core-bundle/src/Resources/contao/library/Contao/Encryption.php
public static function encrypt($varValue, $strKey=null)
	{
		// Recursively encrypt arrays
		if (\is_array($varValue))
		{
			foreach ($varValue as $k=>$v)
			{
				$varValue[$k] = static::encrypt($v);
			}

			return $varValue;
		}
		elseif ($varValue == '')
		{
			return '';
		}

		// Initialize the module
		if (static::$resTd === null)
		{
			static::initialize();
		}

		if (!$strKey)
		{
			$strKey = System::getContainer()->getParameter('contao.encryption_key');
		}

		$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size(static::$resTd));
		mcrypt_generic_init(static::$resTd, md5($strKey), $iv);
		$strEncrypted = mcrypt_generic(static::$resTd, $varValue);
		$strEncrypted = base64_encode($iv.$strEncrypted);
		mcrypt_generic_deinit(static::$resTd);

		return $strEncrypted;
	}
public static function encrypt($varValue, $strKey=null)
	{
		// Recursively encrypt arrays
		if (\is_array($varValue))
		{
			foreach ($varValue as $k=>$v)
			{
				$varValue[$k] = static::encrypt($v);
			}

			return $varValue;
		}
		elseif ($varValue == '')
		{
			return '';
		}

		// Initialize the module
		if (static::$resTd === null)
		{
			static::initialize();
		}

		if (!$strKey)
		{
			$strKey = System::getContainer()->getParameter('contao.encryption_key');
		}

		$iv = mcrypt_create_iv(mcrypt_enc_get_iv_size(static::$resTd));
		mcrypt_generic_init(static::$resTd, md5($strKey), $iv);
		$strEncrypted = mcrypt_generic(static::$resTd, $varValue);
		$strEncrypted = base64_encode($iv.$strEncrypted);
		mcrypt_generic_deinit(static::$resTd);

		return $strEncrypted;
	}
Encrypt a value

@param mixed  $varValue The value to encrypt
@param string $strKey   An optional encryption key

@return string The encrypted value