[53209] in Cypherpunks
Video retraces as a source of entropy...
daemon@ATHENA.MIT.EDU (Deranged Mutant)
Wed Apr 3 21:24:09 1996
From: "Deranged Mutant" <WlkngOwl@UNiX.asb.com>
To: cypherpunks@toad.com
Date: Wed, 3 Apr 1996 20:45:59 +0000
Quickie blurb on using video card retraces as a source of entropy...
I've done some brief testing in the last couple of days on using the
timing drift between video retrace events as a source of randomness.
It seems comparable to truerand() spinners that check the system's
timer ticks [which makes me leary of relying on it since it
has similar strange attractors when plotted in a noise sphere, but
that's another post...].
Assuming one trusts truerand spinners, this method could have some
advantages over a 'pure software' method, since the video controller
(and other hardware controllers which could be adapted to this) runs
in 'parallel' [although it's liable to the samefluctuations in current or
memory-access and other interfaces with the main system, or possibly
tempest attacks for the paranoid...].
In pseudo-C:
int retrace(void) { // test for video retrace
#ifdef __MSDOS__
return (port[0x3da] & 8); // Some VGA, maybe EGA cards
#else
// your OS here
#endif
}
[..]
x = 0;
while (!retrace()) x++;
I've tested it as standalone routines (in Pascal and assembler) as well as
a hook to the DOS idle in the background [See note about strange
atteactors above]. It also seems to work while in Windows (but not
OS/2?!), which is an advantage over using the system's microsecond
timer alarm (which Win3 takes over).
Comments? (other than "truerand is an oxymoron"....)
Rob.
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