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Re: Discouraging :local:

From: Greg A. Woods
Subject: Re: Discouraging :local:
Date: Tue, 28 Jan 2003 16:42:40 -0500 (EST)

[ On Tuesday, January 28, 2003 at 08:38:31 (+0100), Fabian Cenedese wrote: ]
> Subject: Re: Discouraging :local:
> > > Well, generically speaking Ethernet's FCS field is a 32-bit CRC of the
> > > whole data frame.  However if I understand the math correctly that means
> > > that only 32-bit or shorter errors (remember Ethernet is serial) can be
> > > detected reliabliy and "only" about 99.955% of error bursts longer than
> > > 32 bits can be detected.  Ethernet frames containing TCP or UDP IP
> > > packets can be large -- up to ~1500 bytes.
> I think you got that wrong about CRC. It's much better and especially
> suitable for serial streams. Every bit goes into the calculation and it's
> close to impossible (don't know %) that you can correct even one wrong
> bit to get the same result. Similar use is MD5.

Those numbers are true for CRC-16 modulo the error burst length at
16-bits instead of 32-bits -- but I'm not so sure how the percentage was
derived and so how it would work out for CRC-32, which is why I
questioned my understanding of the math....

CRCs of any reasonable integer word length are actually very easy to
fool if you manually frob the errors into the stream, and real-world
errors in a serial data stream can just about as easily emulate such
manufactured errors (I've seen such incidents countless times) -- which
is why people went on to develop other (more) secure hashing algorithms
such as the various MD, SHA, RMD message- digest algorithms and so on.

                                                                Greg A. Woods

+1 416 218-0098;            <address@hidden>;           <address@hidden>
Planix, Inc. <address@hidden>; VE3TCP; Secrets of the Weird <address@hidden>

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