# CRC-32 can't get the correct check sum - any help appreciated

**URL:** <https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627>\
**Category:** General\
**Created:** [16 April 2023 11:32 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627 "2023-04-16T11:32:17Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![dr\_cave\_dweller](https://avatars.discourse-cdn.com/v4/letter/d/71e660/32.png) [@dr\_cave\_dweller](https://discourse.nodered.org/u/dr_cave_dweller)\
**Post date:** [16 April 2023 11:32 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/1 "2023-04-16T11:32:17Z")

</div>

HI, I am trying to write a function node to output CRC-32 values based on a Hex value input, but the calculated CRC values are incorrect. I had no trouble writing function nodes for CRC-8 and CRC-16 so I am not sure why this is not working. I am sure my mistake is something silly.

Here is the Hex value I am trying to find the check sum of \> 01C80101C900000000000000. It is input from an inject node in a buffer.

If I put this Hex value it into an online calculator I get the value of 0x664A6454 (edited the checksum as per below) using CRC-32 algorithm \>\> [https://crccalc.com/](https://crccalc.com/)

This is the function node I have written

```auto

buffer = msg.payload;
var crc = 0xFFFFFFFF; //start crc value
var poly = 0x04C11DB7; //polynomial 
var odd;

for (var i = 0; i < buffer.length; i++) {
    crc = crc ^ buffer[i];

    for (var j = 0; j < 8; j++) {
        odd = crc & 0x0001;
        crc = crc >> 1;
        if (odd) {
            crc = crc ^ poly;
        }
    }
}

```

I have tried a few different variations of the CRC-32 code I have found online, like this, but answer is the same.

```auto
buffer = msg.payload;
var crc = 0xFFFFFFFF; //start crc value
var poly = 0x04C11DB7; //polynomial 

for (var i = 0; i < buffer.length; i++) {
    crc = crc ^ buffer[i];
    for (j = 7; j >= 0; j--) {   
         mask = -(crc & 1);
         crc = (crc >> 1) ^ (poly & mask);
      }
}

```

So I must be doing something incorrect on the node-red side, but I can't figure it out. Thanks!

---

<div class="post-metadata">

**Author:** ![edje11](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/edje11/32/572_2.png) [@edje11](https://discourse.nodered.org/u/edje11)\
**Post date:** [16 April 2023 14:02 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/2 "2023-04-16T14:02:26Z")

</div>

I dont get the checksum you mentioned.

> **[Online CRC-8 CRC-16 CRC-32 Calculator](https://crccalc.com/?crc=01C80101C900000000000000&method=crc32&datatype=hex&outtype=0)**
>
> Calculate CRC-8, CRC-16, CRC-32 checksums online

The outcome is different for me.  
What crc type do you need?

---

<div class="post-metadata">

**Author:** ![dr\_cave\_dweller](https://avatars.discourse-cdn.com/v4/letter/d/71e660/32.png) [@dr\_cave\_dweller](https://discourse.nodered.org/u/dr_cave_dweller)\
**Post date:** [16 April 2023 19:05 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/3 "2023-04-16T19:05:16Z")

</div>

Okay, I had the input selected as ASCII instead of HEX, these are the CRC check sums. I am agnostic on which CRC to use.

For reference the output I am getting from my function node is [FB D8 F5 09] which doesn't match any of these

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/7/a/7a182a46579ee984f14bbe0dcada808383f9cd3d.png)

---

<div class="post-metadata">

**Author:** ![dr\_cave\_dweller](https://avatars.discourse-cdn.com/v4/letter/d/71e660/32.png) [@dr\_cave\_dweller](https://discourse.nodered.org/u/dr_cave_dweller)\
**Post date:** [18 April 2023 22:50 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/4 "2023-04-18T22:50:45Z")

</div>

Any help appreciated. Thanks

---

<div class="post-metadata">

**Author:** ![Steve-Mcl](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/steve-mcl/32/4826_2.png) [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)\
**Post date:** [19 April 2023 09:03 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/5 "2023-04-19T09:03:16Z")

</div>

Based on one of the many examples in this forum: [How can CRC16 XModem accept Hex Input - #2 by Steve-Mcl](https://discourse.nodered.org/t/how-can-crc16-xmodem-accept-hex-input/72466/2)

A common approach is to use a well established lib. e.g. `easy-crc`.

