PHP mcrypt & Perl Crypt::CBC generating different ciphertext

Question

I am trying to learn various encryption methods and came across a problem when trying to generate ciphertext with Perl vs PHP.

If I encrypt a secret with PHP, I can decrypt the resulting ciphertext in both PHP and Perl, but if I encrypt in Perl the ciphertext is 'wrong' and the secret gets garbled by both PHP and Perl...

encrypt.php:

#!/usr/bin/env php
<?php

# Set up vars   
$iv = 'length16length16';
$key = 'length32length32length32length32';
$cleartext = 'password';

if( count( $argv ) > 1 )
{
    $cleartext = $argv[1];
}
# --- ENCRYPTION ---
# Set up cipher
$cipher = mcrypt_module_open( MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');
mcrypt_generic_init( $cipher, $key, $iv );

# Do the encryption
$ciphertext = mcrypt_generic( $cipher, $cleartext );

# Convert to HEX for print/storage
$cipher_block = implode( unpack( 'H*', $iv . $ciphertext ) );

print( "IV " . implode( unpack( 'H*', $iv ) ) );
print( "CIPH " . implode( unpack( 'H*', $ciphertext ) ) );
print( $cipher_block );

# Clean up
mcrypt_generic_deinit( $cipher );
mcrypt_module_close( $cipher );

?>

decrypt.php:

#!/usr/bin/env php
<?php

# Set up vars
$key = 'length32length32length32length32';

if( count( $argv ) > 1 )
{
    # --- DECRYPTION ---
    # Grab the hex-encoded cipherblock & convert it to binary
    $cipher_block = unpack( 'a16iv/a*ciphertext', pack( 'H*', $argv[1] ) );

    # Set up cipher
    $cipher = mcrypt_module_open( MCRYPT_RIJNDAEL_128, '', MCRYPT_MODE_CBC, '');

    mcrypt_generic_init( $cipher, $key, $cipher_block['iv'] );

    # Do the decryption
    $cleartext = mdecrypt_generic( $cipher, $cipher_block['ciphertext'] );

    print( $cleartext );

    # Clean up
    mcrypt_generic_deinit( $cipher );
    mcrypt_module_close( $cipher );
}

?>

encrypt.pl:

#!/usr/bin/env perl

use strict;
use warnings;

use Crypt::CBC;

# Set up vars
my $iv = 'length16length16';
my $key = 'length32length32length32length32';
my $cleartext = shift;

# --- ENCRYPTION ---
# Set up cipher
my $cipher = Crypt::CBC->new(
    -literal_key    => 1,
    -key            => $key,
    -header         => 'none',
    -iv             => $iv,
    -cipher         => 'Crypt::OpenSSL::AES');

# Do the encryption
my $ciphertext = $cipher->encrypt( $cleartext );

# Convert to HEX for print/storage
my $cipher_block = unpack( 'H*', $iv . $ciphertext );

print( "IV " . unpack( 'H*', $iv ) . "\n" );
print( "CIPH " . unpack( 'H*', $ciphertext ) . "\n" );
print( $cipher_block );

decrypt.pl:

#!/usr/bin/env perl

use strict;
use warnings;

use Crypt::CBC;

# Set up vars
my $key = 'length32length32length32length32';

my $cipher_block = shift;

if( $cipher_block )
{
    # --- DECRYPTION ---
    # Grab the hex-encoded cipherblock & convert it to binary
    my ($iv, $ciphertext) = unpack( 'a16a*', pack( 'H*', $cipher_block ) );

    # Set up cipher
    my $cipher = Crypt::CBC->new(
        -literal_key    => 1,
        -key            => $key,
        -header         => 'none',
        -iv             => $iv,
        -cipher         => 'Crypt::OpenSSL::AES');

    my $cleartext = $cipher->decrypt( $ciphertext );

    print( $cleartext );
}

And this is the output I get:

$ ./encrypt.php "Secret Text"
IV 6c656e67746831366c656e6774683136
CIPH 32a47901313f47ed2ca657d3bd0c2e80
6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80

$ ./decrypt.php 6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80
Secret Text

$ ./decrypt.pl 6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80
Secret Text

$ ./encrypt.pl "Secret Text"
IV 6c656e67746831366c656e6774683136
CIPH f3ae0d5f236cea77fa9ac5540d733aef
6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80

$ ./decrypt.php 6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80
sesswrtext

$ ./decrypt.pl 6c656e67746831366c656e677468313632a47901313f47ed2ca657d3bd0c2e80
sesswrtext

As you can see, even with an identical secret, key & IV, the Perl script generates a distinct ciphertext, that the PHP & Perl scripts both decrypt to be the same, but not the original secret...

Thanks in advance.

Try not printing the output, but instead converting it to hex and then outputting it. I have a feeling there are trailing characters at play here (padding). They would be null bytes or low-bytes (unprintable) depending on which mode is used... — ircmaxell
– ircmaxell, Commented Nov 18, 2013 at 15:55
In both the encryption scripts I'm packing the ciphertext into a hex string before output for this reason :D — Tim Jones
– Tim Jones, Commented Nov 18, 2013 at 17:01
I'm not worried about the ciphertext, I'm worried about the plain text after decryption (not having the padding stripped properly)... — ircmaxell
– ircmaxell, Commented Nov 18, 2013 at 18:47

karel-m · Accepted Answer · 2013-11-18 20:42:15Z

4

The trouble is padding. Try:

my $cipher = Crypt::CBC->new(
    -literal_key    => 1,
    -key            => $key,
    -header         => 'none',
    -iv             => $iv,
    -padding        => 'null', #!!!!!!!!!!!!!
    -cipher         => 'Crypt::OpenSSL::AES');

and you should get:

CIPH 32a47901313f47ed2ca657d3bd0c2e80

Anyway -padding=>'null' might not be a good idea. Try to find out whether PHP's mcrypt supports most frequently used PKCS#5/7 padding which corresponds to Crypt::CBC's -padding=>'standard'

edited Nov 18, 2013 at 20:42

answered Nov 18, 2013 at 18:33

karel-m

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1 Comment

Maarten Bodewes Over a year ago

PHP's mcrypt does not support that padding. You will have to do this yourself; there are some implementations in the comments of mcrypt_encrypt. Also note for Crypt::OpenSSL::AES: "Encrypt data. The size of $data must be exactly blocksize in length (16 bytes), otherwise this function will croak." I've tried it but my computer won't croak. Documentation must also be version 0.1 or so.

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