#!/usr/bin/env perl

use strict;
use Crypt::Rijndael;
use MIME::Base64;

my $string = 'QeTEv2804';

sub encrypt {
  my ($plaintext) = @_;
  my $password = "uNsY3WSs0hTd";
  my $trail = 16 - (length($plaintext) % 16 );
  $plaintext .= "\0" x $trail;
  my $cipher = Crypt::Rijndael->new(pack("A32",$password),Crypt::Rijnd
+ael::MODE_CBC());
  $cipher->set_iv(pack("A16",$password));
  my $crypted = $cipher->encrypt($plaintext);
  my $encoded = encode_base64($crypted);
  return $encoded;
}

sub decrypt {
  my ($ciphertext) = @_;
  my $password = "uNsY3WSs0hTd";
  my $cipher_nonbase64 = decode_base64($ciphertext);
  my $cipher = Crypt::Rijndael->new(pack("A32",$password),Crypt::Rijnd
+ael::MODE_CBC());
  $cipher->set_iv(pack("A16",$password));
  my $plaintext = $cipher->decrypt($cipher_nonbase64);
  return $plaintext;
}

my $enc = &encrypt($string);
my $dec = &decrypt("$enc");
print "Decrypted --> $dec <--\n";
[download]

Thanks for posting the code, it confirms GrandFather's suggestion; using my advice from here to use Devel::Peek to inspect $dec, it shows:

SV = PV(0x571d5fd57c40) at 0x560d5ed7d9a0
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x571d5fe15180 "QeTEv2804\0\0\0\0\0\0\0"\0
  CUR = 16
  LEN = 18
  COW_REFCNT = 0
[download]

As per the Crypt::Rijndael docs, the blocksize for Rijndael is 16 bytes.

encrypt() seems to NUL pad the string to the required block size:

  $plaintext .= "\0" x $trail;

That's where your NULs are coming from. If you know that trailing \0s are not ever valid, you can strip them right before your return in decrypt():

$plaintext =~ s/\0+$//;

On the other hand, if trailing NUL are a legitimate possibility, you will have to come up with a different plan, such as encoding the real length within your plaintext in encrypt() so you can trim it in the decrypt() sub.

Ok that's it.
Thanks a lot!