Re: 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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