How to calculate CRC16 CCITT in PHP HEX?

Question

I'm trying to use a PHP CRC16 CCITT function to calculate the checksum.

A device sends me a PACKET with Checksum included:

10 00 00 00 00 00 00 00 12 51 09 08 00 18 00 04 02 14 00 0c 00 0c 02 1c 00 02 00 00 00 00 00 00 a0 77

The checksum is at the end: a0 77

I've tried using

How to calculate crc16 in php

Convert C to PHP for CRC16 Function

With no success, the CRC 16 Calculations return: E6 F4 instead of a0 77

I get the correct Hex information returned, when I lookup:

100000000000000012510908001800040214000c000c021c0002000000000000

on the website http://www.lammertbies.nl/comm/info/crc-calculation.html but I cannot reproduce it. (make sure to select the input type to HEX)

Can you please help me figure out how to get the crc16 CCITT of the string of hex values

100000000000000012510908001800040214000c000c021c0002000000000000

I'm looking for the checksum a0 77

Answer 1

I was able to produce the same checksum with implementation like below:

define('CRC16POLYN', 0x1021);

function CRC16Normal($buffer) {
    $result = 0xFFFF;
    if (($length = strlen($buffer)) > 0) {
        for ($offset = 0; $offset < $length; $offset++) {
            $result ^= (ord($buffer[$offset]) << 8);
            for ($bitwise = 0; $bitwise < 8; $bitwise++) {
                if (($result <<= 1) & 0x10000) $result ^= CRC16POLYN;
                $result &= 0xFFFF;
            }
        }
    }
    return $result;
}

echo dechex(CRC16Normal(hex2bin('100000000000000012510908001800040214000c000c021c0002000000000000')));

Above gives a077 on output.

Code snippet found on https://forums.digitalpoint.com/threads/php-define-function-calculate-crc-16-ccitt.2584389/

Answer 2

I found out from the Deplhi function how it works. It gets an byte Array, instead of Hex string for doing the job. Here is the code:

// $commands = [0x35, 0x02, 0x02, 0x00, 0x10, 0x03];       // => 0x5ba3
$commands = [0x44, 0x02, 0x02, 0x01, 0x10, 0x03];       // => 0x55c0
var_dump(dechex(getChecksum($commands)));

function getChecksum($byteArray) {
    $polynom = 0x8408;
    $in_crc = 0x0000;
    for ($i = 0; $i < sizeof($byteArray); $i++) {
        for ($n = 0; $n < 8; $n++) {
            if((($byteArray[$i] & 0x0001) ^ $in_crc) & 0x0001) 
                $in_crc = ($in_crc >> 1) ^ $polynom;
            else 
                $in_crc = $in_crc >> 1;
            $byteArray[$i] = $byteArray[$i] >> 1;
        }
        $result = $in_crc;
    }
    return $result;
}

The solution can be proved on this Online CRC Calculator. The Algorithm used is CRC-16/KERMIT.

Answer 3

So I was looking for how to calculate checksum according to [ISO/IEC 13239] using the polynomial '1021' (hex) and initial value 'FFFF' (hex). I found this link which can calculate many types of CRC. After submitting my input, I wanted the value of the field CRC-CCITT (0xFFFF), which is something like this:

So I took the JS function (in the .js file, the function name is CRCFFFF ) that calculated the CCITT, and created the PHP version of it.

function crcChecksum($str) {
    // The PHP version of the JS str.charCodeAt(i)
    function charCodeAt($str, $i) {
        return ord(substr($str, $i, 1));
    }

    $crc = 0xFFFF;
    $strlen = strlen($str);
    for($c = 0; $c < $strlen; $c++) {
        $crc ^= charCodeAt($str, $c) << 8;
        for($i = 0; $i < 8; $i++) {
            if($crc & 0x8000) {
                $crc = ($crc << 1) ^ 0x1021;
            } else {
                $crc = $crc << 1;
            }
        }
    }
    $hex = $crc & 0xFFFF;
    $hex = dechex($hex);
    $hex = strtoupper($hex);

    return $hex;
}

$result = crcChecksum('replace with your string here');// In my case, this gave me the desired output, which is '627B' (without '0x')

In case you need the '0x', just add it:

$result = '0x' . crcChecksum('replace with your string here');// result = '0x627B'