Perl DES CBC encryption results in more bytes

Question

I am using Perl to perform CBC DES encryption using the Crypt::CBC library:

#!/usr/bin/perl
use Crypt::CBC;
$key  = "\x4A\x6F\xC2\x2A\x44\xE2\xA4\x48"; 
$iv   = "\x00\x00\x00\x00\x00\x00\x00\x00";
$data = "\x51\x55\x45\x53\x54\x49\x4F\x4E";

print "TXT->", $data, "\n";
print "HEX->", unpack("H*", $data), "\n";
$cipher = Crypt::CBC->new(-literal_key => 1,
                    -key         => $key,
                    -iv          => $iv,
                    -header      => 'none');

$ciphertext = $cipher->encrypt($data);
print "ENC->", unpack("H*", $ciphertext), "\n";

The output of the code is:

TXT->QUESTION
HEX->5155455354494f4e
ENC->8220553e09f1b31ba7691f3f7fb52416

My data is conveniently of size 64bits (16 hex digits) which is in accordance with the DES standard. According to Wikipedia

DES is the archetypal block cipher — an algorithm that takes a fixed-length string of plaintext bits and transforms it through a series of complicated operations into another ciphertext bitstring of the same length

Why is it that the encoded output is of longer byte length than the original input?

Thanks.

Unknown Unknown · Accepted Answer · 2014-02-20T09:29:39

Working backward from the second block (a7691f3f7fb52416) gives 8a285d3601f9bb13, and XORed with the first block (8220553e09f1b31b) gives 0808080808080808 (HEX). Something is producing the block value of 0808080808080808 as a second input block value.

CBC Values

So all you have to do is figure out where the backspace characters came from as a second block input.

See https://metacpan.org/pod/Crypt::CBC

This:

#!/usr/bin/perl

use Crypt::CBC;
$key  = "\x4A\x6F\xC2\x2A\x44\xE2\xA4\x48"; 
$iv   = "\x00\x00\x00\x00\x00\x00\x00\x00";
$data = "\x51\x55\x45\x53\x54\x49\x4F\x4E";

print "TXT->", $data, "\n";
print "HEX->", unpack("H*", $data), "\n";
$cipher = Crypt::CBC->new(-literal_key => 1,
                    -key         => $key,
                    -iv          => $iv,
                    -header      => 'none',
                    -padding     => 'null');

$ciphertext = $cipher->encrypt($data);

print "ENC->", unpack("H*", $ciphertext), "\n";

Gave:

david_koontz@Macbook: cbc_des
TXT->QUESTION
HEX->5155455354494f4e
ENC->8220553e09f1b31b
david_koontz@Macbook:

I made the mistake poking around because I know a fair bit about DES, not so much perl.

Adding the padding null seemed to do the trick, after I learned how to add Crypt::CBC and Crypt::DES to a perl library.

I used http://code.google.com/p/dpades/source/browse/trunk/simu_js/JS-DES.html to do the encryptions and decryptions necessary to figure out what's going on. Use the view raw file button and save JS-DES.html locally, open it with a browser.

Perl DES CBC encryption results in more bytes

2 Answers