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 don't know PHP, but reading through related questions on the sidebar, I see Converting Ruby AES256 decrypt function to PHP. This includes a reference to this page, pointing out that the 128 in MCRYPT_RIJNDAEL_128 refers to the block size of the encryption, not the key size. You'll notice that the key size that you've passed between ruby and PHP is 256 bits in both cases. In other words, this seems to be the expected behavior, and you are using the larger key already.

#!/usr/bin/ruby
require 'base64'

puts((Base64.decode64("RIvFgoi9xZaHS/0Bp0J9WDRyND6Z7jrd3btiAfcQ8Y0=").length * 8).to_s)

HTH