Contributor: GUY MCLOUGLIN { GUY MCLOUGHLIN >I wanted to ask you... would you happen to know how a CRC Check-sum >works? Everytime I go to look this up in a book I see a bunch of >stuff about X^7 + X^12 + X^17..... (and on and on) but nothing that >actually says "Here's what the code looks like" ... just a bunch of >non-sensical bull...Would you happen to know the algorithm that is >used? ...Greg Vigneault is much better at this stuff than I am. I usually know "why" something works, but not always "how".The basic idea is that the data is treated as input to a specific polynomial equation (ie: X^32 + X^26 + X^23 + X^22 + X^16 + X^12), the result of this is then divided by a specific prime number, and the remainder left over is the CRC value. I know that this is easier said than understood, but that's the gist of it. ...if a single bit of a chunk of data is changed, the chances are very good that a CRC check number would catch this change. It's not 100 percent guaranteed, but something more like 99.97 percent, so CRCs are not an entirely bulletproof check. Here's a standard Pascal Implementation of a CRC-16 routine: } Function CRC16(InString: String) : Word; Var CRC : Word; Index1, Index2 : Byte; begin CRC := 0; For Index1 := 1 to length(S) do begin CRC := (CRC xor (ord(InString[Index1]) SHL 8)); For Index2 := 1 to 8 do if ((CRC and $8000) <> 0) then CRC := ((CRC SHL 1) xor $1021) else CRC := (CRC SHL 1) end; CRC16 := (CRC and $FFFF) end;