A Guide to Cryptography in PHP : Page 5

Generating Secret Keys with the Crypt_DiffieHellman PEAR Package
This PEAR package implements the Diffie-Hellman Key Exchange cryptographic protocol in PHP5. You can use the protocol to generate a secret key for two foreign parties, who can then use the generated key for communications even on insecure channels. You can download the latest release, version 0.2.1 (beta) and then install it the same way as any other PEAR package.

   > pear install pear_package_name

The following two code examples show two applications for generating a secret key between two parties: subject_1 and subject_2. The first example shows you the simplest way to obtain a secure key based on Diffie Hellman algorithm:

   <?php
   
   //include Diffie Hellman functions
   require_once 'Crypt/DiffieHellman.php';
   
   //set the required options for two subjects
   $subject_1 = array('prime'=>'123', 'generator'=>'7', 'private'=>'3');
   $subject_2 = array('prime'=>'123', 'generator'=>'7', 'private'=>'34');
   
   //apply Diffie Hellman algorithm
   $subject_1_GK = new Crypt_DiffieHellman(
      $subject_1['prime'], $subject_1['generator'], 
      $subject_1['private']);
   $subject_2_GK = new Crypt_DiffieHellman(
      $subject_2['prime'], $subject_2['generator'], 
      $subject_2['private']);
   
   //generate keys      
   $subject_1_GK->generateKeys();
   $subject_2_GK->generateKeys();
   
   //compute the secret keys     
   $subject_1_SK = $subject_1_GK->computeSecretKey(
      $subject_2_GK->getPublicKey())->getSharedSecretKey();
   $subject_2_SK = $subject_2_GK->computeSecretKey(
      $subject_1_GK->getPublicKey())->getSharedSecretKey();
   
   //displaying the secret keys      
   echo('Subject_1_SK:'.$subject_1_SK.'<br />');
   echo('Subject_2_SK:'.$subject_2_SK);
   ?>      

The second example shows you how to generate a secret key using the Diffie Hellman BINARY mode:

   <?php
   
   //include Diffie Hellman functions
   require_once 'Crypt/DiffieHellman.php';
   
   //set the required options for two subjects
   $subject_1 = array('prime' => 
      '9568094558049898340935098349053', 
      'generator'=>'2', 
      'private' => '2232370277237628823279273723742872289398723');
   $subject_2 = array('prime' => 
      '9568094558049898340935098349053', 
      'generator'=>'2', 
      'private' => '0389237288721323987429834389298232433363463');
   
   //apply Diffie Hellman algorithm
   $subject_1_GK = new Crypt_DiffieHellman(
      $subject_1['prime'], $subject_1['generator'], 
      $subject_1['private']);
   $subject_2_GK = new Crypt_DiffieHellman(
      $subject_2['prime'], $subject_2['generator'], 
      $subject_2['private']);
   
   //generate keys    
   $subject_1_GK->generateKeys();
   $subject_2_GK->generateKeys(); 
   
   //compute the secret keys using BINARY mode      
   $subject_1_SK = $subject_1_GK->computeSecretKey(
     $subject_2_GK->getPublicKey(Crypt_DiffieHellman::BINARY), 
     Crypt_DiffieHellman::BINARY)->
     getSharedSecretKey(Crypt_DiffieHellman::BINARY);
   $subject_2_SK = $subject_2_GK->computeSecretKey(
      $subject_1_GK->getPublicKey(Crypt_DiffieHellman::BINARY), 
      Crypt_DiffieHellman::BINARY)->
      getSharedSecretKey(Crypt_DiffieHellman::BINARY);
   
   //display the secret keys    
   echo('subject_1_SK:'.$subject_1_SK.'<br />');
   echo('subject_2_SK:'.$subject_2_SK.'<br />');
   
   ?>

With the wide variety of cryptographic options you've seen here, you should be able to achieve nearly anything you want. Cryptography is a delicate security problem—and as you can see there are many solutions and implementations. The information in this article can help you get started, but beyond that, only experience and hard work with secure systems will help you to choose the cryptographic implementation that represents the perfect compromise between security, speed, and implementation time.

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