Replace Mcrypt with OpenSSL

Question

currently we have a mcrypt implentation on our systems to crypt some sensible data in our PHP application. Now we have a new requirement that we have to change the crypt module to openssl. Another thing which is important know is that we are using the cipher blowfish and the mode ecb. So I began to test what are differences and how I can decrypt mcrypt encrypted strings with openssl.

I used the standard PHP function:

• mcrypt_encrypt vs. openssl_encrypt
• mcrypt_decrypt vs. openssl_decrypt

Both methods are delivering different results. Second thing is that in the given cipher (blowfish) and mode (ecb) in both types different IV lengthes are required (openssl=0 and mcrypt=56).

Does anybody know how I can easily change the modules without having a big migration effort?

Thanks in advance!

UPDATE:

Here is the code, which I tested it:

<?php 

function say($message){
    if(!is_string($message)){
        if(!isset($_SERVER["HTTP_USER_AGENT"])) echo "<pre>";
        echo var_export($message, true) . ((!isset($_SERVER["HTTP_USER_AGENT"]) ? "\n" : "<br />"));
        if(!isset($_SERVER["HTTP_USER_AGENT"])) echo "</pre>";
    }else{
        echo $message . ((!isset($_SERVER["HTTP_USER_AGENT"]) ? "\n" : "<br />"));
    }
}

say("= Begin raw encryption");
$key    = "anotherpass";
$str    = "does it work";

say("  Params:");
say("  - String to encrypt '".$str."'");
say("  - Key: ".$key);
say("");


$params = array(
    "openssl"  => array(
        "cipher"    => "BF",
        "mode"      => "ECB",
    ),
    "mcrypt" => array(
        "cipher"    => "blowfish", 
        "mode"      => "ecb",
    ),
);

say("= Mcrypt");
$handler = mcrypt_module_open($params['mcrypt']['cipher'], '', $params['mcrypt']['mode'], '');
$iv      = mcrypt_create_iv (mcrypt_enc_get_iv_size($handler), MCRYPT_RAND);
$keysize = mcrypt_enc_get_key_size($handler);
mcrypt_generic_init($handler,$key,"\0\0\0\0\0\0\0\0");
say("  Params:");
say("  - InitVector   ".bin2hex($iv)." (bin2hex)");
say("  - Max keysize  ".$keysize);
say("  - Cipher       ".$params['mcrypt']['cipher']);
say("  - Mode         ".$params['mcrypt']['mode']);
say("");
say("  Encryption:");
$m_encrypted = mcrypt_generic($handler, $str);
$m_decrypted = mdecrypt_generic($handler, $m_encrypted);
say("  - Encrypted   ".bin2hex($m_encrypted)." (bin2hex)");
say("  - Descrypted  ".$m_decrypted);
say("");


say("= Openssl");
say("  Params:");
say("  - InitVector   not needed");
say("  - Max keysize  ".openssl_cipher_iv_length($params['openssl']['cipher']."-".$params['openssl']['mode']));
say("  - Cipher       ".$params['openssl']['cipher']);
say("  - Mode         ".$params['openssl']['mode']);
say("");
say("  Encryption:");
$o_encrypted = openssl_encrypt($str,$params['openssl']['cipher']."-".$params['openssl']['mode'],$key,true);
$o_decrypted = openssl_decrypt($o_encrypted,$params['openssl']['cipher']."-".$params['openssl']['mode'],$key,true);
say("  - Encrypted   ".bin2hex($o_encrypted)." (bin2hex)");
say("  - Descrypted  ".$o_decrypted);

And this is my result:

= Begin raw encryption
  Params:
  - String to encrypt 'does it work'
  - Key: anotherpass

= Mcrypt
  Params:
  - InitVector   06a184909d7bf863 (bin2hex)
  - Max keysize  56
  - Cipher       blowfish
  - Mode         ecb

  Encryption:
  - Encrypted   0e93dce9a6a88e343fe5f90d1307684c (bin2hex)
  - Descrypted  does it work

= Openssl
  Params:
  - InitVector   not needed
  - Max keysize  0
  - Cipher       BF
  - Mode         ECB

  Encryption:
  - Encrypted   213460aade8f9c14d8d51947b8231439 (bin2hex)
  - Descrypted  does it work

Maybe any ideas now?

Thanks!

Salleysalli

You'll either have to run a migration script that decrypts the current data using mcrypt and then encrypts it again using openssl, or else you'll need to implement a method by which you know which encryption/decryption functions to use for each item of data, and change them from mcrypt to openssl as required when next accessing some data encrypted using mcrypt. – Obnubilate

As I understand it, mcrypt and open_ssl use different key derivation methods and so Jon is right, you will need to either migrate by decrypt then encrypt or mark data so that it gets migrated as it is next accessed. – Cm

I wonder why IB is required in ECB mode. See the description of ECB mode at en.wikipedia.org/wiki/Block_cipher_modes_of_operation. – Police

Also see Upgrading my encryption library from Mcrypt to OpenSSL and Preparing for removal of Mcrypt in PHP 7.2 – Florey

