How to make Ruby AES-256-CBC and PHP MCRYPT_RIJNDAEL_128 play well together

Question

I'm generating data to send from a Ruby stack to a PHP stack. I'm using the OpenSSL::Cipher library on the Ruby side and the 'mcrypt' library in PHP. When I encrypt using 'aes-256-cbc' (256-bit block size) in Ruby I need to use MCRYPT_RIJNDAEL_128 (128-bit block size) in PHP to decrypt it. I suspect the Ruby code that is broken, because the cipher.iv_len is 16; I believe it should be 32:

>> cipher = OpenSSL::Cipher::Cipher.new('aes-128-cbc')
=> #<OpenSSL::Cipher::Cipher:0x3067c5c>
>> cipher.key_len
=> 16
>> cipher.iv_len
=> 16
>> cipher = OpenSSL::Cipher::Cipher.new('aes-256-cbc')
=> #<OpenSSL::Cipher::Cipher:0x306de18>
>> cipher.key_len
=> 32
>> cipher.iv_len
=> 16

So here's my test. On the Ruby side, first I generate the key and iv:

>> cipher = OpenSSL::Cipher::Cipher.new('aes-256-cbc')
>> cipher.encrypt
>> iv = cipher.random_iv
>> iv64 = [iv].pack("m").strip
=> "vCkaypm5tPmtP3TF7aWrug=="
>> key = cipher.random_key
>> key64 = [key].pack("m").strip
=> "RIvFgoi9xZaHS/0Bp0J9WDRyND6Z7jrd3btiAfcQ8Y0="

Then I use those keys to do the encryption:

>> plain_data = "Hi, Don, this is a string."
>> cipher = OpenSSL::Cipher::Cipher.new('aes-256-cbc')
>> cipher.encrypt
>> cipher.key = Base64.decode64(key64)
>> cipher.iv = Base64.decode64(iv64)
>> encrypted_data = cipher.update(plain_data)
>> encrypted_data << cipher.final
>> crypt64 = [encrypted_data].pack("m").strip
=> "5gfC/kJcnAV2fJI0haxnLcdraIKWgtu54UoznVxf8K0="

Here's the PHP decryption:

$ruby_crypt = "5gfC/kJcnAV2fJI0haxnLcdraIKWgtu54UoznVxf8K0=";
$encrypted_data = base64_decode($ruby_crypt);
$key = base64_decode("RIvFgoi9xZaHS/0Bp0J9WDRyND6Z7jrd3btiAfcQ8Y0=");
$iv = base64_decode("vCkaypm5tPmtP3TF7aWrug==");
$result = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $encrypted_data, MCRYPT_MODE_CBC, $iv);
$unencrypt = rtrim($result, "\x00..\x1F");
print "\nUnencrypted token:\n'$unencrypt'\n";

RESULT:
Unencrypted token:
'Hi, Don, this is a string.'

I'd prefer to use the longer block size. Clearly I'm misunderstanding the APIs. Help?

Answer 1

I wrote an example that somebody else may find explanatory of the discussion above:

$ cat publisher.rb

#!/usr/bin/env ruby

require 'openssl'
require 'base64'

key = '7fc4d85e2e4193b842bb0541de51a497'

cipher = OpenSSL::Cipher::Cipher.new('aes-128-cbc')
cipher.encrypt()
iv = cipher.random_iv

cipher = OpenSSL::Cipher::Cipher.new('aes-256-cbc')
cipher.encrypt()
cipher.key = key
cipher.iv = iv
crypt = cipher.update('This is my text')
crypt << cipher.final()

puts [Base64.encode64(crypt).strip(), Base64.encode64(iv).strip()].join('|')

$ cat consumer.php

$key256 = '7fc4d85e2e4193b842bb0541de51a497';

$fd = fopen("php://stdin", "r");
$tokens = '';
while (!feof($fd))
  $tokens .= fread($fd, 1024);
fclose($fd);

$tokens = explode('|', trim($tokens));
$crypt = $tokens[0];
$iv = $tokens[1];

$crypttext = base64_decode($crypt);
$iv = base64_decode($iv);
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key256, $crypttext, MCRYPT_MODE_CBC, $iv);

print $decrypted ."\n";

To test it, from command line try:

$ ruby publisher.rb | php consumer.php

This is my text