谷歌在今年年初公布的2因子认证（2FA）的G-邮件提供了一个为Android，iPhone和黑莓的应用程序名为Google的Authenticator生成一个时间登录令牌。 这篇文章将展示如何实现谷歌2FA Web应用程序，以防止被盗的凭据。
谷歌验证器是基于RFC 4226 -基于时间的一次性密码（TOTP）这是使用16位基32（ RFC 4648 ）编码的种子值初始化。 TOTP使用的初始种子可以进入谷歌通过使用QR码相机或通过键盘的Authenticator。 谷歌还提供了一个PAM模块，允许用户集成的sshd 2FA。
可以写一个模块，以支持在任何语言的Google TOTP – 写PHP库唯一需要注意的是缺乏一个RFC 4648兼容的基地32解码功能。 32基本功能是需要解码的初始种子。 这可能是最棘手的部分实施谷歌的2FA。 可以使用以下功能：
<?php
function base32_decode($b32) {
$lut = array("A" => 0, "B" => 1,
"C" => 2, "D" => 3,
"E" => 4, "F" => 5,
"G" => 6, "H" => 7,
"I" => 8, "J" => 9,
"K" => 10, "L" => 11,
"M" => 12, "N" => 13,
"O" => 14, "P" => 15,
"Q" => 16, "R" => 17,
"S" => 18, "T" => 19,
"U" => 20, "V" => 21,
"W" => 22, "X" => 23,
"Y" => 24, "Z" => 25,
"2" => 26, "3" => 27,
"4" => 28, "5" => 29,
"6" => 30, "7" => 31
);
$b32 = strtoupper($b32);
$l = strlen($b32);
$n = 0;
$j = 0;
$binary = "";
for ($i = 0; $i < $l; $i++) {
$n = $n << 5;
$n = $n + $lut[$b32[$i]];
$j = $j + 5;
if ($j >= 8) {
$j = $j - 8;
$binary .= chr(($n & (0xFF << $j)) >> $j);
}
}
return $binary;
}
?>
这个二进制的数据值将被用来生成一个时间标记，以及当前的Unix时间戳在一个SHA1哈希。除以30，口令每30秒改变的Unix时间戳记（基于计数器的当然就是一次换一个啦）。
<?php
function get_timestamp() {
return floor(microtime(true)/30);
}
?>
但是不能只是直入SHA1功能从get_timestamp传递的数量，首先需要的时间戳记将减少到8个字节的二进制字符串，由于数据包不支持64位的整数，我们用两个32位无符号整数，以弥补二进制形式。
<?php
$binary_timestamp = pack('N*', 0) . pack('N*', $timestamp);
?>
一旦你有二进制的seed和你有二进制时间戳传递到他们的“hash_mhac”功能。这给你一个20字节的SHA1字符串。
<?php
$hash = hash_hmac ('sha1', $binary_timestamp, $binary_key, true);
?>
进行哈希处理根据RFC4226获得一次性密码。
<?php
$offset = ord($hash[19]) & 0xf;
$OTP = (
((ord($hash[$offset+0]) & 0x7f) << 24 ) |
((ord($hash[$offset+1]) & 0xff) << 16 ) |
((ord($hash[$offset+2]) & 0xff) << 8 ) |
(ord($hash[$offset+3]) & 0xff)
) % pow(10, 6);
?>
现在TOTP应该包含您的一次性密码。但是仍然有些小问题，如果你想使用这个应用程序，可能有以下的小问题：
您的客户端和服务器的时钟可能不同步 – 这可能意味着，当你来检查您的令牌生成的用户，它会失败。这是你可以规定，客户端和服务器的时钟必须是在完美的同步，或者你需要创建一个函数，检查对那些令牌+ / – 当前服务器时间2分钟。这将允许您的客户端和服务器出长达2分钟，但显然增加了机会，使攻击者能够正确猜测一个时间令牌。
如果没有次数上限，用户可以在猜测令牌，它可能会以蛮力一次性令牌。
如果seed太小了，攻击者可以拦截一些令牌，它可能会蛮力的种子值，允许攻击者产生新的一次性令牌。出于这个原因，谷歌执行最低的16个字符或80位的种子长度。
如果令牌没有标记为无效，因为它已被用于尽快截获令牌的攻击者可能能够快速重新获得。
Google Authenticator: Seed value ‘PEHMPSDNLXIOG65U’
在这里有一个PHP类，实现谷歌TOTP。针对蛮力攻击其失踪的保护，但其他功能齐全。
<?php
<?
/**
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* PHP Google two-factor authentication module.
