What the !@#!@ is a checksum??

[nishantp]

Ive been hearing the term lately...whats a checksum? Is it something used to detect whether your program is being cracked or something??

[chrisjk]

no, it goes on the end of something that verifies the data befoere it. If you look at ISBN numbers, you'll see the last number is 0-9 or X, that is the check digit which verifies the rest of it.

It's mainly used for inputting, when it will detect if you have entered any characters wrongly.

[filburt1]

I believe it is used in a CRC.

[nishantp]

Hmm i feel even stupidier than before...whats a CRC by the way?

[filburt1]

ZIP files have it for that exact reason.

[nishantp]

I dont mean to be a pain...but how do they work? If they're just a few chars at the end of some data then how can they verify the integrity of the entire thing?

[Geoff Gunson]

The check sum is calulated by the value of each byte and their position therefore some simple packet like:

1234

will not have the same checksum as

4321

So what you do is to calcluate a check sum with the incoming data and see if that answer matchs the one sent in the packet.


For instance a real crappy check sum maker would be

For i = 0 to len
sum = sum + data(i) + (1024^i)
next i

Where data is an array of byte values.

HTH

G

[Nucleus]

Another crappy checksum, but you will get the idea:

Function Checksum(path As String) As Byte
 If len(Dir$(path)) Then 
 
   Dim fnum As Long
   Dim ByteVar As Byte
   
   fnum = FreeFile
   
   Open path For Binary As #fnum
   While Not EOF(fnum)
      Get #fnum, , ByteVar 'Get a byte at a time as buffer was defined as a single byte
      Checksum = Checksum Xor ByteVar
   Wend
   Close #fnum
 
 End If
End Function

I think it is faster to do this using asm and then implementing in VB.