gistfile1.pl

#! /usr/bin/perl -w

use Crypt::Rijndael;
use Digest::SHA;

my $cleartext = "Here is some data for the coding"; # 32 bytes
my $iv = 'a2xhcgAAAAAAAAAA';

my $hash_1 = Digest::SHA->new(256);  # SHA256
$hash_1->add("Nixnogen");
my $cryptkey = $hash_1->digest;

sub encode { 
    my ($cryptkey, $iv, $cleartext) = @_;
    my $cipher = new Crypt::Rijndael $cryptkey, Crypt::Rijndael::MODE_CBC;

    $cipher->set_iv($iv); # we assume this will change for each cookie anyway.
    my $encrypted = $cipher->encrypt($cleartext);
    return $encrypted;
}   

sub decode {
    my ($cryptkey, $iv, $secretdata) = @_;
    my $cipher = new Crypt::Rijndael $cryptkey, Crypt::Rijndael::MODE_CBC;
    $cipher->set_iv($iv);
    my $decrypted = $cipher->decrypt($secretdata);
    return $decrypted;
}   

sub flipLastByte {
    my $original = shift;
    my @bytes = unpack("C*", $original);
    $bytes[-1] ^= 0xff;
    return pack("C*", @bytes);
}

my $enc = encode($cryptkey, $iv, $cleartext);
my $dec = decode($cryptkey, $iv, $enc);

my $encFlipped = flipLastByte($enc);
my $decFlipped = decode($cryptkey, $iv, $encFlipped);

print "Encoded length " . length($enc) . "\n";
print "Encoded (original): " . unpack("H*", $enc) . "\n";
print "Decode original 32 bytes: $dec\n";

print "Encoded (flipped) : " . unpack("H*", $encFlipped) . "\n";
print "Decode flipped  32 bytes: $decFlipped\n";