Sample code sharing for implementing OATH dynamic password algorithm in PHP

<?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(&#39;/^[ABCDEFGHIJKLMNOPQRSTUVWXYZ234567]+$/&#39;, $b32, $match))  
            throw new Exception(&#39;Invalid characters in the base32 string.&#39;);  
  
        $l     = strlen($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(&#39;Secret key is too short. Must be at least 16 base 32 characters&#39;);  
  
        $bin_counter = pack(&#39;N*&#39;, 0) . pack(&#39;N*&#39;, $counter);        // Counter must be 64-bit int  
        $hash      = hash_hmac (&#39;sha1&#39;, $bin_counter, $key, true);  
  
        return str_pad(self::oath_truncate($hash), self::otpLength, &#39;0&#39;, 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 = "LFLFMU2SGVCUIUCZKBMEKRKLIQ";                    // 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);  
  
?>