AES-256 encryption in PHP

Look at the mcrypt module

AES-Rijndael example taken from here

$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
$iv = mcrypt_create_iv($iv_size, MCRYPT_DEV_URANDOM);
$key = pack('H*', "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3");
# show key size use either 16, 24 or 32 byte keys for AES-128, 192
# and 256 respectively
$key_size =  strlen($key);
echo "Key size: " . $key_size . "\n";
$text = "Meet me at 11 o'clock behind the monument.";
echo strlen($text) . "\n";

$crypttext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $text, MCRYPT_MODE_CBC, $iv);
echo strlen($crypttext) . "\n";

This is the decrypt function

MCRYPT_RIJNDAEL_256 is not equivalent to AES_256.

The way to make RIJNDAEL be decrypted from AES is to use MCRYPT_RIJNDAEL_128 and padd the string to encrypt before encrypting

AES-256 has BlockSize=128bit and KeySize=256bit Rijndael-256 has BlockSize=256bit and KeySize=256bit

Just AES/Rijndael 128bit are identical. Rijndael-192 and Rijndael-256 are not identical to AES-192 and AES-256 (block sizes and number of rounds differ).

I need a PHP function, AES256_encode($dataToEcrypt) to encrypt the $data into AES-256 and another one AES256_decode($encryptedData) do the opposite. Does anyone know what code should this functions have?

There is a difference between encrypting and encoding.

Do you really need AES-256? The security of AES-256 versus AES-128 isn't that significant; you're more likely to screw up at the protocol layer than get hacked because you used a 128-bit block cipher instead of a 256-bit block cipher.

Important - Use A Library

• defuse/php-encryption
• PECL libsodium
• Halite (libsodium wrapper, now stable)

A Quick and Dirty AES-256 Implementation

If you're interested in building your own not for the sake of deploying it in production but rather for the sake of your own education, I've included a sample AES256

/**
 * This is a quick and dirty proof of concept for StackOverflow.
 * 
 * @ref http://stackoverflow.com/q/6770370/2224584
 * 
 * Do not use this in production.
 */
abstract class ExperimentalAES256DoNotActuallyUse
{
    /**
     * Encrypt with AES-256-CTR + HMAC-SHA-512
     * 
     * @param string $plaintext Your message
     * @param string $encryptionKey Key for encryption
     * @param string $macKey Key for calculating the MAC
     * @return string
     */
    public static function encrypt($plaintext, $encryptionKey, $macKey)
    {
        $nonce = random_bytes(16);
        $ciphertext = openssl_encrypt(
            $plaintext,
            'aes-256-ctr',
            $encryptionKey,
            OPENSSL_RAW_DATA,
            $nonce
        );
        $mac = hash_hmac('sha512', $nonce.$ciphertext, $macKey, true);
        return base64_encode($mac.$nonce.$ciphertext);
    }

    /**
     * Verify HMAC-SHA-512 then decrypt AES-256-CTR
     * 
     * @param string $message Encrypted message
     * @param string $encryptionKey Key for encryption
     * @param string $macKey Key for calculating the MAC
     */
    public static function decrypt($message, $encryptionKey, $macKey)
    {
        $decoded = base64_decode($message);
        $mac = mb_substr($message, 0, 64, '8bit');
        $nonce = mb_substr($message, 64, 16, '8bit');
        $ciphertext = mb_substr($message, 80, null, '8bit');

        $calc = hash_hmac('sha512', $nonce.$ciphertext, $macKey, true);
        if (!hash_equals($calc, $mac)) {
            throw new Exception('Invalid MAC');
        }
        return openssl_decrypt(
            $ciphertext,
            'aes-256-ctr',
            $encryptionKey,
            OPENSSL_RAW_DATA,
            $nonce
        );
    }
}

Usage

First, generate two keys (yes, two of them) and store them somehow.

$eKey = random_bytes(32);
$aKey = random_bytes(32);

Then to encrypt/decrypt messages:

$plaintext = 'This is just a test message.';
$encrypted = ExperimentalAES256DoNotActuallyUse::encrypt($plaintext, $eKey, $aKey);
$decrypted = ExperimentalAES256DoNotActuallyUse::decrypt($encrypted, $eKey, $aKey);

If you don't have random_bytes(), get random_compat.