*
* See http://www.idontplaydarts.com/2011/07/google-totp-two-factor-authentication-for-php/
* for more details
*
* @author Phil
**/
class Google2FA {
const keyRegeneration = 30; // Interval between key regeneration
const otpLength = 6; // Length of the Token generated
private static $lut = array( // Lookup needed for Base32 encoding
"A" => 0, "B" => 1,
"C" => 2, "D" => 3,
"E" => 4, "F" => 5,
"G" => 6, "H" => 7,
"I" => 8, "J" => 9,
"K" => 10, "L" => 11,
"M" => 12, "N" => 13,
"O" => 14, "P" => 15,
"Q" => 16, "R" => 17,
"S" => 18, "T" => 19,
"U" => 20, "V" => 21,
"W" => 22, "X" => 23,
"Y" => 24, "Z" => 25,
"2" => 26, "3" => 27,
"4" => 28, "5" => 29,
"6" => 30, "7" => 31
);
/**
* Generates a 16 digit secret key in base32 format
* @return string
**/
public static function generate_secret_key($length = 16) {
$b32 = "234567QWERTYUIOPASDFGHJKLZXCVBNM";
$s = "";
for ($i = 0; $i < $length; $i++)
$s .= $b32[rand(0,31)];
return $s;
}
/**
* Returns the current Unix Timestamp devided by the keyRegeneration
* period.
* @return integer
**/
public static function get_timestamp() {
return floor(microtime(true)/self::keyRegeneration);
}
/**
* Decodes a base32 string into a binary string.
**/
public static function base32_decode($b32) {
$b32 = strtoupper($b32);
if (!preg_match('/^[ABCDEFGHIJKLMNOPQRSTUVWXYZ234567]+$/', $b32, $match))
throw new Exception('Invalid characters in the base32 string.');
$l = strlen($b32);
$n = 0;
$j = 0;
$binary = "";
for ($i = 0; $i < $l; $i++) {
$n = $n << 5; // Move buffer left by 5 to make room
$n = $n + self::$lut[$b32[$i]]; // Add value into buffer
$j = $j + 5; // Keep track of number of bits in buffer
if ($j >= 8) {
$j = $j - 8;
$binary .= chr(($n & (0xFF << $j)) >> $j);
}
}
return $binary;
}
/**
* Takes the secret key and the timestamp and returns the one time
* password.
*
* @param binary $key - Secret key in binary form.
* @param integer $counter - Timestamp as returned by get_timestamp.
* @return string
**/
public static function oath_hotp($key, $counter)
{
if (strlen($key) < 8)
throw new Exception('Secret key is too short. Must be at least 16 base 32 characters');
$bin_counter = pack('N*', 0) . pack('N*', $counter); // Counter must be 64-bit int
$hash = hash_hmac ('sha1', $bin_counter, $key, true);
return str_pad(self::oath_truncate($hash), self::otpLength, '0', STR_PAD_LEFT);
}
/**
* Verifys a user inputted key against the current timestamp. Checks $window
* keys either side of the timestamp.
*
* @param string $b32seed
* @param string $key - User specified key
* @param integer $window
* @param boolean $useTimeStamp
* @return boolean
**/
public static function verify_key($b32seed, $key, $window = 4, $useTimeStamp = true) {
$timeStamp = self::get_timestamp();
if ($useTimeStamp !== true) $timeStamp = (int)$useTimeStamp;
$binarySeed = self::base32_decode($b32seed);
for ($ts = $timeStamp - $window; $ts <= $timeStamp + $window; $ts++)
if (self::oath_hotp($binarySeed, $ts) == $key)
return true;
return false;
}
/**
* Extracts the OTP from the SHA1 hash.
* @param binary $hash
* @return integer
**/
public static function oath_truncate($hash)
{
$offset = ord($hash[19]) & 0xf;
return (
((ord($hash[$offset+0]) & 0x7f) << 24 ) |
((ord($hash[$offset+1]) & 0xff) << 16 ) |
((ord($hash[$offset+2]) & 0xff) << 8 ) |
(ord($hash[$offset+3]) & 0xff)
) % pow(10, self::otpLength);
}
}
$InitalizationKey = "PEHMPSDNLXIOG65U"; // Set the inital key
$TimeStamp = Google2FA::get_timestamp();
$secretkey = Google2FA::base32_decode($InitalizationKey); // Decode it into binary
$otp = Google2FA::oath_hotp($secretkey, $TimeStamp); // Get current token
echo("Init key: $InitalizationKey\n");
echo("Timestamp: $TimeStamp\n");
echo("One time password: $otp\n");
// Use this to verify a key as it allows for some time drift.
$result = Google2FA::verify_key($InitalizationKey, "123456");
var_dump($result);
?>
你可以检查其工作由安装谷歌的Authenticator应用QR码和扫描的权利 – 由应用程序生成的代码应匹配类生成的代码。
这个function Google2FA::verify_key 应该用来验证用户一次令牌，因为它允许客户端时钟漂移2分钟服务器时间两侧。