fast 386 DES code figures

[Phil Karn <karn@unix.ka9q.ampr.org>]

To see if software DES could really be made acceptable in a IP
security protocol, I've been bumming cycles out of my old DES code.

I've completely translated the encrypt and decrypt routines to
assembler, with no calls or jumps inside either routine.  I picked up
Richard Outerbridge's seriously clever initial and final permutation
algorithm from Schneier, along with a few of his other tricks.

The bottom line: about 38,373 encryptions/sec (2.456 megabits/sec) on
a 50 Mhz Intel 486 running in 16-bit real mode. This includes the
overhead of the C loop that calls the encrypt function and prints a
status line every 10,000 loops.

The code would probably run faster if assembled and run in 32-bit
native mode, as this would eliminate a lot of 1-clock operand size
prefixes (I do many 32-bit operations).

Oh, by the way, if I eliminate the permutations the speed goes up to
about 42,986 encryptions/sec (2.751 megabits/sec), an increase of
about 12%. That says I should be able to do triple-DES at about 13,777
blocks/sec (881.7 kbit/sec) although I haven't tried it yet.

What still bugs me is that Schneier lists the speed of one commercial
DES implementation as 40,600 encryptions/sec on a 33 Mhz 486.  I just
don't see how that's possible without using a lot more memory for
lookup table space (I use only 2K, which is nice in a DOS
environment).

In any event, this should be enough for a T1 link (half duplex) as
long as too many cycles aren't needed for things like routing
packets. :-)

Phil