javascript - How to use CRC exactly? - Stack Overflow

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I've read the famous article A PAINLESS GUIDE TO CRC ERROR DETECTION ALGORITHMS, now I want to try this out. I'm using a nodejs module node-crc, it looks easy to use:

var crc = require('crc');

crc.crc32('hello');
# => "3610a686"

The result of function crc32, I believe, is the checksum of hello using polynomial 0x04C11DB7. Please correct me if I' wrong.

So the next step is appending the checksum to the original data. Here's my code:

var crc = require('crc');

var bf = Buffer("test");

charlist = crc.crc32(bf).toString();
var k = []
for (var i=0; i< 8; i++) {
    k.push(parseInt(charlist[i], 16));
}
console.log(k)
var checksum = Buffer(k);    
console.log(crc.crc32(Buffer.concat([bf, checksum])));

output

[ 13, 8, 7, 15, 7, 14, 0, 12 ]
646b4f9b

I'm expecting the second output to be 0, but it's not.
Therefore I'm here asking for help. What is the correct way to do this(appending and checking if the message was correctly received)? Thank you.

Edit

So I understand that the second output isn't necessarilly 0. Here's the final solution:

var crc = require('crc');

var bf = Buffer("test");

charlist = crc.crc32(bf).toString();
var k = []
for (var i=0; i< 8; i++) {
    k.push(parseInt(charlist[i], 16));
}
var checksum = Buffer(k);

transfer_data = Buffer.concat([bf, checksum]);

////////////////////// Network Transfer /////////////////////////

received_data = transfer_data

checksum = received_data.slice(-8);
charlist = '';
for (var i=0; i<8; i++) {
    charlist += checksum.readUInt8(i).toString(16);
}

// these two should be equal if data received correctly
console.log(crc.crc32(received_data.slice(0, -8)));
console.log(charlist)

Output

d87f7e0c
d87f7e0c

I've read the famous article A PAINLESS GUIDE TO CRC ERROR DETECTION ALGORITHMS, now I want to try this out. I'm using a nodejs module node-crc, it looks easy to use:

var crc = require('crc');

crc.crc32('hello');
# => "3610a686"

The result of function crc32, I believe, is the checksum of hello using polynomial 0x04C11DB7. Please correct me if I' wrong.

So the next step is appending the checksum to the original data. Here's my code:

var crc = require('crc');

var bf = Buffer("test");

charlist = crc.crc32(bf).toString();
var k = []
for (var i=0; i< 8; i++) {
    k.push(parseInt(charlist[i], 16));
}
console.log(k)
var checksum = Buffer(k);    
console.log(crc.crc32(Buffer.concat([bf, checksum])));

output

[ 13, 8, 7, 15, 7, 14, 0, 12 ]
646b4f9b

I'm expecting the second output to be 0, but it's not.
Therefore I'm here asking for help. What is the correct way to do this(appending and checking if the message was correctly received)? Thank you.

Edit

So I understand that the second output isn't necessarilly 0. Here's the final solution:

var crc = require('crc');

var bf = Buffer("test");

charlist = crc.crc32(bf).toString();
var k = []
for (var i=0; i< 8; i++) {
    k.push(parseInt(charlist[i], 16));
}
var checksum = Buffer(k);

transfer_data = Buffer.concat([bf, checksum]);

////////////////////// Network Transfer /////////////////////////

received_data = transfer_data

checksum = received_data.slice(-8);
charlist = '';
for (var i=0; i<8; i++) {
    charlist += checksum.readUInt8(i).toString(16);
}

// these two should be equal if data received correctly
console.log(crc.crc32(received_data.slice(0, -8)));
console.log(charlist)

Output

d87f7e0c
d87f7e0c

• javascript
• node.js
• crc
• crc32

Share Follow edited Jul 10, 2014 at 9:49 laike9m asked Jul 8, 2014 at 17:23 laike9mlaike9m 19.5k2323 gold badges116116 silver badges152152 bronze badges 3

• 1 "I'm expecting the second output to be 0" Why's that? – Lightness Races in Orbit Commented Jul 8, 2014 at 17:26
• @LightnessRacesinOrbit From cs.jhu.edu/~scheideler/courses/600.344_S02/CRC.html, When a message is received the corresponding polynomial is divided by G(x). If the remainder is non-zero, an error is detected. Otherwise, the message is assumed to be correct. – laike9m Commented Jul 9, 2014 at 2:08
• @LightnessRacesinOrbit I was wrong, I know. – laike9m Commented Jul 9, 2014 at 2:32

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1 Answer 1

Sorted by: Reset to default 3

Yes, the crc32 function uses the polynomial you quoted, but it also is pre and post processed by taking the one's plement. That eliminates the property that the (pure) CRC of a message with its CRC appended is zero.

The correct way to check a CRC, which is extensible to any manner of check value, is to simply pute the specified check algorithm on the message, and then pare the result with the check value appended to the message.

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