Algorithm Help CRC32

[coldfusion244]

I was curious about CRC32 so I did a quick google search and it brought up some examples. They weren't all that exceptional so I figured I'd post here to get more information. From what I gathered, you take a message that is 32 bit of data in length, then shift it left bitwise 3 places. Take the lowest 5 bits from that message and add 1 to it which would become the checksum. you then append the checksum on the end of the message. To check you would take the message modulo the checksum and shouldn't have a remainder.Is that correct or did I not fully understand it?

[IngisKahn]

CRC32 is an algorithm that produces a 32 bit number for a given byte stream using polynomials. (Here's a good description: http://www34.brinkster.com/dizzyk/math-crc.asp ) It is used to verify that the byte stream hasn't been changed since a minor change in the stream would produce a completely different cheksum. Generally CRC32 is being phased out. If speed is important then Adler32 is favored otherwise the more secure MD5 or SHA1 is used mostly now days.

[coldfusion244]

Ok, I followed it (I think) and for some reason it doesn't come out right.. I'm thinking this is because you can choose any polynomial that you wish (Although I used the one for Ethernet). Any Ideas? Here is the code I used (I did it the slow way I guess, no lookup table).

 

#include "stdafx.h"

#include <iostream>

#include <fstream>



using namespace std;



int _tmain(int argc, _TCHAR* argv[])

{

	ifstream mf;

	char *b = new char();

	unsigned long r = 0x04c11db7L;

	unsigned long s = 0;

	unsigned long t = 0;

	unsigned long temp = 0;



	



	mf.open("C:\\test.txt",ios::binary);



	while(mf.peek() != -1)

	{

	s = r & 0xff000000;

	cout<<mf.read(b,1);

	s = s ^ *b;

	temp = r & 0xFF000000;

	temp = temp>>24;

	r = r<<8;

	r = r | temp;

	s = s ^ r;

	t = t + s;

	}

	mf.close();

	cout<<endl<<r<<"|"<<s<<"|"<<t<<endl;

	cin.get();

	return 0;

}

[/Code]