bcrypt is a hashing algorithm which is scalable with hardware (via a configurable number of rounds). Its slowness and multiple rounds ensures that an attacker must deploy massive funds and hardware to be able to crack your passwords. Add to that per-password salts (bcrypt REQUIRES salts) and you can be sure that an attack is virtually unfeasible without either ludicrous amount of funds or hardware.

bcrypt uses the Eksblowfish algorithm to hash passwords. While the encryption phase of Eksblowfish and Blowfish are exactly the same, the key schedule phase of Eksblowfish ensures that any subsequent state depends on both salt and key (user password), and no state can be precomputed without the knowledge of both. Because of this key difference, bcrypt is a one-way hashing algorithm. You cannot retrieve the plain text password without already knowing the salt, rounds and key (password). [Source]

How to use bcrypt:

Using PHP >= 5.5-DEV

Password hashing functions have now been built directly into PHP >= 5.5. You may now use password_hash() to create a bcrypt hash of any password:

<?php// Usage 1:echo password_hash('rasmuslerdorf', PASSWORD_DEFAULT)."\n";// $2y$10$xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx// For example:// $2y$10$.vGA1O9wmRjrwAVXD98HNOgsNpDczlqm3Jq7KnEd1rVAGv3Fykk1a// Usage 2:$options = [  'cost' => 11];echo password_hash('rasmuslerdorf', PASSWORD_BCRYPT, $options)."\n";// $2y$11$6DP.V0nO7YI3iSki4qog6OQI5eiO6Jnjsqg7vdnb.JgGIsxniOn4C

To verify a user provided password against an existing hash, you may use the password_verify() as such:

<?php// See the password_hash() example to see where this came from.$hash = '$2y$07$BCryptRequires22Chrcte/VlQH0piJtjXl.0t1XkA8pw9dMXTpOq';if (password_verify('rasmuslerdorf', $hash)) {    echo 'Password is valid!';} else {    echo 'Invalid password.';}

Using PHP >= 5.3.7, < 5.5-DEV (also RedHat PHP >= 5.3.3)

There is a compatibility library on GitHub created based on the source code of the above functions originally written in C, which provides the same functionality. Once the compatibility library is installed, usage is the same as above (minus the shorthand array notation if you are still on the 5.3.x branch).

Using PHP < 5.3.7 (DEPRECATED)

You can use crypt() function to generate bcrypt hashes of input strings. This class can automatically generate salts and verify existing hashes against an input. If you are using a version of PHP higher or equal to 5.3.7, it is highly recommended you use the built-in function or the compat library. This alternative is provided only for historical purposes.

class Bcrypt{  private $rounds;  public function __construct($rounds = 12) {    if (CRYPT_BLOWFISH != 1) {      throw new Exception("bcrypt not supported in this installation. See http://php.net/crypt");    }    $this->rounds = $rounds;  }  public function hash($input){    $hash = crypt($input, $this->getSalt());    if (strlen($hash) > 13)      return $hash;    return false;  }  public function verify($input, $existingHash){    $hash = crypt($input, $existingHash);    return $hash === $existingHash;  }  private function getSalt(){    $salt = sprintf('$2a$%02d$', $this->rounds);    $bytes = $this->getRandomBytes(16);    $salt .= $this->encodeBytes($bytes);    return $salt;  }  private $randomState;  private function getRandomBytes($count){    $bytes = '';    if (function_exists('openssl_random_pseudo_bytes') &&        (strtoupper(substr(PHP_OS, 0, 3)) !== 'WIN')) { // OpenSSL is slow on Windows      $bytes = openssl_random_pseudo_bytes($count);    }    if ($bytes === '' && is_readable('/dev/urandom') &&       ($hRand = @fopen('/dev/urandom', 'rb')) !== FALSE) {      $bytes = fread($hRand, $count);      fclose($hRand);    }    if (strlen($bytes) < $count) {      $bytes = '';      if ($this->randomState === null) {        $this->randomState = microtime();        if (function_exists('getmypid')) {          $this->randomState .= getmypid();        }      }      for ($i = 0; $i < $count; $i += 16) {        $this->randomState = md5(microtime() . $this->randomState);        if (PHP_VERSION >= '5') {          $bytes .= md5($this->randomState, true);        } else {          $bytes .= pack('H*', md5($this->randomState));        }      }      $bytes = substr($bytes, 0, $count);    }    return $bytes;  }  private function encodeBytes($input){    // The following is code from the PHP Password Hashing Framework    $itoa64 = './ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';    $output = '';    $i = 0;    do {      $c1 = ord($input[$i++]);      $output .= $itoa64[$c1 >> 2];      $c1 = ($c1 & 0x03) << 4;      if ($i >= 16) {        $output .= $itoa64[$c1];        break;      }      $c2 = ord($input[$i++]);      $c1 |= $c2 >> 4;      $output .= $itoa64[$c1];      $c1 = ($c2 & 0x0f) << 2;      $c2 = ord($input[$i++]);      $c1 |= $c2 >> 6;      $output .= $itoa64[$c1];      $output .= $itoa64[$c2 & 0x3f];    } while (true);    return $output;  }}

You can use this code like this:

$bcrypt = new Bcrypt(15);$hash = $bcrypt->hash('password');$isGood = $bcrypt->verify('password', $hash);

Alternatively, you may also use the Portable PHP Hashing Framework.

