CRC32 with custom Algorithm

1

Hi,
i would have a crc code with the polynomial as follows:
0xEDB88320
I tried to customize the crc32 algorithm using a predefined crate:

const CUSTOM_ALG: Algorithm<u32> = Algorithm {
    width: 32,
    poly: 0xEDB88320,
    init: 0x00000000,
    refin: false,
    refout: false,
    xorout: 0xffffffff,
    check: 0x00000000,
    residue: 0x00000000,
};

fn crc_gen(){
    let crc = Crc::<u32>::new(&CUSTOM_ALG);
    let mut digest = crc.digest();
     digest.update(&[0xDA; 0x16;0x22;0xB4]); //DA 16 22 B4 in bytes
     let mycrc_calc = digest.finalize();
     println!("my first byte {:x}",218 as u8); // 
    println!("crc final {:x}",mycrc_calc);
}

So, when i obtain the result it is wrong. I don't undersrtand if the customize algorithm is uncorrect or it is uncorrect other parts.

2

What is the expected output? What library are you using to obtain the expected output?

3

the solution is 96 68 B8 1B.

i'm using this use crc::{Crc, Algorithm};

4

I get the same result from your code as from an online JavaScript CRC calculator (0x6F6EE1BF). I would reconsider why you expect the output that you do, it seems unlikely that two unrelated implementations are wrong in the same exact way (though not impossible). I did modify your code a little before running it, because it doesn't compile as is:

error: literal out of range for `u8`
  --> src/main.rs:17:21
   |
17 |     digest.update(&[0xDA1622B4]); //DA 16 22 B4 in bytes
   |                     ^^^^^^^^^^
   |
   = note: the literal `0xDA1622B4` (decimal `3658883764`) does not fit into the type `u8` and will become `180u8`
   = help: consider using the type `u32` instead
   = note: `#[deny(overflowing_literals)]` on by default

So I changed the input to &[0xDA, 0x16, 0x22, 0xB4] instead. Though I don't get 96 68 B8 1B either way so this probably isn't the root of the issue.

1 Like
5

So it looks like your 0xedb88320 is not a normal polynomial but a reversed polynomial. The corresponding normal polynomial is 0x04c11db7, perhaps the most common CRC-32 polynomial. Therefore, you can use one of the predefined algorithms for this:

use crc::{Crc, CRC_32_ISO_HDLC};

fn main() {
    let crc = Crc::<u32>::new(&CRC_32_ISO_HDLC);
    let checksum = crc.checksum(&[0xda, 0x16, 0x22, 0xb4]);
    println!("{checksum:#x}");
}

This prints 0x1bb86896, which in little-endian order is 96 68 B8 1B, as expected. (To get the checksum in little-endian form, you can call checksum.to_le_bytes().)

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6

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