Simplest two-way encryption using PHP

Question

What is the simplest way of doing two way encryption in common PHP installs?

I need to be able to encrypt data with a string key, and use the same key to decrypt on the other end.

The security isn't as big of a concern as the portability of the code, so I'd like to be able to keep things as simple as possible. Currently, I am using an RC4 implementation, but if I can find something natively supported I figure I can save a lot of unnecessary code.

Answer 1

Important: Unless you have a very particular use-case, do not encrypt passwords, use a password hashing algorithm instead. When someone says they encrypt their passwords in a server-side application, they're either uninformed or they're describing a dangerous system design. Safely storing passwords is a totally separate problem from encryption.

Be informed. Design safe systems.

Portable Data Encryption in PHP

If you're using PHP 5.4 or newer and don't want to write a cryptography module yourself, I recommend using an existing library that provides authenticated encryption. The library I linked relies only on what PHP provides and is under periodic review by a handful of security researchers. (Myself included.)

If your portability goals do not prevent requiring PECL extensions, libsodium is highly recommended over anything you or I can write in PHP.

Update (2016-06-12): You can now use sodium_compat and use the same crypto libsodium offers without installing PECL extensions.

If you want to try your hand at cryptography engineering, read on.

---

First, you should take the time to learn the dangers of unauthenticated encryption and the Cryptographic Doom Principle.

• Encrypted data can still be tampered with by a malicious user.
• Authenticating the encrypted data prevents tampering.
• Authenticating the unencrypted data does not prevent tampering.

Encryption and Decryption

Encryption in PHP is actually simple (we're going to use openssl_encrypt() and openssl_decrypt() once you have made some decisions about how to encrypt your information. Consult openssl_get_cipher_methods() for a list of the methods supported on your system. The best choice is AES in CTR mode:

• aes-128-ctr
• aes-192-ctr
• aes-256-ctr

There is currently no reason to believe that the AES key size is a significant issue to worry about (bigger is probably not better, due to bad key-scheduling in the 256-bit mode).

Note: We are not using mcrypt because it is abandonware and has unpatched bugs that might be security-affecting. Because of these reasons, I encourage other PHP developers to avoid it as well.

Simple Encryption/Decryption Wrapper using OpenSSL

class UnsafeCrypto
{
    const METHOD = 'aes-256-ctr';

    /**
     * Encrypts (but does not authenticate) a message
     * 
     * @param string $message - plaintext message
     * @param string $key - encryption key (raw binary expected)
     * @param boolean $encode - set to TRUE to return a base64-encoded 
     * @return string (raw binary)
     */
    public static function encrypt($message, $key, $encode = false)
    {
        $nonceSize = openssl_cipher_iv_length(self::METHOD);
        $nonce = openssl_random_pseudo_bytes($nonceSize);

        $ciphertext = openssl_encrypt(
            $message,
            self::METHOD,
            $key,
            OPENSSL_RAW_DATA,
            $nonce
        );

        // Now let's pack the IV and the ciphertext together
        // Naively, we can just concatenate
        if ($encode) {
            return base64_encode($nonce.$ciphertext);
        }
        return $nonce.$ciphertext;
    }

    /**
     * Decrypts (but does not verify) a message
     * 
     * @param string $message - ciphertext message
     * @param string $key - encryption key (raw binary expected)
     * @param boolean $encoded - are we expecting an encoded string?
     * @return string
     */
    public static function decrypt($message, $key, $encoded = false)
    {
        if ($encoded) {
            $message = base64_decode($message, true);
            if ($message === false) {
                throw new Exception('Encryption failure');
            }
        }

        $nonceSize = openssl_cipher_iv_length(self::METHOD);
        $nonce = mb_substr($message, 0, $nonceSize, '8bit');
        $ciphertext = mb_substr($message, $nonceSize, null, '8bit');

        $plaintext = openssl_decrypt(
            $ciphertext,
            self::METHOD,
            $key,
            OPENSSL_RAW_DATA,
            $nonce
        );

        return $plaintext;
    }
}

Usage Example

$message = 'Ready your ammunition; we attack at dawn.';
$key = hex2bin('000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f');

$encrypted = UnsafeCrypto::encrypt($message, $key);
$decrypted = UnsafeCrypto::decrypt($encrypted, $key);

var_dump($encrypted, $decrypted);

Demo: https://3v4l.org/jl7qR

---

The above simple crypto library still is not safe to use. We need to authenticate ciphertexts and verify them before we decrypt.

