最近看到的一篇文章，作者详细讲解了加盐哈希密码，http://blog.jobbole.com/61872/#php，深有感触，结合一些其他的见闻，写点东西。
首先，大部分网站都不会直接存储用户的明文密码。因为一旦数据库暴露，后果很严重。所以一般都会进行哈希加密。哈希加密是一种单向的加密算法，任何长度的输入，都会输出一个固定长度的指纹。常见的有MD5,sha1等函数。但随着技术的飞速发展，这种算法已经出现很多安全的漏洞，甚至可以很轻易的破解。比如常见的有字典法，查表法，彩虹表等，说白了就是去不断的去试，直到得到正确的结果。
于是，加盐哈希的概念就引出了，在对密码哈希前，再随机生成一段字符串，混入密码中再进行加密。这个随机的字串称作为“盐”，这个“盐”需要同密码一起保存进数据库，以便验证。
看似简单，但是如何加盐，又有很多误区，比如说：
使用一样的盐值：这样做两个一样的密码就生成了一样的哈希密码。一旦泄露，破解者一样可以用字典法等进行暴力破解，只不过带上这个不变的盐值。
短盐值：如果盐值太短，攻击者可以构造一个查询表包含所有可能的盐值。以只有3个ASCII字符的盐值为例，一共有95x95x95=857,375种可能。这看起来很多，但是如果对于每个盐值查询表只包含1MB最常见的密码，那么总共只需要837GB的储存空间。一个不到100美元的1000GB硬盘就能解决问题。
组合使用不同的哈希函数：这个其实是有争议的，但是基本上是否定的意见了。
这里有些低端的组合哈希函数，我在网上某些论坛看到它们被推荐使用：
- md5(sha1(password))
- md5(md5(salt) + md5(password))
- sha1(sha1(password))
- sha1(str_rot13(password + salt))
- md5(sha1(md5(md5(password) + sha1(password)) + md5(password)))
不要使用其中任何一种。
那如何正确使用加盐哈希密码呢？
盐值应该使用基于加密的伪随机数生成器（Cryptographically Secure Pseudo-Random Number Generator – CSPRNG）来生成，该生成专门被设计用于加密，所以生成随机字串具有很高的随机性和不可预测性，下面列出一些主流语言对应的加密函数：
关于加盐哈希的几个建议:
- 盐值至少哈希函数的输出一样长
- 盐值保存进数据库
- 永远在服务端进行加密和验证
应该使用那些加密函数：
- 本文末尾的PHP source code, Java source code, C# source code or the Ruby source code
- OpenWall的Portable PHP password hashing framework，即phpass类。
- 任何先进的、被良好测试过的哈希加密算法，比如SHA256，SHA512，RipeMD，WHIRLPOOL，SHA3等等
- 设计良好的密钥扩展算法，如PBKDF2，bcrypt，scrypt
- 安全的crypt()版本（$2y$，$5$，$6$）
- pass_hash()等函数，5.5.0版本以上。
关于phpass类，给出一个简单的例子：
主要有几个函数
- 构造函数
- 生成哈希密码 HashPassword($str)
- 验证密码 CheckPassword($hashedpass);
<?php
// 引入passwordhash.php
require "PasswordHash.php";
// 初始化散列器为不可移植(这样更安全)
$hash = new PasswordHash(8, false);
$str = "hello world";
$pass = $hash->HashPassword($str);
$hash->CheckPassword("hellow world", $pass); //true
贴出一份代码php版本的，下载：
<?php
/*
* Password Hashing With PBKDF2 (http://crackstation.net/hashing-security.htm).
* Copyright (c) 2013, Taylor Hornby
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
// These constants may be changed without breaking existing hashes.
define("PBKDF2_HASH_ALGORITHM", "sha256");
define("PBKDF2_ITERATIONS", 1000);
define("PBKDF2_SALT_BYTE_SIZE", 24);
define("PBKDF2_HASH_BYTE_SIZE", 24);
define("HASH_SECTIONS", 4);
define("HASH_ALGORITHM_INDEX", 0);
define("HASH_ITERATION_INDEX", 1);
define("HASH_SALT_INDEX", 2);
define("HASH_PBKDF2_INDEX", 3);
function create_hash($password)
{
// format: algorithm:iterations:salt:hash
$salt = base64_encode(mcrypt_create_iv(PBKDF2_SALT_BYTE_SIZE, MCRYPT_DEV_URANDOM));
return PBKDF2_HASH_ALGORITHM . ":" . PBKDF2_ITERATIONS . ":" . $salt . ":" .
base64_encode(pbkdf2(
PBKDF2_HASH_ALGORITHM,
$password,
$salt,
PBKDF2_ITERATIONS,
PBKDF2_HASH_BYTE_SIZE,
true
));
}
function validate_password($password, $correct_hash)
{
$params = explode(":", $correct_hash);
if(count($params) < HASH_SECTIONS)
return false;
$pbkdf2 = base64_decode($params[HASH_PBKDF2_INDEX]);
return slow_equals(
$pbkdf2,
pbkdf2(
$params[HASH_ALGORITHM_INDEX],
$password,
$params[HASH_SALT_INDEX],
(int)$params[HASH_ITERATION_INDEX],
strlen($pbkdf2),
true
)
);
}
// Compares two strings $a and $b in length-constant time.
function slow_equals($a, $b)
{
$diff = strlen($a) ^ strlen($b);
for($i = 0; $i < strlen($a) && $i < strlen($b); $i++)
{
$diff |= ord($a[$i]) ^ ord($b[$i]);
}
return $diff === 0;
}
/*
* PBKDF2 key derivation function as defined by RSA's PKCS #5: https://www.ietf.org/rfc/rfc2898.txt
* $algorithm - The hash algorithm to use. Recommended: SHA256
* $password - The password.
* $salt - A salt that is unique to the password.
* $count - Iteration count. Higher is better, but slower. Recommended: At least 1000.
* $key_length - The length of the derived key in bytes.
* $raw_output - If true, the key is returned in raw binary format. Hex encoded otherwise.
* Returns: A $key_length-byte key derived from the password and salt.
*
* Test vectors can be found here: https://www.ietf.org/rfc/rfc6070.txt
*
* This implementation of PBKDF2 was originally created by https://defuse.ca
* With improvements by http://www.variations-of-shadow.com
*/
function pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)
{
$algorithm = strtolower($algorithm);
if(!in_array($algorithm, hash_algos(), true))
trigger_error('PBKDF2 ERROR: Invalid hash algorithm.', E_USER_ERROR);
if($count <= 0 || $key_length <= 0)
trigger_error('PBKDF2 ERROR: Invalid parameters.', E_USER_ERROR);
if (function_exists("hash_pbkdf2")) {
// The output length is in NIBBLES (4-bits) if $raw_output is false!
if (!$raw_output) {
$key_length = $key_length * 2;
}
return hash_pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output);
}
$hash_length = strlen(hash($algorithm, "", true));
$block_count = ceil($key_length / $hash_length);
$output = "";
for($i = 1; $i <= $block_count; $i++) {
// $i encoded as 4 bytes, big endian.
$last = $salt . pack("N", $i);
// first iteration
$last = $xorsum = hash_hmac($algorithm, $last, $password, true);
// perform the other $count - 1 iterations
for ($j = 1; $j < $count; $j++) {
$xorsum ^= ($last = hash_hmac($algorithm, $last, $password, true));
}
$output .= $xorsum;
}
if($raw_output)
return substr($output, 0, $key_length);
else
return bin2hex(substr($output, 0, $key_length));
}
?>
