partitions. This time your partitions 
  247: will reflect the `cgd` disk names, for example:
  248: 
  249:     # newfs /dev/rcgd0h
  250: 
  251: ### Modifying configuration files
  252: 
  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:
  257: 
  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
  264: 
  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.
  268: 
  269: Each time you reboot, you're going to need your `cgd` configured early, before 
  270: [fsck(8)](http://netbsd.gw.com/cgi-bin/man-cgi?fsck+8+NetBSD-current) runs and 
  271: filesystems are mounted.
  272: 
  273: Put the following line in `/etc/cgd/cgd.conf`:
  274: 
  275:     cgd0    /dev/wd0e
  276: 
  277: This will use `/etc/cgd/wd0e` as config file for `cgd0`.
  278: 
  279: To finally enable cgd on each boot, put the following line into `/etc/rc.conf`:
  280: 
  281:     cgd=YES
  282: 
  283: You should now be prompted for `/dev/cgd0`'s passphrase whenever `/etc/rc` 
  284: starts.
  285: 
  286: ### Restoring data
  287: 
  288: Next, mount your new filesystems, and 
  289: [restore(8)](http://netbsd.gw.com/cgi-bin/man-cgi?restore+8+NetBSD-current) 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.
  293: 
  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.
  299: 
  300: ## Example: encrypted CDs/DVDs
  301: 
  302: ### Introduction
  303: 
  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.
  310: 
  311: ### Creating an encrypted CD/DVD
  312: 
  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`](http://ftp.NetBSD.org/pub/pkgsrc/current/pkgsrc/sysutils/cdrtools/README.html) 
  319: package. ISO images must *not* contain disklabels or cgd partitions.
  320: 
  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.
  325: 
  326: The basic steps when creating an encrypted CD are:
  327: 
  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
  334: 
  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).
  344: 
  345: Invoke all following commands as root!
  346: 
  347: For a CD:
  348: 
  349:     # dd if=/dev/zero of=image.img bs=1m count=700
  350: 
  351: or, for a DVD:
  352: 
  353:     # dd if=/dev/zero of=image.img bs=1m count=4482
  354: 
  355: Now configure a 
  356: [vnd(4)](http://netbsd.gw.com/cgi-bin/man-cgi?vnd+4+NetBSD-5.0.1+i386)-pseudo 
  357: disk with the image:
  358: 
  359:     # vnconfig vnd0 image.img
  360: 
  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).
  366: 
  367: I'll use AES-CBC with a keylength of 256 bits. Refer to 
  368: [cgd(4)](http://netbsd.gw.com/cgi-bin/man-cgi?cgd+4+NetBSD-5.0.1+i386) and 
  369: [cgdconfig(8)](http://netbsd.gw.com/cgi-bin/man-cgi?cgdconfig+8+NetBSD-5.0.1+i386) 
  370: for details and alternatives.
  371: 
  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).
  379: 
  380:     # cgdconfig -g -V disklabel aes-cbc 256 > /etc/cgd/image
  381: 
  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):
  386: 
  387:     # cgdconfig -V re-enter cgd1 /dev/vnd0d /etc/cgd/image
  388: 
  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.
  392: 
  393: Now it's time to create a disklabel inside the cgd. The defaults of the label 
  394: are ok, so invoking disklabel with
  395: 
  396:     # disklabel -e -I cgd1
  397: 
  398: and leaving vi with `:wq` immediately will do.
  399: 
  400: Let's create a filesystem on the cgd, and finally mount it somewhere:
  401: 
  402:     # newfs /dev/rcgd1a
  403:     # mount /dev/cgd1a /mnt
  404: 
  405: The cgd is alive! Now fill `/mnt` with content. When finished, reverse the 
  406: configuration process. The steps are:
  407: 
  408: 1.  Unmounting the cgd1a:
  409: 
  410:         # umount /mnt
  411: 
  412: 2.  Unconfiguring the cgd:
  413: 
  414:         # cgdconfig -u cgd1
  415: 
  416: 3.  Unconfiguring the vnd:
  417: 
  418:         # vnconfig -u vnd0
  419: 
  420: 
  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`](http://ftp.NetBSD.org/pub/pkgsrc/current/pkgsrc/sysutils/dvd+rw-tools/README.html) 
  425: package. Again, use `c` instead of `d` if this is the raw partition on your 
  426: platform.
