Custom CRC16 with bit reflection

Question

To expand and give explanation that may also go with the link posted above, here is a function in PHP for getting a CRC16 checksum for string data:

    function crc16($data) {
        $crc = 0xAC6F;
        $len = strlen($data);
        $i = 0;
        while ($len--) {
            $crc ^= reversebyte(ord($data[$i++])) << 8;
            $crc &= 0xffff;
            for ($j = 0; $j < 8; $j++){
                $crc = ($crc & 0x8000) ? ($crc << 1) ^ 0x8005 : $crc << 1;
                $crc &= 0xffff;
            }
        }
        $crc ^= 0x0000;
        $crc = reversebits($crc);
        return $crc;
    }

    function reversebyte($byte) {
        $ob = 0;
        $b = (1 << 7);
        for ($i = 0; $i <= 7; $i++) {
            if (($byte & $b) !== 0) {
                $ob |= (1 << $i);
            }
            $b >>= 1;
        }
        return $ob;
    }

    function reversebits($cc) {
        $ob = 0;
        $b = (1 << 15);
        for ($i = 0; $i <= 15; $i++) {
            if (($cc & $b) !== 0) {
                $ob |= (1 << $i);
            }
            $b >>= 1;
        }
        return $ob;
    }

    Poly = 0x8005
    Init = 0xAC6F
    Xor = 0x0000
    Refin = True
    Refout = True

If you don't want to add reflection then replace these lines in function crc16():

Replace Refin = True:

    $crc ^= reversebyte(ord($data[$i++])) << 8;

With Refin = False:

    $crc ^= ord($data[$i++]) << 8;

Replace Refout = True:

    $crc = reversebits($crc);

with Refout = False (remove this whole line):

    $crc = reversebits($crc); //remove me

Since you've done no effort to solve the problem yourself - what's your budget? — h2ooooooo, Apr 14 '14 at 09:46
Do you have any examples what they should output if they're checksummed correctly? If so, add it to the OP. (`foo` gives `xxx`, `bar` gives `yyy`) — h2ooooooo, Apr 14 '14 at 10:18
Can you also check out what settings would give correct results? http://www.zorc.breitbandkatze.de/crc.html As far as I can tell none of them give the correct result according to what you've said they should give. — h2ooooooo, Apr 14 '14 at 10:46

score 0 · Answer 1 · answered Apr 14 '14 at 11:20

To write your own CRC function, I recommend you read The Painless Guide to CRC Error Detection Algorithms. It provides pseudo-code for different kind of implementations, including a simple, naive one and improved table-driven implementations. The guide also covers your requirements such as refin, refout and so on.

The only real difficulty left is then to translate the pseudo-code or the C code to php, but since php has just the same bitwise operators as C, this shouldn't really be a challenge.

Custom CRC16 with bit reflection

1 Answers1