lopsided Feistel (was: cryptographic ergodic sequence generators)

[Tim Dierks]

At 08:28 PM 9/6/2003, John S. Denker wrote:
>On 09/06/2003 02:33 PM, Tim Dierks wrote:
> > I'm sure that it would be possible to design a Feistel-based block
> > cipher with variable block size, supporting some range of even values
> > of n.
>
>There's no need to exclude odd n.

Of course, I'd forgotten about unbalanced Feistel networks.

Here's an updated script:

use Crypt::DES;

$n = shift @ARGV;
if (!defined($n) || $n < 2 || $n > 32 || $#ARGV > 0) {
     die "Usage: $0 n\n2 <= n <= 32\n";
}

$key = pack("A8", rand());

$cipher = new Crypt::DES $key;

$tb = int(($n+1)/2);
$sb = $n - $tb;
$tmask = (1 << ($tb)) - 1;
$smask = (1 << ($sb)) - 1;

sub f($$) {
     my ($round, $v) = @_;

     my $pt = pack("LL", $round, $v);
     my $ct = $cipher->encrypt($pt);
     my ($high, $low) = unpack("LL", $ct);
     return $low & $tmask;
}

sub E($) {
     my ($p) = @_;
     my $L, $R, $Ln, $Rn, $round;

     for $round (1..2) {
         $L = $p >> $sb;
         $R = $p & $smask;
         $Ln = $R;
         $Rn = $L ^ f($r, $R);
         $p = ($Ln << $tb) | $Rn;
     }

     return $p;
}

foreach $v (0..(1<<$n)-1) {
     $o = E($v);
     print "$v => $o\n";
     if ($o >= 1<<$n) {
         die "Too big";
     }
     if ($retvals{$o}++) {
         die "Duplicate";
     }
}



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