>The CRC, or any hash or encryption should be done on the file contents - not the size

Very well

>A CRC is a hash. Since the whole file has to be read in order to calculate the hash, the longer the file the longer it takes the calculate the hash
>
>Note that the time is not that significant. I can encrypt ( Rijndael) at speeds of 25 to 35 MB/sec. And this is a block cipher with multiple rounds per block (10, 12 or 14) and lookup tables

Ok, thanks, if I ever have to implement such detection, I will make it OS free specific.