# I'm positive there's a better way to do this, but it works for a quick
# example.

require "bigint.pl";

# X^A mod Y, X^B mod Y
# A is Alice's secret, B is Bob's secret
# N1 is sent from Alice to Bob, N2 is sent from Bob to Alice
# X and Y can be transferred in cleartext if necessary
# Resulting K1 and K2 are the same, can be used as a crypto key
#
$X=123; $Y=711;
($A,$B) = (103,107);

$N1 = DH($X,$A,$Y);
$N2 = DH($X,$B,$Y);

$K1 = DH($N2,$A,$Y);
$K2 = DH($N1,$B,$Y);

print "$X^$A % $Y = $N1\n";
print "$X^$B % $Y = $N2\n";

print "\n";

print "$N2^$A % $Y = $K1\n";
print "$N1^$B % $Y = $K2\n";

sub DH {
	my($X,$A,$Y) = @_;

	my $R = $X;
	for ( my $i = $A; $i > 1; $i-- ) {
		$R = bmul($R,$X);
	}

	return bmod($R,$Y);
}