> That's a good idea. I never trust a burglar alarm I haven't made
 > myself, and the same applies to viruses. Some clever bastard can
 > always find a way around the well-known alarms and locks.

 NP> Yeah. Inoticed that some had posted a warning about a WORD
 NP> macro with a virus in it :-( 

  Yair, I know about those. Bloody hell!

 NP> How does a CRC work. I just use a checksum of the ascii value
 NP> of each byte of the file.

  A CRC is just a more foolproof way to do a checksum that improves
the odds, mathematically. It's easy to see how two different files
could have the same ASCII checksum using different characters. CRC
does it bit-by-bit rather than byte-by-byte, and in order. Rather than
just adding up the numbers, CRC goes through the bits one character at
a time *in order* so that each bit changes what happens to the next.
If one character is wrong, you can't make it up later because the
whole bloody thing gets thrown out. You start it off with a seed
(usually 0) and each character returns a new seed, and so on until
you end up with a number when it stops at the end.

  For memory CRC-32 gives odds of a million... you'd need a file a
million long to get an even-money chance of duplicating the CRC with
two files one-bit different. Or something like that. It's thousands
of times more secure than checksum, and it's fast because it's just
bit-shifting.

  Paul wrote an explanation in PDCRC200.ZIP (28K) that's pretty good,
but it's all in C code. There's another on TML one named CRC_V3.LZH 
(34K) written by a uni wanker who explains it in the finest detail.

Regards,
Bob
 
___ Blue Wave/QWK v2.12
@EOT:

---