
                    Scatterlist Cryptographic API
                   
INTRODUCTION

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.


DETAILS

At the lowest level are algorithms, which register dynamically with the
API.

'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.

An asynchronous scheduling interface is in planning but not yet
implemented, as we need to further analyze the requirements of all of
the possible hardware scenarios (e.g. IPsec NIC offload).

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)
		fail();
		
	/* ... set up the scatterlists ... */
	
	crypto_digest_init(tfm);
	crypto_digest_update(tfm, &sg, 2);
	crypto_digest_final(tfm, result);
	
	crypto_free_tfm(tfm);

    
Many real examples are available in the regression test module (tcrypt.c).


CONFIGURATION NOTES

As Triple DES is part of the DES module, for those using modular builds,
add the following line to /etc/modules.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


DEVELOPER NOTES

None of this code should be called from hardirq context, only 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.


ADDING NEW ALGORITHMS

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.


BUGS

Send bug reports to:
James Morris <jmorris@intercode.com.au>
Cc: David S. Miller <davem@redhat.com>


FURTHER INFORMATION

For further patches and various updates, including the current TODO
list, see:
http://samba.org/~jamesm/crypto/

Ongoing development discussion may also be found on
kerneli cryptoapi-devel, 
see http://www.kerneli.org/mailman/listinfo/cryptoapi-devel


AUTHORS

James Morris
David S. Miller
Jean-Francois Dive (SHA1 algorithm module)


CREDITS

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 (http://www.kerneli.org/)
    Alexander Kjeldaas
    Herbert Valerio Riedel
    Kyle McMartin
    Jean-Luc Cooke
    David Bryson
    Clemens Fruhwirth
    Tobias Ringstrom
    Harald Welte

and;
  
  Nettle (http://www.lysator.liu.se/~nisse/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)
  Kazunori Miyazawa / USAGI (HMAC)
  Matthew Skala (Twofish)
  Dag Arne Osvik (Serpent)
  
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 algorithm contributors:
  Andrew McDonald

Please send any credits updates or corrections to:
James Morris <jmorris@intercode.com.au>