So, you want to use bcrypt? Awesome! However, like other areas of cryptography, you shouldn't be doing it yourself. If you need to worry about anything like managing keys, or storing salts or generating random numbers, you're doing it wrong.

The reason is simple: it's so trivially easy to screw up bcrypt. In fact, if you look at almost every piece of code on this page, you'll notice that it's violating at least one of these common problems.

Face It, Cryptography is hard.

Leave it for the experts. Leave it for people whose job it is to maintain these libraries. If you need to make a decision, you're doing it wrong.

Instead, just use a library. Several exist depending on your requirements.

Libraries

Here is a breakdown of some of the more common APIs.

PHP 5.5 API - (Available for 5.3.7+)

Starting in PHP 5.5, a new API for hashing passwords is being introduced. There is also a shim compatibility library maintained (by me) for 5.3.7+. This has the benefit of being a peer-reviewed and simple to use implementation.

function register($username, $password) {    $hash = password_hash($password, PASSWORD_BCRYPT);    save($username, $hash);}function login($username, $password) {    $hash = loadHashByUsername($username);    if (password_verify($password, $hash)) {        //login    } else {        // failure    }}

Really, it's aimed to be extremely simple.

Resources:

• Documentation: on PHP.net
• Compatibility Library: on GitHub
• PHP's RFC: on wiki.php.net

Zend\Crypt\Password\Bcrypt (5.3.2+)

This is another API that's similar to the PHP 5.5 one, and does a similar purpose.

function register($username, $password) {    $bcrypt = new Zend\Crypt\Password\Bcrypt();    $hash = $bcrypt->create($password);    save($user, $hash);}function login($username, $password) {    $hash = loadHashByUsername($username);    $bcrypt = new Zend\Crypt\Password\Bcrypt();    if ($bcrypt->verify($password, $hash)) {        //login    } else {        // failure    }}

Resources:

• Documentation: on Zend
• Blog Post: Password Hashing With Zend Crypt

PasswordLib

This is a slightly different approach to password hashing. Rather than simply supporting bcrypt, PasswordLib supports a large number of hashing algorithms. It's mainly useful in contexts where you need to support compatibility with legacy and disparate systems that may be outside of your control. It supports a large number of hashing algorithms. And is supported 5.3.2+

function register($username, $password) {    $lib = new PasswordLib\PasswordLib();    $hash = $lib->createPasswordHash($password, '$2y$', array('cost' => 12));    save($user, $hash);}function login($username, $password) {    $hash = loadHashByUsername($username);    $lib = new PasswordLib\PasswordLib();    if ($lib->verifyPasswordHash($password, $hash)) {        //login    } else {        // failure    }}

References:

• Source Code / Documentation: GitHub

PHPASS

This is a layer that does support bcrypt, but also supports a fairly strong algorithm that's useful if you do not have access to PHP >= 5.3.2... It actually supports PHP 3.0+ (although not with bcrypt).

function register($username, $password) {    $phpass = new PasswordHash(12, false);    $hash = $phpass->HashPassword($password);    save($user, $hash);}function login($username, $password) {    $hash = loadHashByUsername($username);    $phpass = new PasswordHash(12, false);    if ($phpass->CheckPassword($password, $hash)) {        //login    } else {        // failure    }}

Resources

• Code: cvsweb
• Project Site: on OpenWall
• A review of the < 5.3.0 algorithm: on StackOverflow

Note: Don't use the PHPASS alternatives that are not hosted on openwall, they are different projects!!!

About BCrypt

If you notice, every one of these libraries returns a single string. That's because of how BCrypt works internally. And there are a TON of answers about that. Here are a selection that I've written, that I won't copy/paste here, but link to:

• Fundamental Difference Between Hashing And Encryption Algorithms - Explaining the terminology and some basic information about them.
• About reversing hashes without rainbow tables - Basically why we should use bcrypt in the first place...
• Storing bcrypt Hashes - basically why is the salt and algorithm included in the hash result.
• How to update the cost of bcrypt hashes - basically how to choose and then maintain the cost of the bcrypt hash.
• How to hash long passwords with bcrypt - explaining the 72 character password limit of bcrypt.
• How bcrypt uses salts
• Best practices of salting and peppering passwords - Basically, don't use a "pepper"
• Migrating old md5 passwords to bcrypt

Wrap Up

There are many different choices. Which you choose is up to you. However, I would HIGHLY recommend that you use one of the above libraries for handling this for you.

Again, if you're using crypt() directly, you're probably doing something wrong. If your code is using hash() (or md5() or sha1()) directly, you're almost definitely doing something wrong.

Just use a library...

You'll get a lot of information in Enough With The Rainbow Tables: What You Need To Know About Secure Password Schemes or Portable PHP password hashing framework.

The goal is to hash the password with something slow, so someone getting your password database will die trying to brute force it (a 10 ms delay to check a password is nothing for you, a lot for someone trying to brute force it). Bcrypt is slow and can be used with a parameter to choose how slow it is.