<?php
/**
 * HOTP Class
 * Based on the work of OAuth, and the sample implementation of HMAC OTP
 * http://tools.ietf.org/html/draft-mraihi-oath-hmac-otp-04#appendix-D
 * @author Jakob Heuser (firstname)@felocity.com
 * @copyright 2011
 * @license BSD
 * @version 1.0
 */
class HOTP {
    /**
     * Generate a HOTP key based on a counter value (event based HOTP)
     * @param string $key the key to use for hashing
     * @param int $counter the number of attempts represented in this hashing
     * @return HOTPResult a HOTP Result which can be truncated or output
     */
    public static function generateByCounter($key, $counter) {
        // the counter value can be more than one byte long,
        // so we need to pack it down properly.
        $cur_counter = array(0, 0, 0, 0, 0, 0, 0, 0);
        for($i = 7; $i >= 0; $i--) {
            $cur_counter[$i] = pack ('C*', $counter);
            $counter = $counter >> 8;
        }
        
        $bin_counter = implode($cur_counter);
        
        // Pad to 8 chars
        if (strlen($bin_counter) < 8) {
            $bin_counter = str_repeat (chr(0), 8 - strlen ($bin_counter)) . $bin_counter;
        }

        // HMAC
        $hash = hash_hmac('sha1', $bin_counter, $key);
        
        return new HOTPResult($hash);
    }
    
    /**
     * Generate a HOTP key based on a timestamp and window size
     * @param string $key the key to use for hashing
     * @param int $window the size of the window a key is valid for in seconds
     * @param int $timestamp a timestamp to calculate for, defaults to time()
     * @return HOTPResult a HOTP Result which can be truncated or output
     */
    public static function generateByTime($key, $window, $timestamp = false) {
        if (!$timestamp && $timestamp !== 0) {
            $timestamp = HOTP::getTime();
        }
        
        $counter = intval($timestamp / $window);
        
        return HOTP::generateByCounter($key, $counter);
    }
    
    /**
     * Generate a HOTP key collection based on a timestamp and window size
     * all keys that could exist between a start and end time will be included
     * in the returned array
     * @param string $key the key to use for hashing
     * @param int $window the size of the window a key is valid for in seconds
     * @param int $min the minimum window to accept before $timestamp
     * @param int $max the maximum window to accept after $timestamp
     * @param int $timestamp a timestamp to calculate for, defaults to time()
     * @return array of HOTPResult
     */
    public static function generateByTimeWindow($key, $window, $min = -1, $max = 1, $timestamp = false) {
        if (!$timestamp && $timestamp !== 0) {
            $timestamp = HOTP::getTime();
        }
        
        $counter = intval($timestamp / $window);
        $window = range($min, $max);
        
        $out = array();
        for ($i = 0; $i < count($window); $i++) {
            $shift_counter = $window[$i];
            $out[$shift_counter] = HOTP::generateByCounter($key, $counter + $shift_counter);
        }
        
        return $out;
    }
    
    /**
     * Gets the current time
     * Ensures we are operating in UTC for the entire framework
     * Restores the timezone on exit.
     * @return int the current time
     */
    public static function getTime() {
        return time(); // PHP's time is always UTC
    }
}

/**
 * The HOTPResult Class converts an HOTP item to various forms
 * Supported formats include hex, decimal, string, and HOTP
 * @author Jakob Heuser (firstname)@felocity.com
 */
class HOTPResult {
    protected $hash;
    protected $binary;
    protected $decimal;
    
    /**
     * Build an HOTP Result
     * @param string $value the value to construct with
     */
    public function __construct($value) {
        // store raw
        $this->hash = $value;
    }
    
    /**
     * Returns the string version of the HOTP
     * @return string
     */
    public function toString() {
        return $this->hash;
    }
    
    /**
     * Returns the hex version of the HOTP
     * @return string
     */
    public function toHex() {
        if( !$this->hex ) 
        {
            $this->hex = dechex($this->toDec());
        }
        return $this->hex;
    }
    
    /**
     * Returns the decimal version of the HOTP
     * @return int
     */
    public function toDec() {
        if( !$this->decimal ) 
        {
            // store calculate decimal
            $hmac_result = array();
        
            // Convert to decimal
            foreach(str_split($this->hash,2) as $hex)
            {
               $hmac_result[] = hexdec($hex);
            }
        
            $offset = $hmac_result[19] & 0xf;
        
            $this->decimal = (
                (($hmac_result[$offset+0] & 0x7f) << 24 ) |
                (($hmac_result[$offset+1] & 0xff) << 16 ) |
                (($hmac_result[$offset+2] & 0xff) << 8 ) |
                ($hmac_result[$offset+3] & 0xff)
            );
        }
        return $this->decimal;
    }
    
    /**
     * Returns the truncated decimal form of the HOTP
     * @param int $length the length of the HOTP to return
     * @return string
     */
    public function toHOTP($length) {
        $str = str_pad($this->toDec(), $length, "0", STR_PAD_LEFT);
        $str = substr($str, (-1 * $length));
        return $str;
    }

}