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/* crc5.c
* CRC-5 routine
*
* 2019 Tomasz Mon <desowin@gmail.com>
*
* Wireshark - Network traffic analyzer
* By Gerald Combs <gerald@wireshark.org>
* Copyright 1998 Gerald Combs
*
* SPDX-License-Identifier: GPL-2.0-or-later
*/
#include "config.h"
#include <glib.h>
#include <wsutil/crc5.h>
static guint8 crc5_usb_bits(guint32 v, int vl, guint8 ival)
{
/* This function is based on code posted by John Sullivan to Wireshark-dev
* mailing list on Jul 21, 2019.
*
* "One of the properties of LFSRs is that a 1 bit in the input toggles a
* completely predictable set of register bits *at any point in the
* future*. This isn't often useful for most CRC caculations on variable
* sized input, as the cost of working out which those bits are vastly
* outweighs most other methods."
*
* In USB 2.0, the CRC5 is calculated on either 11 or 19 bits inputs,
* and thus this approach is viable.
*/
guint8 rv = ival;
static const guint8 bvals[19] = {
0x1e, 0x15, 0x03, 0x06, 0x0c, 0x18, 0x19, 0x1b,
0x1f, 0x17, 0x07, 0x0e, 0x1c, 0x11, 0x0b, 0x16,
0x05, 0x0a, 0x14
};
for (int i = 0; i < vl; i++) {
if (v & (1 << i)) {
rv ^= bvals[19 - vl + i];
}
}
return rv;
}
guint8 crc5_usb_11bit_input(guint16 input)
{
return crc5_usb_bits(input, 11, 0x02);
}
guint8 crc5_usb_19bit_input(guint32 input)
{
return crc5_usb_bits(input, 19, 0x1d);
}
/*
* Editor modelines - https://www.wireshark.org/tools/modelines.html
*
* Local variables:
* c-basic-offset: 4
* tab-width: 8
* indent-tabs-mode: nil
* End:
*
* vi: set shiftwidth=4 tabstop=8 expandtab:
* :indentSize=4:tabSize=8:noTabs=true:
*/
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