12 Nov
2007
12 Nov
'07
5:44 a.m.
From: cclist at sydex.com
On 11 Nov 2007 at 10:37, dwight elvey wrote:
Hi Chuck
As I recall, you just play the CRC forward with 0's in the
two bytes. The value in the CRC is then just put into
the place of the two bytes. It the will cancel to zero.
I'd have to check that but that is what I recall.
I recall it wasn't all that complicated. Remember,
CRC circuits in controller chip do this for floppy disk
without complicated circuits.
It has been some time since I played with CRC's. I once
had to write test to check CRC chips that used
the Chinese Remainder Theorem to make data correction.
I had to make sure that it corrected correctly by checking
each part of the error correction. That was many years
ago.
Dwight
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To my knowledge, I don't know if any of the
assemblers had
a CRC calculator built in. The final check number was most
likely calculated after the assembly and just added to the code.
If in the listing, this may have been a second pass through
the assembler.
MAC certainly didn't have one. I assume that the last 2 bytes of the
ROM were the zero-CRC-makers. The "quick and dirty way" to get a
zero sum is to compuate the CRC for the first n-2 bytes of the size n
ROM, then iterate through the 65536 possible 16-bit combinations of
the "corrector" stopping when a zero CRC is hit. It shouldn't take
long, particularly if you're using an emulator on a modern PC.
You could also work out the reverse polynomial, but that sounds like
too much work for what probably will amount to a one-off deal.
Cheers,
Chuck 
12 Nov
12 Nov
6:09 a.m.
On 11 Nov 2007 at 21:44, dwight elvey wrote:
It has been some time since I played with CRC's.
I once
had to write test to check CRC chips that used
the Chinese Remainder Theorem to make data correction.
I had to make sure that it corrected correctly by checking
each part of the error correction. That was many years
ago.
I've still got the docs for the WD chip that would do both ECC and
CRC. When I was securing data for computer forensics, I started with
two interlocked CRCs using different polynomials. Eventually, the
gummint expressed a preference for SHA hashes, so the CRC went the
way of the dodo. SHA was a LOT slower than CRC or even MD5.
Cheers,
Chuck
3:03 p.m.
From: cclist at sydex.com
On 11 Nov 2007 at 21:44, dwight elvey wrote:
Hi Chuck
What are SHA hashes? I've not heard that term before.
The CRC chips I was looking at were from AMD when I
worked for Systems Industries. Now I work for AMD
but the only thing they work on today are processors.
Dwight
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It has been some time since I played with
CRC's. I once
had to write test to check CRC chips that used
the Chinese Remainder Theorem to make data correction.
I had to make sure that it corrected correctly by checking
each part of the error correction. That was many years
ago.
I've still got the docs for the WD chip that would do both ECC and
CRC. When I was securing data for computer forensics, I started with
two interlocked CRCs using different polynomials. Eventually, the
gummint expressed a preference for SHA hashes, so the CRC went the
way of the dodo. SHA was a LOT slower than CRC or even MD5.
4:01 p.m.
On Mon, Nov 12, 2007 at 07:03:22AM -0800, dwight elvey wrote:
SHA ist a family of secure hash functions. Usually, SHA-1 is used as a
cryptographically strong hash function.
General take on SHA hash functions:
http://en.wikipedia.org/wiki/SHA-1
FIPS publication 180-1 on the Secure Hash Standard:
http://www.itl.nist.gov/fipspubs/fip180-1.htm
Unfortunately, SHA-1 has been broken more than 2 years ago, so while it
is fine for error detection, it is not a good idea to use SHA-1 alone
for signatures.
At a project were we needed digital signature to authenticate documents,
we computed three different hash functions (SHA-1, MD5 and RMD160) and
signed the resulting signature file with a PGP key.
From: cclist at sydex.com
On 11 Nov 2007 at 21:44, dwight elvey wrote:
Hi Chuck
What are SHA hashes? I've not heard that term before. It has been some time since I played with
CRC's. I once
had to write test to check CRC chips that used
the Chinese Remainder Theorem to make data correction.
I had to make sure that it corrected correctly by checking
each part of the error correction. That was many years
ago.
I've still got the docs for the WD chip that would do both ECC and
CRC. When I was securing data for computer forensics, I started with
two interlocked CRCs using different polynomials. Eventually, the
gummint expressed a preference for SHA hashes, so the CRC went the
way of the dodo. SHA was a LOT slower than CRC or even MD5.
The CRC chips I was looking at were from AMD when I
worked for Systems Industries. Now I work for AMD
but the only thing they work on today are processors.
Regards,
Alex.
--
"Opportunity is missed by most people because it is dressed in overalls and
looks like work." -- Thomas A. Edison
5:34 p.m.
On 12 Nov 2007 at 7:03, dwight elvey wrote:
What are SHA hashes? I've not heard that term
before.
The CRC chips I was looking at were from AMD when I
worked for Systems Industries. Now I work for AMD
but the only thing they work on today are processors.
SHA = "Secure Hash Algorithm"--really a family of algorithms. The
most commonly encountered are the SHA-1 group. Here's the FIPS pub
detailing the whole family:
http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf
These are most useful when one wants to reduce the possibility of
data being altered. The algorithms have a cryptographic basis,
rather than a simple polynomial as in a CRC and so it's *very*
difficult to modify data in such a manner as to yield the same hash
as the unmodified data.
Cheers,
Chuck
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