  427: 
  428: Finally, write the image file to a CD:
  429: 
  430:     # cdrecord dev=/dev/rcd0d -v image.img
  431: 
  432: ...or to a DVD:
  433: 
  434:     # growisofs -dvd-compat -Z /dev/rcd0d=image.img
  435: 
  436: Congratulations! You've just created a really secure CD!
  437: 
  438: ### Using an encrypted CD/DVD
  439: 
  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.
  443: 
  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.
  447: 
  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:
  451: 
  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
  471:     
  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+)
  476: 
  477: Now as the disklabel is in 512 b/s format, accessing the CD is as easy as:
  478: 
  479:     # cgdconfig cgd1 /dev/cd0d /etc/cgd/image
  480:     # mount -o ro /dev/cgd1a /mnt
  481: 
  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!
  485: 
  486: Now we're done! Enjoy your secure CD!
  487: 
  488:     # ls /mnt
  489: 
  490: Remember you have to reverse all steps to remove the CD:
  491: 
  492:     # umount /mnt
  493:     # cgdconfig -u cgd1
  494:     # eject cd0
  495: 
  496: ## Suggestions and Warnings
  497: 
  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.
  503: 
  504:  * Use multiple `cgd`'s for different kinds of data, one mounted all the time 
  505:    and others mounted only when needed.
  506: 
  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]].
  511: 
  512: ### Using a random-key cgd for swap
  513: 
  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.
  518: 
  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.
  522: 
  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.
  526: 
  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`:
  530: 
  531:     # cgdconfig -g -o /etc/cgd/wd0b -V none -k randomkey blowfish-cbc
  532: 
  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.
  538: 
  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)](http://netbsd.gw.com/cgi-bin/man-cgi?disklabel+8+NetBSD-5.0.1+i386) 
  543: for information about how to use `disklabel` to set up a swap partition.
  544: 
  545: On each reboot, to restore this saved label to the new `cgd`, create the 
  546: `/etc/rc.conf.d/cgd` file as below:
  547: 
  548:     swap_device="cgd1"
  549:     swap_disklabel="/etc/cgd/wd0b.disklabel"
  550:     start_postcmd="cgd_swap"
  551:     
  552:     cgd_swap()
  553:     {
  554:         if [ -f $swap_disklabel ]; then
  555:             disklabel -R -r $swap_device $swap_disklabel
  556:         fi
  557:     }
  558: 
  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`.
  561: 
  562: ### Warnings
  563: 
  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.
  568: 
  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 
  572: encryption tools like `gpg` can be one way of addressing this issue, but make 
  573: sure you have all the keys and tools you need to decrypt it to `restore` after 
  574: a disaster.
  575: 
  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.
  580: 
  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.
  584: 
  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.
  589: 
  590: ## Further Reading
  591: 
  592: The following resources contain more information on CGD:
  593: 
  594: ### Bibliography
  595: 
  596:  * [smackie-cgd]: *[NetBSD CGD Setup](http://www.bsdguides.org/guides/netbsd/misc/cgd_setup.php)*. Stuart Mackie.
  597:  * [nycbug-cgd]: *[I want my cgd](http://genoverly.com/articles/view/5/) aka: I want an encrypted pseudo-device on my laptop*.
  598:  * [elric-cgd]: *The original paper on [The CryptoGraphic Disk Driver](http://www.imrryr.org/~elric/cgd/cgd.pdf)*. Roland Dowdeswell and John Ioannidis.
  599:  * [biancuzzi-cgd]: *[Inside NetBSD's CGD](http://onlamp.com/pub/a/bsd/2005/12/21/netbsd_cgd.html) - an interview with CGD creator Roland Dowdeswell*. Biancuzzi Federico.
  600:  * [hubertf-cgd]: *[CryptoGraphicFile (CGF)](http://www.feyrer.de/NetBSD/blog.html/nb_20060823_2311.html), or how to keep sensitive data on your laptop*. Feyrer Hubert.
  601: 

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