Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!uw-entropy!dataio!pilchuck!seahcx!phred!jimo
From: jimo@phred.UUCP (Jim Osborn)
Newsgroups: comp.sys.mac.programmer
Subject: Re: crc functions
Message-ID: <3005@phred.UUCP>
Date: 12 Mar 90 22:35:49 GMT
References: <8548@netcom.UUCP> <1990Mar7.065639.11927@Neon.Stanford.EDU> <2981@murtoa.cs.mu.oz.au>
Reply-To: jimo@phred.UUCP (Jim Osborn)
Organization: <2981@murtoa.cs.mu.oz.au>o
Lines: 21
Summary: Always init to -1

rob@murtoa.UUCP (Robert Wallen) writes:
-kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes:
--The way CRCs are meant to be used is:  Initialize the CRC to all 1's, then
--compute the CRC of the message.  Send the message with the CRC appended to it.
--On the receiving side, initialize the CRC to all 0's.  Compute the CRC of the
--entire message PLUS the message CRC.  The result should be 0.  If not, you
-
-Can someone confirm this?  Or preferably deny it?  I have never used a seed
-value of all 1's (except one weirdo case where it wasnt CCITT CRC's being 
-used anyway)

You must use the same initial value on both ends.  Some people seem to like
zero, but that leaves you vulnerable to computing a good CRC on an all-zero
message.  All-ones is a better choice, though I suppose you could use any
non-zero value, as long as both ends agree to it.

jimo
-- 
pilchuck!jimo@phred		Jim Osborn, Physio Control Corp
				11811 Willows Rd, Redmond, WA, 98073
				206-867-4704 direct to my desk