wsutil/crc5.c

   1 /* crc5.c
   2  * CRC-5 routine
   3  *
   4  * 2019 Tomasz Mon <desowin@gmail.com>
   5  *
   6  * Wireshark - Network traffic analyzer
   7  * By Gerald Combs <gerald@wireshark.org>
   8  * Copyright 1998 Gerald Combs
   9  *
  10  * SPDX-License-Identifier: GPL-2.0-or-later
  11  */
  12
  13 #include "config.h"
  14
  15 #include <wsutil/crc5.h>
  16
  17 static uint8_t crc5_usb_bits(uint32_t v, int vl, uint8_t ival)
  18 {
  19     /* This function is based on code posted by John Sullivan to Wireshark-dev
  20      * mailing list on Jul 21, 2019.
  21      *
  22      * "One of the properties of LFSRs is that a 1 bit in the input toggles a
  23      *  completely predictable set of register bits *at any point in the
  24      *  future*. This isn't often useful for most CRC caculations on variable
  25      *  sized input, as the cost of working out which those bits are vastly
  26      *  outweighs most other methods."
  27      *
  28      * In USB 2.0, the CRC5 is calculated on either 11 or 19 bits inputs,
  29      * and thus this approach is viable.
  30      */
  31     uint8_t rv = ival;
  32     static const uint8_t bvals[19] = {
  33         0x1e, 0x15, 0x03, 0x06, 0x0c, 0x18, 0x19, 0x1b,
  34         0x1f, 0x17, 0x07, 0x0e, 0x1c, 0x11, 0x0b, 0x16,
  35         0x05, 0x0a, 0x14
  36     };
  37
  38     for (int i = 0; i < vl; i++) {
  39         if (v & (1 << i)) {
  40             rv ^= bvals[19 - vl + i];
  41         }
  42     }
  43     return rv;
  44 }
  45
  46 uint8_t crc5_usb_11bit_input(uint16_t input)
  47 {
  48     return crc5_usb_bits(input, 11, 0x02);
  49 }
  50
  51 uint8_t crc5_usb_19bit_input(uint32_t input)
  52 {
  53     return crc5_usb_bits(input, 19, 0x1d);
  54 }
  55
  56 /*
  57  * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
  58  *
  59  * Local variables:
  60  * c-basic-offset: 4
  61  * tab-width: 8
  62  * indent-tabs-mode: nil
  63  * End:
  64  *
  65  * vi: set shiftwidth=4 tabstop=8 expandtab:
  66  * :indentSize=4:tabSize=8:noTabs=true:
  67  */