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

NAME

cgd -- cryptographic disk driver

SYNOPSIS

pseudo-device cgd

DESCRIPTION

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.

IOCTLS
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.

WARNINGS

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.

FILES

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