An AES encryption implementation in PHP with Mcrypt module.

McryptAes256.php

<?php
/**
 * An AES encryption implementation in PHP with Mcrypt module.
 *
 * Key length is 256, CBC mode, and use PKCS#7 padding.
 *
 * Example:
 *     $aes = new McryptAes256;
 *     $encryptedString = $aes->encrypt($rawString);
 *     $decryptedString = $aes->decrypt($encryptedString);
 *
 * Reference:
 *     http://my.oschina.net/u/435872/blog/131799
 */
class McryptAes256 {
	/**
	 * Cipher name. MCRYPT_RIJNDAEL_128 is compliant with AES.
	 *
	 * @var string
	 */
	private $cipher = MCRYPT_RIJNDAEL_128;

	/**
	 * Mode of operation. Choose CBC.
	 *
	 * @var string
	 */
	private $mode = MCRYPT_MODE_CBC;

	/**
	 * Encryption key. The key length must be 256 bits to meet AES-256 requirements.
	 *
	 * @var string
	 */
	private $key = '';

	/**
	 * Initialization vector. 128 bits long is good.
	 *
	 * @var string
	 */
	private $iv = '';

	/**
	 * Setting encryption key and initialization vector.
	 *
	 * @param string $key
	 * @param string $iv
	 */
	public function __construct($key = '', $iv = '')
	{
		$this->setKey($key);
		$this->setIv($iv);
	}

	/**
	 * Encryption key setter.
	 *
	 * @param string $key
	 */
	private function setKey($key)
	{
		$this->key = (empty($key)) ? $this->getRandomString(mcrypt_get_key_size($this->cipher, $this->mode)) : $key;
	}

	/**
	 * Initialization vector setter.
	 *
	 * @param string $iv
	 */
	private function setIv($iv)
	{
		$this->iv = (empty($iv)) ? mcrypt_create_iv(mcrypt_get_iv_size($this->cipher, $this->mode), MCRYPT_RAND) : $iv;
	}

	/**
	 * Random string generator.
	 *
	 * @param int $length
	 *
	 * @return string
	 */
	private function getRandomString($length)
	{
		return substr(md5(time()), 0, $length);
	}

	/**
	 * Encrypt input string with Mcrypt.
	 *
	 * @param string $rawString
	 *
	 * @return string
	 */
	public function encrypt($rawString)
	{
		$paddedString = $this->pkcs7Padding($rawString);
		$encryptedBin = mcrypt_encrypt($this->cipher, $this->key, $paddedString, $this->mode, $this->iv);
		return base64_encode($encryptedBin);
	}

	/**
	 * Decrypt Mcrypt encrypted string.
	 *
	 * @param string $encryptedString
	 *
	 * @return string
	 */
	public function decrypt($encryptedString)
	{
		$encryptedBin = base64_decode($encryptedString);
		$decryptedString = mcrypt_decrypt($this->cipher, $this->key, $encryptedBin, $this->mode, $this->iv);

		// Remove padding bytes
		$blockSize = mcrypt_get_block_size($this->cipher, $this->mode);
		$lastChar = substr($decryptedString, -1, 1);
		$paddingSize = ord($lastChar);
		$originSize = $blockSize - $paddingSize;

		return substr($decryptedString, 0, $originSize);
	}

	/**
	 * Use PKCS#7 padding. See https://en.wikipedia.org/wiki/Padding_(cryptography)#PKCS7 for more information.
	 *
	 * @param string $rawString
	 *
	 * @return string
	 */
	private function pkcs7Padding($rawString)
	{
		$blockSize = mcrypt_get_block_size($this->cipher, $this->mode);
		$paddingSize = $blockSize - (strlen($rawString) % $blockSize);
		$paddingChar = chr($paddingSize);
		return $rawString . str_repeat($paddingChar, $paddingSize);
	}
}