PHP decrypt not working using mcrypt_decrypt?

Question

Seems that mcrypt_decrypt can't proper decrypt my string (all works fine in var_dump except when decrypting - load_decrypted value is wrong). Any help would be appreciated.

array
  'salve_plain' => string 'a:1:{s:8:"modified";i:1321974656;}' (length=34)
  'save_encrypted' => string '^ånÄc¥JŸRæk®»}J%áR–y #‡nwZX\µÚ™È§œ‘5‚<_¹M¿ÔT9î»ªk)…ª  Ø' (length=64)
  'save_encoded' => string 'XuVuxGOlA0qfUuYXa667fUoSEyXhBVKWeSAjh253EFpYXLUS2pnIp5yRNa3LgjxfuRNNv9RUOe67qmsphaoJ2A==' (length=88)

array
  'load_undecoded' => string 'XuVuxGOlA0qfUuYXa667fUoSEyXhBVKWeSAjh253EFpYXLUS2pnIp5yRNa3LgjxfuRNNv9RUOe67qmsphaoJ2A==' (length=88)
  'load_decoded' => string '^ånÄc¥JŸRæk®»}J%áR–y #‡nwZX\µÚ™È§œ‘5‚<_¹M¿ÔT9î»ªk)…ª    Ø' (length=64)
  'load_decrypted' => string '-dœÞ{*€ ¥ûü(1À�ðú-›(!*»ÓÍW¦;}' (length=34)

PHP load() function:

private function load()
{

    // Decoding
    $plain = file_get_contents($this->filename);
    $decoded = base64_decode($plain);

    // Decrypting
    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    $decrypted = trim(mcrypt_decrypt(MCRYPT_RIJNDAEL_256, md5($this->secret),
        $decoded, MCRYPT_MODE_CBC, $iv));

    // Deserializing & loading
    $this->data = unserialize($decrypted);
    var_dump(array('load_undecoded' => $plain, 'load_decoded' => $decoded,
        'load_decrypted' => $decrypted));
}

PHP save() function:

private function save()
{

    // Serialization
    $serialized = serialize($this->data);

    // Encrypting
    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    $encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, md5($this->secret),
        $serialized, MCRYPT_MODE_CBC, $iv);

    // Encoding & saving
    $encoded = base64_encode($encrypted);
    file_put_contents($this->filename, $encoded);

    var_dump(array('salve_plain' => $serialized,
        'save_encrypted' => $encrypted, 'save_encoded' => $encoded));

}

I have started using a decryption software for all of this... Decry.pt is what I use for all of my php now. I mainly use it to decode IonCube.. Hope this is helpful. — user2023102
– user2023102, Commented Jan 29, 2013 at 20:29

halfer · Accepted Answer · 2011-11-22 17:10:31Z

2

Unfortunately you need the same $iv in both the load and the save.

// Creates a random value so that the same message encoded with the same key
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);

A way around it is to use

$mode = MCRYPT_MODE_CFB;

Encrypt with

$encrypted = mcrypt_encrypt($cipher, $key, $iv . $message, $mode, $iv);

Prefix the encoded message with the IV, and then in decoding use

$decrypted = mcrypt_decrypt($cipher, $key, $encrypted, $mode, str_pad('', $iv_size));
$decrypted = substr(rtrim($decrypted, "\0"), $iv_size);

CFB has the ability to re-sync during decryption, and can use this 'recovery' feature to put your IV in.

Also, depending on the number of IVs you're creating, you may want to use urandom rather then random.

$iv = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM);

edited Nov 22, 2011 at 17:10

halfer

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answered Nov 22, 2011 at 15:21

rrehbein

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1 Comment

Paŭlo Ebermann Over a year ago

Actually, you can prefix your IV to the ciphertext with about any IV-using mode (like CBC, CTR, ...) - you just have to split it off before decrypting.

Marc B · Accepted Answer · 2011-11-22 15:21:12Z

0

You're using a different IV for encrypting and decrypting each time. So basically you're locking up your data with one key, then trying to unlock it with a completely different key.

answered Nov 22, 2011 at 15:21

Marc B

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