A PHP implementation of Google two-factor authentication.

class.Google2FA.php

<?
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);
	}
}