The rand() function in most C libraries is small and fast, but not particularly random. Below is a perl script that can take a few numbers from rand() and use them to predict the rest of the sequence, if rand() is a linear congruential generator of some sort. Actually, it isn't guaranteed to find the solution, but it usually does. It's based on the L3 lattice basis reduction algorithm. For more information, take a look at this introductory paper.

The script needs to know the parameters of your particular random number generator. I've included information for drand48() (which is the default in Linux) and the rand() function in ActiveState perl for Windows. It's generally not too hard to reverse engineer an unknown RNG by feeding chosen values into srand() and looking at the first couple of outputs.

$n is the number of random output bytes that are used to solve for the generator's internal state. With drand48(), it takes at least 7 bytes. That's just slightly inefficient in terms of information, since the internal state of the generator is 6 bytes long.

In order to run this, you'll need a recent version of Math::BigInt, Math::BigRat, and Math::BigInt::Pari or Math::BigInt::GMP. The running time increases as $n**6 (with a big multiplicative constant), so you'll need a reasonably fast BigInt library. On my machine, it takes 30-40 seconds with $n==7.

use strict;
use warnings;
use Math::BigInt lib => "Pari,GMP";
use Math::BigRat;
# Random number generator parameters
# $x = ($a*$x + $b) % $m
# These need to match the RNG your perl binary was compiled with.
# Run this one-liner and check to see if the output matches one
# of the signatures below.
# srand 1;@x=map int(256*rand),1..10;print "@x\n"
# drand48() -- the default on Linux
# Signature: 10 116 213 86 144 0 48 253 192 93
my $a = Math::BigInt->new("0x5DEECE66D");
my $b = Math::BigInt->new("0xB");
my $m = Math::BigInt->new(1) << 48;
my $n = 7; # number of input bytes needed
# ActiveState perl for windows
# Signature: 0 144 49 207 149 122 89 229 210 191
#my $a = Math::BigInt->new("0x343FD");
#my $b = Math::BigInt->new("0x269EC3");
#my $m = Math::BigInt->new(1) << 31;
#my $n = 4; # number of bytes needed
my $one_half = Math::BigRat->new("1/2");
my $three_fourths = Math::BigRat->new("3/4");
my @y = map int(256*rand), 1..$n;
print "rand = @y\n";
my @mat = map [(0) x ($n+1)], 0..$n;
my $k = $m >> 8;
$mat[0][0] = $k;
$mat[1][1] = 1;
foreach (2..$n) {
$mat[0][$_] = ($mat[0][$_-1] * $a + $b) % $m;
$mat[1][$_] = ($mat[1][$_-1] * $a) % $m;
$mat[$_][$_] = $m;
}
$mat[0][$_] -= $y[$_-1] * $k for 1..$n;
l3_reduce(\@mat);
CHECK: foreach my $row (@mat) {
if ($row->[0] < 0) { $_ = -$_ for @$row }
next CHECK unless $row->[0] == $k;
foreach (1..$n) {
next CHECK unless $row->[$_] >= 0 && $row->[$_] < $k;
}
my $x = $y[$n-1]*$k + $row->[$n];
my @p;
foreach (1..10) {
$x = ($a*$x + $b) % $m;
push @p, $x / $k;
}
print "prediction = @p\n";
}
my @z = map int(256*rand), 1..10;
print "rand = @z\n";
# l3 lattice basis reduction algorithm
# @$m is an array of lattice basis vectors
# $m is reduced in-place (the original is destroyed)
sub l3_reduce {
my ($m) = @_;
my $n = @$m;
my (@m2, @mag, @coeff);
foreach my $i (0..$n-1) {
my @v = map Math::BigRat->new($_), @{$m->[$i]};
foreach my $j (0..$i-1) {
my $w = $m2[$j];
my $c = dot_product(\@v, $w) / $mag[$j];
$coeff[$i][$j] = $c;
foreach my $k (0..$#v) {
$v[$k] -= $c * $w->[$k];
}
}
$m2[$i] = \@v;
$mag[$i] = dot_product(\@v, \@v);
}
undef @m2;
my $i = 1;
while ($i < $n) {
reduce_one($m, \@coeff, $i, $i-1);
my $u = $coeff[$i][$i-1];
my $u2 = $u*$u;
if (rcmp($mag[$i], ($three_fourths - $u2) * $mag[$i-1]) < 0) {
my $t = $mag[$i] + $u2*$mag[$i-1];
my $s = $mag[$i-1] / $t;
$mag[$i-1] = $t;
$mag[$i] *= $s;
$coeff[$i][$i-1] *= $s;
@$m[$i, $i-1] = @$m[$i-1, $i];
foreach my $j (0..$i-2) {
my $t = $coeff[$i][$j];
$coeff[$i][$j] = $coeff[$i-1][$j];
$coeff[$i-1][$j] = $t;
}
foreach my $j ($i+1..$n-1) {
my $t = $coeff[$j][$i];
$coeff[$j][$i] = $coeff[$j][$i-1] - $u*$t;
$coeff[$j][$i-1] = $t + $coeff[$i][$i-1]*$coeff[$j][$i];
}
$i-- if $i > 1;
}
else {
foreach my $j (reverse 0..$i-2) {
reduce_one($m, \@coeff, $i, $j);
}
$i++;
}
}
return $m;
} # l3_reduce
sub reduce_one {
my ($m, $coeff, $i, $j) = @_;
if (rcmp(abs($coeff->[$i][$j]), $one_half) > 0) {
my $c = $coeff->[$i][$j] + $one_half;
my $d = $c->numerator() / $c->denominator();
my $r = Math::BigRat->new($d);
if (rcmp($r, $c) > 0) {
--$d;
$r = Math::BigRat->new($d);
}
my ($u, $v) = @$m[$i, $j];
foreach my $k (0..$#$u) {
$u->[$k] -= $d * $v->[$k];
}
foreach my $k (0..$j-1) {
$coeff->[$i][$k] -= $r * $coeff->[$j][$k];
}
$coeff->[$i][$j] -= $r;
}
} # reduce_one
sub dot_product {
my ($x, $y) = @_;
my $sum = $x->[0] * $y->[0];
foreach my $i (1..$#$x) {
$sum += $x->[$i] * $y->[$i];
}
return $sum;
} # dot_product
# comparison not implemented in current Math::BigRat
sub rcmp {
my ($r, $s) = @_;
my $t = $r->numerator() * $s->denominator();
my $u = $s->numerator() * $r->denominator();
return $t->bcmp($u);
} # rcmp