CGD(4)                  NetBSD Kernel Interfaces Manual                 CGD(4)

     cgd -- cryptographic disk driver

     pseudo-device cgd

     The cgd driver provides the capability of encrypting blocks on their way
     to and from a disk or partition.

     In order to compile support for the cgd into your kernel, you must add
     the driver to your kernel configuration file.  To do this, add a line
     similar to:

           pseudo-device   cgd     # cryptographic disk driver

     The cgd devices are allocated as needed.

   Encryption Algorithms
     Currently the following cryptographic algorithms are supported:

     aes-cbc        AES in CBC mode.  AES uses a 128 bit blocksize and can
                    accept keys of length 128, 192, or 256.  The default key
                    length is 128.

     aes-xts        AES in XTS mode.  AES-XTS uses a 128 bit blocksize and can
                    accept keys of length 256 or 512.  Note that an AES-XTS
                    key consists of two AES keys of equal size.  The second
                    key is used solely to encrypt the block number of the
                    physical disk block.  The default key length is 256.

     3des-cbc       Triple DES in CBC mode.  Triple DES uses a 64 bit block-
                    size and is performed in EDE3 mode with a 168 bit key.
                    The key passed to the kernel is 192 bits but the parity
                    bits are ignored.

     blowfish-cbc   Blowfish in CBC mode.  Blowfish uses a 64 bit blocksize
                    and can accept keys between 40 and 448 bits in multiples
                    of 8.  It is strongly encouraged that keys be at least 128
                    bits long.  There are no performance advantages of using
                    shorter keys.  The default key length is 128 bits.

   IV Methods
     Currently, the following IV Methods are supported:

     encblkno1  This method  encrypts the block number of the physical disk
                block once with the cipher and key provided and uses the
                result as the IV for CBC mode.  This method should ensure that
                each block has a different IV and that the IV is reasonably
                unpredictable.  This is the default method used by
                cgdconfig(8) when configuring a new cgd.

     encblkno8  This is the original IV method used by cgd and provided for
                backward compatibility.  It repeatedly encrypts the block num-
                ber of the physical disk block eight times and uses the result
                as the IV for CBC mode.  This method should ensure that each
                block has a different IV and that the IV is reasonably unpre-
                dictable.  The eightfold encryption was not intended and
                causes a notable performance loss with little (if any)
                increase in security over a single encryption.

     A cgd responds to all of the standard disk ioctl(2) calls defined in
     sd(4), and also defines the following:

           CGDIOCSET  Configure the cgd.  This ioctl(2) sets up the encryption
                      parameters and points the cgd at the underlying disk.

           CGDIOCCLR  Unconfigure the cgd.

           CGDIOCGET  Get info about the cgd.

     These ioctl(2)'s and their associated data structures are defined in
     <dev/cgdvar.h> header.

     It goes without saying that if you forget the passphrase that you used to
     configure a cgd, then you have irrevocably lost all of the data on the
     disk.  Please ensure that you are using an appropriate backup strategy.

     A cgd device doesn't authenticate data and thus it can't guarantee
     integrity of the encrypted data.  In particular, if the plaintext is
     known to an adversary, it is possible to change every second block on a
     disk encrypted in the CBC mode to plaintext blocks of their choice.  The
     XTS mode isn't vulnerable to this particular attack but a lack of
     integrity should be taken into account when evaluating security risks.

     /dev/{,r}cgd*      cgd device special files.

     config(1), ioctl(2), sd(4), cgdconfig(8), MAKEDEV(8)

     Roland C. Dowdeswell and John Ioannidis, "The CryptoGraphic Disk Driver",
     Proceedings of the FREENIX Track: 2003 USENIX Annual Technical
     Conference, USENIX Association,,
     179-186, June 9-14, 2003.

     The cgd driver was written by Roland C. Dowdeswell for NetBSD.  The cgd
     driver originally appeared in NetBSD 2.0.

NetBSD 8.1                      August 31, 2018                     NetBSD 8.1

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