Note: By default, UnsafeCrypto::encrypt() will return a raw binary string. Call it like this if you need to store it in a binary-safe format (base64-encoded):

$message = 'Ready your ammunition; we attack at dawn.';
$key = hex2bin('000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f');

$encrypted = UnsafeCrypto::encrypt($message, $key, true);
$decrypted = UnsafeCrypto::decrypt($encrypted, $key, true);

var_dump($encrypted, $decrypted);

Demo: http://3v4l.org/f5K93

Simple Authentication Wrapper

class SaferCrypto extends UnsafeCrypto
{
    const HASH_ALGO = 'sha256';

    /**
     * Encrypts then MACs a message
     * 
     * @param string $message - plaintext message
     * @param string $key - encryption key (raw binary expected)
     * @param boolean $encode - set to TRUE to return a base64-encoded string
     * @return string (raw binary)
     */
    public static function encrypt($message, $key, $encode = false)
    {
        list($encKey, $authKey) = self::splitKeys($key);

        // Pass to UnsafeCrypto::encrypt
        $ciphertext = parent::encrypt($message, $encKey);

        // Calculate a MAC of the IV and ciphertext
        $mac = hash_hmac(self::HASH_ALGO, $ciphertext, $authKey, true);

        if ($encode) {
            return base64_encode($mac.$ciphertext);
        }
        // Prepend MAC to the ciphertext and return to caller
        return $mac.$ciphertext;
    }

    /**
     * Decrypts a message (after verifying integrity)
     * 
     * @param string $message - ciphertext message
     * @param string $key - encryption key (raw binary expected)
     * @param boolean $encoded - are we expecting an encoded string?
     * @return string (raw binary)
     */
    public static function decrypt($message, $key, $encoded = false)
    {
        list($encKey, $authKey) = self::splitKeys($key);
        if ($encoded) {
            $message = base64_decode($message, true);
            if ($message === false) {
                throw new Exception('Encryption failure');
            }
        }

        // Hash Size -- in case HASH_ALGO is changed
        $hs = mb_strlen(hash(self::HASH_ALGO, '', true), '8bit');
        $mac = mb_substr($message, 0, $hs, '8bit');

        $ciphertext = mb_substr($message, $hs, null, '8bit');

        $calculated = hash_hmac(
            self::HASH_ALGO,
            $ciphertext,
            $authKey,
            true
        );

        if (!self::hashEquals($mac, $calculated)) {
            throw new Exception('Encryption failure');
        }

        // Pass to UnsafeCrypto::decrypt
        $plaintext = parent::decrypt($ciphertext, $encKey);

        return $plaintext;
    }

    /**
     * Splits a key into two separate keys; one for encryption
     * and the other for authenticaiton
     * 
     * @param string $masterKey (raw binary)
     * @return array (two raw binary strings)
     */
    protected static function splitKeys($masterKey)
    {
        // You really want to implement HKDF here instead!
        return [
            hash_hmac(self::HASH_ALGO, 'ENCRYPTION', $masterKey, true),
            hash_hmac(self::HASH_ALGO, 'AUTHENTICATION', $masterKey, true)
        ];
    }

    /**
     * Compare two strings without leaking timing information
     * 
     * @param string $a
     * @param string $b
     * @ref https://paragonie.com/b/WS1DLx6BnpsdaVQW
     * @return boolean
     */
    protected static function hashEquals($a, $b)
    {
        if (function_exists('hash_equals')) {
            return hash_equals($a, $b);
        }
        $nonce = openssl_random_pseudo_bytes(32);
        return hash_hmac(self::HASH_ALGO, $a, $nonce) === hash_hmac(self::HASH_ALGO, $b, $nonce);
    }
}

Usage Example

$message = 'Ready your ammunition; we attack at dawn.';
$key = hex2bin('000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f');

$encrypted = SaferCrypto::encrypt($message, $key);
$decrypted = SaferCrypto::decrypt($encrypted, $key);

var_dump($encrypted, $decrypted);

Demos: raw binary, base64-encoded

---

If anyone wishes to use this SaferCrypto library in a production environment, or your own implementation of the same concepts, I strongly recommend reaching out to your resident cryptographers for a second opinion before you do. They'll be able tell you about mistakes that I might not even be aware of.

You will be much better off using a reputable cryptography library.