CGD(4) | Device Drivers Manual | CGD(4)

# NAME

`cgd` —
cryptographic disk driver

# SYNOPSIS

`pseudo-device cgd`

# DESCRIPTION

The `cgd` driver, configured with the cgdconfig(8) tool, implements a logical disk device by encrypting or decrypting disk sectors on their way to and from a physical backing disk or partition.

## Security model

As long as you keep the key secret, `cgd` keeps the content of the disk secret from a passive adversary, such as a thief who steals your disk or a border patrol agent who detains you and takes a snapshot of your laptop's disk while you are crossing a border.

`cgd` does not detect tampering by an active adversary who can modify the content of the backing store, such as a man-in-the-middle between you and an iSCSI target, or after the border patrol returns your laptop to you.

## Ciphers

The following ciphers are supported:

`adiantum (key size: 256 bits)`
  The Adiantum tweakable wide-block cipher. The Adiantum tweak for each disk sector is taken to be the little-endian encoding of the disk sector number.

  Adiantum provides the best security by encrypting entire disk sectors at a time (512 bytes), and generally provides the best performance on machines without CPU support for accelerating AES.

`aes-cbc (key sizes: 128, 192, or 256 bits)`
  AES in CBC mode. The CBC initialization vector for each disk sector is chosen to be the encryption under AES of the little-endian encoding of the disk sector number. The default key length is 128 bits. CBC mode is expected to provide marginally better theoretical security than XTS mode.
`aes-xts (key sizes: 256 or 512 bits)`
  AES in XTS mode. The XTS tweak for each disk sector is chosen to be the little-endian encoding of the disk sector number. AES-XTS uses a 256-bit or 512-bit key, composed of a pair of AES-128 or AES-256 keys. The default key length is 256, meaning AES-128. XTS mode is expected to provide marginally better theoretical performance than CBC mode.

## Obsolete Ciphers

The following obsolete ciphers are supported for compatibility with old disks.

WARNING: These obsolete ciphers are implemented without timing side channel protection, so, for example, JavaScript code in a web browser that can measure the timing of disk activity may be able to recover the secret key. These are also based on 64-bit block ciphers and are therefore unsafe for disks much larger than a gigabyte. You should not use these except where compatibility with old disks is necessary.

`3des-cbc (key size: 192 bits)`
  3DES (Triple DES with EDE3) in CBC mode. The CBC initialization vector for each disk sector is chosen to be the encryption under 3DES of the little-endian encoding of the disk sector number.

  Note: Internally, the ‘parity bits’ of the 192-bit key are ignored, so there are only 168 bits of key material, and owing to generic attacks on 64-bit block ciphers and to meet-in-the-middle attacks on compositions of ciphers as in EDE3 the security is much lower than one might expect even for a 168-bit key.

`blowfish-cbc (key sizes: 40, 48, 56, 64, ..., 432, 440, or 448 bits)`
  Blowfish in CBC mode. The CBC initialization vector for each disk sector is chosen to be the encryption under Blowfish of the little-endian encoding of the disk sector number. 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

A very early version of `cgd` had a bug in the CBC-based ciphers `aes-cbc`, `3des-cbc`, and `blowfish-cbc`: the CBC initialization vector was chosen to be the eight-fold encryption under the block cipher of the little-endian encoding of the disk sector number, which has no impact on security but reduces performance. For compatibility with such disks, the ‘IV method’ must be set to `encblkno8`. Otherwise the ‘IV method’ should always be `encblkno1`. The parameter is meaningless for `adiantum` and `aes-xts`.

# 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.

# FILES

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

# SEE ALSO

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, https://www.usenix.org/event/usenix03/tech/freenix03/full_papers/dowdeswell/dowdeswell.pdf, 179-186, June 9-14, 2003.

Paul Crowley and Eric Biggers, Adiantum: length-preserving encryption for entry-level processors, International Association of Cryptologic Research, Transactions on Symmetric Cryptology, 4, 2018, https://doi.org/10.13154/tosc.v2018.i4.39-61, 39-61.

FIPS PUB 46-3: Data Encryption Standard (DES), National Institute of Standards and Technology, https://csrc.nist.gov/publications/detail/fips/46/3/archive/1999-10-25, United States Department of Commerce, October 25, 1999, withdrawn May 19, 2005.

FIPS PUB 197: Advanced Encryption Standard (AES), National Institute of Standards and Technology, https://csrc.nist.gov/publications/detail/fips/197/final, United States Department of Commerce, November 2001.

Morris Dworkin, Recommendation for Block Cipher Modes of Operation: Methods and Techniques, National Institute of Standards and Technology, https://csrc.nist.gov/publications/detail/sp/800-38a/final, United States Department of Commerce, December 2001, NIST Special Publication 800-38A.

Morris Dworkin, Recommendation for Block Cipher Modes of Operation: the XTS-AES Mode for Confidentiality on Storage Devices, National Institute of Standards and Technology, https://csrc.nist.gov/publications/detail/sp/800-38e/final, United States Department of Commerce, January 2010, NIST Special Publication 800-38E.

Bruce Schneier, The Blowfish Encryption Algorithm, https://www.schneier.com/academic/blowfish, superseded by Twofish, superseded by Threefish.

Karthikeyan Bhargavan and Gaëtan Leurent, Sweet32: Birthday attacks on 64-bit block ciphers in TLS and OpenVPN, https://sweet32.info.

# HISTORY

The `cgd` driver was written by Roland C. Dowdeswell for NetBSD. The `cgd` driver originally appeared in NetBSD 2.0. The `aes-xts` cipher was added in NetBSD 8.0. The `adiantum` cipher was added in NetBSD 10.0.

September 27, 2024 | NetBSD 10.1