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

Answer 1

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'

Answer 2

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 3

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.