#### Demo of `easy-crc`

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/7/d/7d2a12bc93b47425f60afab8ff8e62c1516d76ea.png)

#### Flow (use `CTRL-I` to import)

```auto
[{"id":"e2beeb574d982801","type":"inject","z":"b0865f2b12f5f1a6","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"01C80101C900000000000000","payloadType":"str","x":1090,"y":140,"wires":[["4bf21b40a1445e15"]]},{"id":"4bf21b40a1445e15","type":"function","z":"b0865f2b12f5f1a6","name":"easy-crc","func":"//See https://www.npmjs.com/package/easy-crc?activeTab=readme for list of supported algorithms\n\n/*\nSupported CRC32 algorithms:\nCRC-32\nCRC-32C\nCRC-32D\nCRC-32Q\nBZIP2\nJAMCRC\nMPEG-2\nPOSIX\nXFER\n*/\n\nconst { crc32 } = easyCrc;\nconst data = Buffer.from(msg.payload, 'hex');\nmsg.payload = {} \n\nmsg.payload.CRC32 = crc32('CRC-32', data)\nmsg.payload.CRC32C = crc32('CRC-32C', data)\nmsg.payload.CRC32D = crc32('CRC-32D', data)\nmsg.payload.CRC32Q = crc32('CRC-32Q', data)\nmsg.payload.BZIP2 = crc32('BZIP2', data)\nmsg.payload.JAMCRC = crc32('JAMCRC', data)\nmsg.payload.MPEG2 = crc32('MPEG-2', data)\nmsg.payload.POSIX = crc32('POSIX', data)\nmsg.payload.XFER = crc32('XFER', data)\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[{"var":"easyCrc","module":"easy-crc"}],"x":1100,"y":200,"wires":[["25cd665db5eaa0d9"]]},{"id":"25cd665db5eaa0d9","type":"debug","z":"b0865f2b12f5f1a6","name":"","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"payload","statusType":"auto","x":1130,"y":260,"wires":[]}]

```

  

If you are adverse to using NPM modules in your function nodes then here is a hand coded "CRC-32"

 ![image](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/8/0/8020de7dd8a8810b3dc76e9b15218d99cb94e8a2.png)

```auto
[{"id":"8cfbeeef9e2ee878","type":"inject","z":"b0865f2b12f5f1a6","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"01C80101C900000000000000","payloadType":"str","x":1090,"y":400,"wires":[["d72681dcdc9878a5"]]},{"id":"d72681dcdc9878a5","type":"function","z":"b0865f2b12f5f1a6","name":"hand coded CRC32","func":"\nconst buffer = Buffer.from(msg.payload, 'hex');\n\nconst makeCRCTable = function () {\n let c;\n const crcTable = [];\n for (let n = 0; n < 256; n++) {\n c = n;\n for (let k = 0; k < 8; k++) {\n c = ((c & 1) ? (0xEDB88320 ^ (c >>> 1)) : (c >>> 1));\n }\n crcTable[n] = c;\n }\n return crcTable;\n}\n\nconst crc32 = function (buffer) {\n const crcTable = context.crcTable || (context.crcTable = makeCRCTable());\n context.crcTable = crcTable\n let crc = 0 ^ (-1);\n for (let i = 0; i < buffer.length; i++) {\n crc = (crc >>> 8) ^ crcTable[(crc ^ buffer[i]) & 0xFF];\n }\n return (crc ^ (-1)) >>> 0;\n};\n\n\nmsg.payload = crc32(buffer)\nreturn msg\n\n\n","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":1130,"y":460,"wires":[["76586e5b0b84bcc8"]]},{"id":"76586e5b0b84bcc8","type":"debug","z":"b0865f2b12f5f1a6","name":"","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"payload","statusType":"auto","x":1130,"y":520,"wires":[]}]

```

---

<div class="post-metadata">

**Author:** ![dr\_cave\_dweller](https://avatars.discourse-cdn.com/v4/letter/d/71e660/32.png) [@dr\_cave\_dweller](https://discourse.nodered.org/u/dr_cave_dweller)\
**Post date:** [20 April 2023 13:54 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/6 "2023-04-20T13:54:10Z")

</div>

Awesome, thanks so much Steve! Life saver.

If you have the time to answer one more question. For learning purposes so I can better understand the input and output reflection, how do you modify the code for the CRC32Q implementation?

---

<div class="post-metadata">

**Author:** ![Steve-Mcl](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/steve-mcl/32/4826_2.png) [@Steve-Mcl](https://discourse.nodered.org/u/Steve-Mcl)\
**Post date:** [20 April 2023 14:09 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/7 "2023-04-20T14:09:46Z")

</div>

> [@dr\_cave\_dweller](#):
>
> how do you modify the code for the CRC32Q implementation

The first version is very easy. Just specify `"CRC-32Q"` instead of `"CRC-32"` inside the function.

For the hand coded version, you will need to adjust it for a different constant and start value (where applicable)

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [19 June 2023 14:09 UTC](https://discourse.nodered.org/t/crc-32-cant-get-the-correct-check-sum-any-help-appreciated/77627/8 "2023-06-19T14:09:48Z")

</div>

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.