Hi, Bob.

[...]

BL>   For memory CRC-32 gives odds of a million... you'd need a file a
BL> million long to get an even-money chance of duplicating the CRC with
BL> two files one-bit different. Or something like that. It's thousands

If 2 files are 1 bit different you will *never* get the same CRC. Or the
same checksum either, using Niels' approach. The likelihood of getting
the same CRC for 2 files with *multiple* bits different depends on the
size of the file. There are, for example, 8,192 different possible 1k
byte files, every possible 32-bit CRC could have been generated from 256
of those files.

BL> of times more secure than checksum, and it's fast because it's just
BL> bit-shifting.

It's certainly not faster than a checksum, if that's what you're saying.
But I can CRC over 200meg in over 5,000 files at nearly 40meg/minute.

BL>   Paul wrote an explanation in PDCRC200.ZIP (28K) that's pretty good,
BL> but it's all in C code. There's another on TML one named CRC_V3.LZH

I've done 3 in Pascal/BASM - the 32-bit used in PKZIP & ARJ, the 16-bit
used in the other archivers, and the 16-bit used in XModem.

BL> (34K) written by a uni wanker who explains it in the finest detail.

Thanks, I'll grab all those.

Regards, fIM.

 * * Liberal's economy equals 100% tax on EVERYTHING
@EOT:

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