php calculated CRC-CCITT (XModem)

Question

I'm trying to implement a CRC-CCITT (XModem) check in php without success. Does anyone know how to do it? I expected crc16('test') will return 0x9B06.

Answer 1

Here is a simple bit-wise calculation of the XMODEM 16-bit CRC, in C:

#include <stdint.h>

unsigned crc16xmodem_bit(unsigned crc, void const *data, size_t len) {
    if (data == NULL)
        return 0;
    while (len--) {
        crc ^= (unsigned)(*(unsigned char const *)data++) << 8;
        for (unsigned k = 0; k < 8; k++)
            crc = crc & 0x8000 ? (crc << 1) ^ 0x1021 : crc << 1;
    }
    crc &= 0xffff;
    return crc;
}

This was generated by my crcany software, which also generates byte-wise and word-wise versions for speed.

This can be easily converted to php.