TOTP two factor authentication using PHP

I needed to implemented TOTP two-factor authentication in a project.
After searching a found a php class somewhere and used it in my project.

Time based one time password works based on current timestamp.
If you face problem time problem in your maching, you can fetch timestamp from ntp server.
For fetching timestamp from ntp server, see my previous post.

I am writing the class here so you can use it.

class TOTP {

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) + 10) /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 = "M5UIUKQBTRCU7IQG"; // Set the inital key

$TimeStamp  = TOTP::get_timestamp();

$secretkey  = TOTP::base32_decode($InitalizationKey); // Decode it into binary

$otp        = TOTP::oath_hotp($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.

$result = TOTP::verify_key($InitalizationKey, "123456");

var_dump($result);