Encrypting / Decrypting file with Mcrypt

Question

Trying to write a couple of functions that will encrypt or decrypt a file and am using the class found here to try and accomplish this:

http://www.itnewb.com/v/PHP-Encryption-Decryption-Using-the-MCrypt-Library-libmcrypt

The encryption function below seems to work, in that it appears to encrypt the file and place it in the intended directory. I'm trying to decrypt the file now, and it just dies with the message "Failed to complete decryption" (which is coded in there...) There's nothing in the php error logs, so I'm not sure why it's failing, but as mcrypt is entirely new to me, I'm more than inclined to believe I'm doing something wrong here...

Here are the functions:

//ENCRYPT FILE
    function encryptFile() {
        global $cryptastic;
        $pass = PGPPASS;
        $salt = PGPSALT;
        $key = $cryptastic->pbkdf2($pass, $salt, 1000, 32) or die("Failed to generate secret key.");

        if ($handle = opendir(PATH.'/ftpd')) {
            while (false !== ($file = readdir($handle))) {
                if ($file != "." && $file != "..") {
                    $newfile = PATH.'/encrypted/'.$file.'.txt';
                    $msg = file_get_contents(PATH.'/ftpd/'.$file);
                    $encrypted = $cryptastic->encrypt($msg, $key) or die("Failed to complete encryption.");
                    $nfile = fopen($newfile, 'w');
                    fwrite($nfile, $encrypted);
                    fclose($nfile);
                    unlink(PATH.'/ftpd/'.$file);

                }
            }
            closedir($handle);
        }       


//DECRYPT FILE
    function inFTP() {
        global $cryptastic;
        $pass = PGPPASS;
        $salt = PGPSALT;
        $key = $cryptastic->pbkdf2($pass, $salt, 1000, 32) or die("Failed to generate secret key.");

        if ($handle = opendir(PATH.'/encrypted')) {
            while (false !== ($file = readdir($handle))) {
                if ($file != "." && $file != "..") {
                    $newfile = PATH.'/decrypted/'.$file;
                    $msg = PATH.'/encrypted/'.$file;
                    $decrypted = $cryptastic->decrypt($msg, $key) or die("Failed to complete decryption.");
                    $nfile = fopen($newfile, 'w');
                    fwrite($nfile, $decrypted);
                    fclose($nfile);
                    //unlink(PATH.'/encrypted/'.$file);

                }
            }
            closedir($handle);
        }       
        //$crypt->decrypt($file);
    }

Answer 1

Try this PHP5 class for encryption using mcrypt. In this case it's using AES encryption. You'll want to change the key for each site you use it on. If you don't use it at least it may guide you on writing your own version of it.

<?php

class Encryption
{
    const CIPHER = MCRYPT_RIJNDAEL_128; // Rijndael-128 is AES
    const MODE   = MCRYPT_MODE_CBC;

    /* Cryptographic key of length 16, 24 or 32. NOT a password! */
    private $key;
    public function __construct($key) {
        $this->key = $key;
    }

    public function encrypt($plaintext) {
        $ivSize = mcrypt_get_iv_size(self::CIPHER, self::MODE);
        $iv = mcrypt_create_iv($ivSize, MCRYPT_DEV_URANDOM);
        $ciphertext = mcrypt_encrypt(self::CIPHER, $this->key, $plaintext, self::MODE, $iv);
        return base64_encode($iv.$ciphertext);
    }

    public function decrypt($ciphertext) {
        $ciphertext = base64_decode($ciphertext);
        $ivSize = mcrypt_get_iv_size(self::CIPHER, self::MODE);
        if (strlen($ciphertext) < $ivSize) {
            throw new Exception('Missing initialization vector');
        }

        $iv = substr($ciphertext, 0, $ivSize);
        $ciphertext = substr($ciphertext, $ivSize);
        $plaintext = mcrypt_decrypt(self::CIPHER, $this->key, $ciphertext, self::MODE, $iv);
        return rtrim($plaintext, "\0");
    }
}

Usage:

$key = /* CRYPTOGRAPHIC!!! key */;
$crypt = new Encryption($key);
$encrypted_string = $crypt->encrypt('this is a test');
$decrypted_string = $crypt->decrypt($encrypted_string); // this is a test

Notes:

• This class is not safe for use with binary data (which may end in NUL bytes)
• This class does not provide authenticated encryption.

Answer 2

While Johns answer is good, using base64 encoding just to fix the binary safety issue is overkill and will make your encrypted files 33% larger than the original. Here is my PHP Implementation of the AES Crypt file format which solves all the above issues transparently.

https://github.com/philios33/PHP-AES-File-Encryption

It is binary safe and includes authenticated encryption. Since it uses the open source aes crypt file format (.aes) it is fully compatible with other .aes software.

https://www.aescrypt.com/

The interface is pretty simple whether you are encrypting or decrypting. You just give it a source file and password.

Answer 3

You should not be using Mcrypt to encrypt/decrypt data. As shown in your question, and in the accepted answer, the data is not authenticated, which means it will fall victim to chosen ciphertext attacks.

Further, a great deal of effort has been done to make sure that developers put together cryptographic primitives correctly. As such, instead of Mcrypt, you should be using libsodium for your PHP projects. libsodium is a fork of NaCl. NaCl/libsodium is written to remove a lot of the cryptographic pitfalls that developers find themselves in, such as timing attacks with verification of MAC tags.

Mcrypt is deprecated in PHP 7.1, and libsodim is the preferred way to handle cryptography in PHP.

Using libsodium in your PHP project is easy, and secure. Scott Arciszewski has written an extensive ebook on using libsodium with PHP at https://paragonie.com/book/pecl-libsodium. It's worth the read for anyone doing PHP cryptography.

Answer 4

CakePHP has a pretty good implementation of rijndael. I'm not posting code directly here because not sure the legal ramifications.

Here are the api docs for the Security::rijndael() method.

If encoding a file, you will want to base64_encode() before calling this method with 'encrypt', and base64_decode() after calling this method with 'decrypt'