How do I detect if there is a CRC in a sequence of bytes? [closed]

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Here's the data I have, I'm not sure the last 2 bytes are the CRC but it's my best guess. I've tried CRC-RevEng with no luck. What would be the best approach to try to figure out if the last 2 bytes are a CRC for sure? and then try to crack it?

Each line is a different hex string with the las 2 bytes being the CRC (my guess).

305B80DB3D009F04D9D848000110097C
305BA9983E00A304D9D852000110F3A3
305CA40340009F04D9D83C0001108730
305D364741009F04D9D84A000110C4F1
3066380443009F04CD883A000110B6B7
3066364842000254FFFF54870100528A
30CE692D4400002AFFFF0C870300F85B
30487DA239000254FFFF2495000006FD
305B7E6A3C000254FFFFC4A4000076FA
305C7D063F000254FFFF64B400005D30
30E2570E78009A08400017000110195D
30E4558679009904400030000110846C
30E16A7575009F04400037000110031A
30E18AD27600A30440005000011074E7
30E23B0D77009704400001000110FFEE
30E4558679009904400030000110846C

Do you have a way to generate buffers for different inputs? If so, you could study how the last two bytes change based on the inputs, and potentially guess the algorithm used to calculate them. — Jeremy Friesner
– Jeremy Friesner, Commented Jul 16, 2024 at 4:55
Of course, if you have the program that generates the bytes running on your own computer (and you don't mind getting your hands dirty) you could run the program in a debugger and step through the code it uses to generate the buffer, and probably directly observe the algorithm that generates the last two bytes. (Decompiling that algorithm from machine-code/assembly into C or similar would be the next step after that) — Jeremy Friesner
– Jeremy Friesner, Commented Jul 16, 2024 at 4:59
Sadly I do not. I think the beginning of the strings are a timestamp of sorts but I have no control over the data. — user737329
– user737329, Commented Jul 16, 2024 at 5:00
Well, it's hard to determine much from a single example; there are an infinite number of algorithms that could have produced that sequence. — Jeremy Friesner
– Jeremy Friesner, Commented Jul 16, 2024 at 5:02
I edited the post there to clarify, each line is a different hex string that I think it has a CRC in the last 2 bytes, that's why I think the last one may give a clue since it's all zeroes until the CRC. — user737329
– user737329, Commented Jul 16, 2024 at 5:11

Mark Adler · Accepted Answer · 2024-07-16 16:16:27Z

0

Yes, the last two bytes are a 16-bit CRC. This will compute it (in C):

#include <stddef.h>
#include <stdint.h>

uint16_t crc16(uint8_t const *data, size_t len) {
    uint16_t crc = 0;
    for (size_t i = 0; i < len; i++) {
        crc ^= (uint16_t)data[i] << 8;
        for (int k = 0; k < 8; k++)
            crc = crc & 0x8000 ? (crc << 1) ^ 0x1021 : crc << 1;
    }
    return crc ^ 0xa96a;
}

The result is placed at the end of the message in little-endian order.

Since all of the messages are of the same length, it is not possible to tell what both the initial value and the final exclusive-or are. I arbitrarily set the initial value to zero. If you have one message of a different length with the CRC, then those two can be solved for.

edited Jul 16, 2024 at 16:16

answered Jul 16, 2024 at 16:10

Mark Adler

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