Answer 1

Blowfish is the block cipher. It requires the data to be padded before encryption. OpenSSL uses PKCS#7 and mcrypt uses PKCS#5. Different padding algorythms for data. Minimal PKCS#5 padding length is 0, for PKCS#7 it's 1 (wikipedia). Take a look at this example (i've manually padded input data for mcrypt_encrypt() in PKCS#7 style):

<?php 

$key = "anotherpassword1";
$str = "does it work 12";

$enc = mcrypt_encrypt(MCRYPT_BLOWFISH, $key, $str."\1", MCRYPT_MODE_ECB);
$dec = mcrypt_decrypt(MCRYPT_BLOWFISH, $key, $enc, MCRYPT_MODE_ECB);
echo(bin2hex($enc).PHP_EOL);
var_dump($dec);

$enc = openssl_encrypt($str, 'bf-ecb', $key, true);
$dec = openssl_decrypt($enc, 'bf-ecb', $key, true);
echo(bin2hex($enc).PHP_EOL);
var_dump($dec);

?>

It's impossible to openssl_decrypt() data encrypted with mcrypt_encrypt(), unless manual data padding was made with PKCS#7 before mcrypt_encrypt() was called.

There is only one way in your case - recrypt the data.

PS: There is an error in your source - ECB mode does not uses IV at all (wikipedia)

Pumpkinseed

Thanks, this hint helped me for making Rijndael-128/AES-128 compatible between MCrypt and OpenSSL.Unfortunately though, it doesn't work for Blowfish - – Geehan

Turns out it does work for Blowfish, BUT the key size must be at least 16 bytes (Wikipedia article for Blowfish says it supports less). – Geehan

It's because of padding in PKCS#7 – Pumpkinseed

Padding is applied to the plain-text data, I'm talking about the key. Regardless, +1 for a useful answer. :) – Geehan

@Geehan - It sounds like a bug if a Blowfish key of 4 bytes is rejected since the keysize is 32-bits to 448-bits. – Florey

@Pumpkinseed PKCS#7 and PKCS#5 are essentially the same and are the same in implementation, it is just that lazy developers re-used the PKCS#5 identifier instead of add a PKCS#7 identifier. Please fix the answer and comment. See PKCS#7 padding: PKCS#5 padding is identical to PKCS#7 padding, except that it has only been defined for block ciphers that use a 64-bit (8-byte) block size. In practice the two can be used interchangeably. – Slavism

@Pumpkinseed Also see: What is the difference between PKCS#5 padding and PKCS#7 padding: Many cryptographic libraries use an identifier indicating PKCS#5 or PKCS#7 to define the same padding mechanism. – Slavism

Answer 2

In case you want to encrypt with openssl and still get the same result as if you had encrypted it with mcrypt when decrypting with mcrypt, you need to manually null-pad the input string prior to encrypting it with openssl_encrypt and pass the OPENSSL_RAW_DATA | OPENSSL_ZERO_PADDING options.

$str = 'encrypt me';
$cipher = 'AES-256-CBC';
$key = '01234567890123456789012345678901';
$opts = OPENSSL_RAW_DATA | OPENSSL_ZERO_PADDING;
$iv_len = 16;
$str_len = mb_strlen($str, '8bit');
$pad_len = $iv_len - ($str_len % $iv_len);
$str .= str_repeat(chr(0), $pad_len);
$iv = openssl_random_pseudo_bytes($iv_len);


$encrypted = openssl_encrypt($str, $cipher, $key, $opts, $iv);

Decrypting with mcrypt_decrypt will then work just as if you had also used mcrypt for encryption.

mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $encrypted, MCRYPT_MODE_CBC, $iv)

Sideband

GREAT! this way I could create a replacement function for the deprecated mycrypt_encrypt() function. Can I also create a replacement for m̀ysql_decrypt()` – Sylvanite

Answer 3

@clover is right that the default padding for Blowfish is different between mcrypt and Openssl, but is wrong that it can't be done. If you use the OPENSSL_ZERO_PADDING option for the decrypt the two actually are compatible:

openssl_decrypt($data, 'bf-ecb', $key, OPENSSL_RAW_DATA | OPENSSL_ZERO_PADDING);

Selection

OPENSSL_ZERO_PADDING does not add any padding, any non-standard padding will need to be added manually prior to encryption and removed on decryption. From the docs openssl_encrypt, perhaps you are using a different version. From the doc comments: So, OPENSSL_ZERO_PADDING disables padding for the context, which means that you will have to manually apply your own padding out to the block size. Without using OPENSSL_ZERO_PADDING, you will automatically get PKCS#7 padding. – Slavism

@Slavism Maybe you should try the code before you downvote? Works for me. – Selection

Answer 4

For shorter keys, you should make cycled keys for openssl when migrating mcrypt's blowfish.

function make_openssl_blowfish_key($key)
{
    if("$key" === '')
        return $key;

    $len = (16+2) * 4;
    while(strlen($key) < $len) {
        $key .= $key;
    }
    $key = substr($key, 0, $len);
    return $key;
}

See: https://bugs.php.net/bug.php?id=72362

See: Moving from mcrypt with Blowfish & ECB to OpenSSL

Natatory