CryptoDev for Linux

Device /dev/crypto (aka CryptoDev) is a way for userspace processes to use cryptographic algorithms provided by kernel CryptoAPI modules. For example a process that needs to AES-encrypt some data can either:

Have the AES algorithm built in, or
Use an external library like OpenSSL that will do the encryption, or
Ask the kernel to do the encryption

Although in most cases using the external library for cryptographic tasks would be the best option, sometimes it may come handy to have a kernel interface for it as well. Using it one may create a really lightweight programs (see cryptodev-demo.c) supporting cryptography without any additional libraries.

The CryptoDev was originally introduced in OpenBSD (at least as far as I know :-), later ported to FreeBSD and on this page you can find my port for Linux 2.6. Because kernel internals differ a lot between BSD and Linux I only attempted to keep the API (i.e. the semantics used when talking with /dev/crypto) and wrote the driver itself from scratch.

Linux kernel patches

To enable /dev/crypto device patch your kernel with the following patch and configure with Cryptographic options -> Cryptodev (/dev/crypto) interface either built-in or as a module. If you are using udev the device /dev/crypto should be automatically created upon loading the module to the kernel.

Linux kernel 2.6.27+

cryptodev-20091126.tar.bz2 — 2023-10-27 16:02
Standalone module, doesn't require kernel patching, simply unpack and run make && make install.
Known to work with 2.6.32 and probably later kernels as well.
Thanks Nikos Mavrogiannopoulos for the port to 2.6.27

Linux kernel 2.6.8

cryptodev-2.6.8.diff — 2023-10-27 16:02, MD5, gpg signature

The module takes two parameters:

verbosity=: This is 0 by default, i.e. no messages about CryptoDev usage are printed. Use 1 if you are developing or using a program that accesses /dev/crypto and is having problems with it. This way some detailed error messages will be print if things go wrong. If you really want to have your log full of messages about CryptoDev usage, set verbosity to 2.
enable_stats=: The driver can collect statistics about CryptoDev usage. For now it can tell you how many bytes was encrypted or decrypted during each session, what size had the biggest request and what was the average size per request. By default this is 0, i.e. no statistics are collected. Set enable_stats=1 and verbosity=2 if you want to be getting the statistics described above.

The parameters can be changed even when the module is already loaded by writing to /sys/module/cryptodev/{enable_stats,verbosity}.

Userspace demo

The basic structure of a program using CryptoDev is following:

	fd = open("/dev/crypto");
	ioctl(fd, CIOCGSESSION);
	ioctl(fd, CIOCRYPT);
	[... repeat CIOCRYPT ioctls ...]
	ioctl(fd, CIOCFSESSION);
	close(fd);

On OpenBSD it is needed to clone the filedescriptor before actually using it. This is also supported on Linux, but not required:

	fd = open("/dev/crypto");
	ioctl(fd, CRIOGET, &fd_new);
	ioctl(fd_new, CIOCGSESSION);
	ioctl(fd_new, CIOCRYPT);
	[... repeat CIOCRYPT ioctls ...]
	ioctl(fd_new, CIOCFSESSION);
	close(fd_new);
	close(fd);

Indeed, there are some structures floating around, but the for the overview this is enough. See the following demo for details:

CryptoDev usage demo: cryptodev-demo1.c (Colorized)
CryptoDev with OpenSSL: cryptodev-demo2.c (Colorized)

CryptoDev support for OpenSSL

If you want to combine the (dis:-)advantages of using both OpenSSL and CryptoDev you may want to give the following patch a try:

openssl-0.9.8k-cryptodev.diff — 2023-10-27 16:02: Apply this patch to the original openssl-0.9.8k.tar.gz and recompile.
openssl-0.9.7d-cryptodev.diff — 2023-10-27 16:02, MD5, gpg signature: Apply this patch to the original openssl-0.9.7d.tar.gz and recompile.

Using CryptoAPI in the Linux kernel

(This is a bit off-topic here :-)
CryptoAPI is a Linux in-kernel infrastructure that offers cryptography to all other subsystems. It provides a single API for accessing different ciphers and digests.

Simple Linux kernel module that shows how the encryption and decryption can be used:: cryptoapi-demo.c (Colorized)

Useful CryptoAPI-related links:

CryptoAPI mailing list: Archive and subscription page
VIA PadLock support for Linux: Patches for Kernel and OpenSSL