n an ideal world, words like cryptography and security wouldn't even exist, but the real world is far from perfect, so software developers have to spend a good deal of time building security into applications. Cryptography is just one piece of the security puzzle, along with SSL/TLS, certificates, digital signatures, and so on. This article explains how to use PHP to implement the most common cryptographic algorithms. In addition to describing PHP's default encryption functions, you'll see how to use a wide variety of cryptographic libraries and packages.

The code examples in this article use the contents of a short text file, textfile.txt, which contains the following plain-text content:

   For every difficult and complicated 
   question there is an answer 
   that is simple, easily understood, 
   and wrong. H.L. Mencken

Default PHP Encryption Functions
PHP ships with three built-in encryption functions: md5(), crypt(), and sha1(). The md5() function prototype is:

   string md5(string $str [, bool $raw_output ])

The function calculates the MD5 hash of a supplied string using the MD5 Message-Digest algorithm. The $str argument represents the string to be encrypted. If you pass FALSE in the $raw_output argument (the default), the function returns the hash as a 32-character hexadecimal number. If you pass TRUE then the function returns a 16-byte raw binary value.

The PHP crypt() function is a one-way encryption function that lets you confirm that an entered password matches a stored encrypted one—without having to decrypt anything. The crypt() function prototype is:

   string crypt (string $str [, string $salt ])

It returns an encrypted string using the standard Unix DES-based encryption algorithm (or alternative algorithms that may be available on the system). The $str argument is the string to be encrypted and the optional $salt argument is a string on which to base the encryption. If you don't provide the salt string, PHP will randomly generate one each time you call this function.

The PHP sha1() function calculates the SHA-1 hash of a string. The sha1() function prototype is:

   string sha1 (string $str [, bool $raw_output ])

Figure 1. Encrypted File: The encrypted.txt file contains a password encrypted with md5, crypt, and sha1 PHP default functions.

The function returns the SHA-1 hash as a string. Again, the $str argument represents the input string. If you set the optional $raw_output argument to TRUE, the function returns the sha1 hash in raw binary format with a length of 20 characters; if you set it to FALSE, it returns a 40-character hexadecimal number.

As an example, the following code shows how to use the PHP default encryption functions to encrypt the contents of texfile.txt file and write the encrypted result in the file encrypted.txt (see Figure 1):

   <?php
    
   $file = 'textfile.txt';
   $initial_contents = file_get_contents($file);  
   
   if($initial_contents){
   
      $password = 'OctaviaAnghel';
   
      //Calculates the md5 hash 
      $md5_data = md5($password);
   
      //This function encrypts data
      $crypt = crypt($password);
   
      //Calculate the sha1 hash
      $sha1 = sha1($password);
   
      $encrypted_file = @fopen('encrypted.txt','w');
      $ok_encrypt = @fwrite($encrypted_file,'md5: '. $md5_data."\r\n".'crypt: 
          '.$crypt."\r\n".'sha1: '.$sha1);
   
      if($ok_encrypt){
         echo 'The encrypted code was succesfully created'.
         ' in encrypted_file.txt!!!'.'
';
      }
      else{
         echo ("The write of this file failed!");
      }
      
      @fclose($encrypted_file);
   } 
   ?>  

In addition to the built-in functions, PHP supports encryption via external libraries and packages. Table 1 shows the libraries and packages described in the rest of this article.

Package/Library	Description
MCrypt	Use MCrypt to encrypt large files or data streams using any of a wide range of encryption functions. You can find more information at http://mcrypt.sourceforge.net/.
MHash	Use the MHash library to obtain hashes. MHash supports the most popular algorithms and implementations, such as SHA, MD5, and CRC. You can use these algorithms to compute checksums, message digests, and create other signatures. MHash is often used to obtain password hashes for passwords entered into HTML password fields. You can find more information at http://mhash.sourceforge.net/.
Crypt_Blowfish	Use Crypt_Blowfish for quick two-way encryption both with or without a secret key. You don't need the MCrypt PHP extension to use Crypt_Blowfish; however, the package can use MCrypt if it's installed. More details at: http://pear.php.net/package/Crypt_Blowfish.
Crypt_RSA	Crypt_RSA provides RSA-like key generation, encryption/decryption, signing and signature checking. More details here: http://pear.php.net/package/Crypt_RSA.
Crypt_ HMAC	This class calculates RFC 2104-compliant hashes. You'll find complete information at: http://pear.php.net/package/Crypt_HMAC.
Crypt_DiffieHellman	This package is a PHP5 implementation of the Diffie-Hellman Key Exchange cryptographic protocol. You can find more information at http://pear.php.net/package/Crypt_DiffieHellman.