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Scatterlist Cryptographic API
The Scatterlist Crypto API takes page vectors (scatterlists) as
arguments, and works directly on pages. In some cases (e.g. ECB
mode ciphers), this will allow for pages to be encrypted in-place
with no copying.
One of the initial goals of this design was to readily support IPsec,
so that processing can be applied to paged skb's without the need
for linearization.
At the lowest level are algorithms, which register dynamically with the
'Transforms' are user-instantiated objects, which maintain state, handle all
of the implementation logic (e.g. manipulating page vectors), provide an
abstraction to the underlying algorithms, and handle common logical
operations (e.g. cipher modes, HMAC for digests). However, at the user
level they are very simple.
Conceptually, the API layering looks like this:
[transform api] (user interface)
[transform ops] (per-type logic glue e.g. cipher.c, digest.c)
[algorithm api] (for registering algorithms)
The idea is to make the user interface and algorithm registration API
very simple, while hiding the core logic from both. Many good ideas
from existing APIs such as Cryptoapi and Nettle have been adapted for this.
The API currently supports three types of transforms: Ciphers, Digests and
Compressors. The compression algorithms especially seem to be performing
very well so far.
Support for hardware crypto devices via an asynchronous interface is
under development.
Here's an example of how to use the API:
#include <linux/crypto.h>
struct scatterlist sg[2];
char result[128];
struct crypto_tfm *tfm;
tfm = crypto_alloc_tfm("md5", 0);
if (tfm == NULL)
/* ... set up the scatterlists ... */
crypto_digest_update(tfm, &sg, 2);
crypto_digest_final(tfm, result);
Many real examples are available in the regression test module (tcrypt.c).
As Triple DES is part of the DES module, for those using modular builds,
add the following line to /etc/modprobe.conf:
alias des3_ede des
The Null algorithms reside in the crypto_null module, so these lines
should also be added:
alias cipher_null crypto_null
alias digest_null crypto_null
alias compress_null crypto_null
The SHA384 algorithm shares code within the SHA512 module, so you'll
also need:
alias sha384 sha512
Transforms may only be allocated in user context, and cryptographic
methods may only be called from softirq and user contexts.
When using the API for ciphers, performance will be optimal if each
scatterlist contains data which is a multiple of the cipher's block
size (typically 8 bytes). This prevents having to do any copying
across non-aligned page fragment boundaries.
When submitting a new algorithm for inclusion, a mandatory requirement
is that at least a few test vectors from known sources (preferably
standards) be included.
Converting existing well known code is preferred, as it is more likely
to have been reviewed and widely tested. If submitting code from LGPL
sources, please consider changing the license to GPL (see section 3 of
the LGPL).
Algorithms submitted must also be generally patent-free (e.g. IDEA
will not be included in the mainline until around 2011), and be based
on a recognized standard and/or have been subjected to appropriate
peer review.
Also check for any RFCs which may relate to the use of specific algorithms,
as well as general application notes such as RFC2451 ("The ESP CBC-Mode
Cipher Algorithms").
It's a good idea to avoid using lots of macros and use inlined functions
instead, as gcc does a good job with inlining, while excessive use of
macros can cause compilation problems on some platforms.
Also check the TODO list at the web site listed below to see what people
might already be working on.
Send bug reports to:
James Morris <>
Cc: David S. Miller <>
For further patches and various updates, including the current TODO
list, see:
James Morris
David S. Miller
The following people provided invaluable feedback during the development
of the API:
Alexey Kuznetzov
Rusty Russell
Herbert Valerio Riedel
Jeff Garzik
Michael Richardson
Andrew Morton
Ingo Oeser
Christoph Hellwig
Portions of this API were derived from the following projects:
Kerneli Cryptoapi (
Alexander Kjeldaas
Herbert Valerio Riedel
Kyle McMartin
Jean-Luc Cooke
David Bryson
Clemens Fruhwirth
Tobias Ringstrom
Harald Welte
Nettle (
Niels Möller
Original developers of the crypto algorithms:
Dana L. How (DES)
Andrew Tridgell and Steve French (MD4)
Colin Plumb (MD5)
Steve Reid (SHA1)
Jean-Luc Cooke (SHA256, SHA384, SHA512)
Kazunori Miyazawa / USAGI (HMAC)
Matthew Skala (Twofish)
Dag Arne Osvik (Serpent)
Brian Gladman (AES)
Kartikey Mahendra Bhatt (CAST6)
Jon Oberheide (ARC4)
Jouni Malinen (Michael MIC)
SHA1 algorithm contributors:
Jean-Francois Dive
DES algorithm contributors:
Raimar Falke
Gisle Sælensminde
Niels Möller
Blowfish algorithm contributors:
Herbert Valerio Riedel
Kyle McMartin
Twofish algorithm contributors:
Werner Koch
Marc Mutz
SHA256/384/512 algorithm contributors:
Andrew McDonald
Kyle McMartin
Herbert Valerio Riedel
AES algorithm contributors:
Alexander Kjeldaas
Herbert Valerio Riedel
Kyle McMartin
Adam J. Richter
Fruhwirth Clemens (i586)
Linus Torvalds (i586)
CAST5 algorithm contributors:
Kartikey Mahendra Bhatt (original developers unknown, FSF copyright).
TEA/XTEA algorithm contributors:
Aaron Grothe
Khazad algorithm contributors:
Aaron Grothe
Whirlpool algorithm contributors:
Aaron Grothe
Jean-Luc Cooke
Anubis algorithm contributors:
Aaron Grothe
Tiger algorithm contributors:
Aaron Grothe
Generic scatterwalk code by Adam J. Richter <>
Please send any credits updates or corrections to:
James Morris <>