Re: Related Signature Analysis-Type Stuff

This was originally called a Fletcher's Checksum after the creator. I agree it
is a nice checksum, I've been using this to checksum things since the mid 80's!

It's well documented in a back issue of Dr Dobb's journal, you could probably
find it on their site, or at least in their back issue CD.

FWIW: It's only "very close" to a CRC in security on blocks below 255 bytes,
it's comparison to CRC falls off quickly after that. But for small packets it's
very good! And for anything larger it's a heck of a lot better than a standard
checksum.

-Zonn

On Thu, 9 May 2002 00:34:24 +0100, "Phillip Eaton" <inbox@phillipeaton.com>
wrote:

>
>For those interested, here's another checksum algorithm designed by BT in
>the UK, called a Frame Check Sequence number (FCS).
>
>It's not quite as secure as a CRC, but it is very close and it is much, much
>easier to calculate. (You can make out the algorithm from the code).
>
>I implemented it in Z80 assembly using a Forth cross-compiler (i.e. it's all
>backwards) for some embedded stuff I was working on.
>
>A CRC on the other hand, is much harder to calculate. See the comparable
>listing at the end. It also shows my start value as 8408.
>
>I'm a bit confused myself now, actually, as the CRC code seems to process
>the whole 16 bit old CRC in one go (or at least it would if the Z80 was a
>true 16 bit processor), which is a bit different from running 1 bit at a
>time. I'll hae to do some testing...when I get some time!
>
>Phil.
>
>\ --------------------------------------------------------------------------
>-
>\ FCS
>\
>\ Descripton: Piece of Z80/H64180 code to implement the FCS algorithm as
>\ defined by British Telecom New Networks Technical Forum -
>\ Document CP(83)/12 04/02/83. Call with the old FCS and data
>\ byte to be FCS'd and a new FCS is returned. Note the 'exx'
>\ at the start and end of the code - MPE Z80 forth crashes if
>\ you use the standard BC registers. You should be able to
>\ remove these if you are using any other compiler.
>\ To parse a complete buffer of information, you need an
>\ external loop which calls this function for each data byte.
>\ FCS = File Check Sequence
>\
>\ Parameters: fcs_c0c1 - Old FCS to modify.
>\ char - Value to add to FCS.
>\
>\ Returns: new_fcs_c0c1 - New FCS.
>\
>\ --------------------------------------------------------------------------
>-
>
>Code FCS \ fcs_c0c1 char --- new_fcs_c0c1 ;
>
> EXX
>
> BC POP \ Contains new data byte in C
> DE POP \ Contains old FCS, D=c0, E=c1
>
> A, D LD \ Calc new c0, c0=c0+char
> A, C ADD
> NC, fcs1 JP \ If result > $ff then
> A INC \ add 1 to result
>L: fcs1 D, A LD \ Store back to c0
>
> A, E LD \ Calc new c1, c1=c1+c0
> A, D ADD
> NC, fcs2 JP \ If result > $ff then
> A INC \ add 1 to result
>L: fcs2 E, A LD \ Store back to c1
>
> DE PUSH
>
> EXX
> NEXT,
>
>End-Code
>
>
>
>************ CRC CODE STARTS HERE ***********
>HEX
>
>$8408 equ CRC-Seed \ Good initial value for CRC calculation.
>
>Code CRC-16 \ crc char --- new_crc ;
>
> EXX
>
> BC POP \ Contains new data byte in C
> DE POP \ Contains old CRC
> HL, # 0 LD \ Define Work Area
>
> A, E LD \ XOR bottom 8 bits of old CRC with data byte
> A, C XOR
> E, A LD
>
> A, H LD \ Swap bottom 8 bits of CRC with top 8 bits of
> H, E LD \ Work Area
> E, A LD
>
>
> A, D LD \ Swap top 8 bits of CRC with bottom 8 bits
> D, E LD
> E, A LD
>
> PE, L1 JP \ If parity of XOR even, miss out the CRC XOR
>
> A, D LD \ XOR the CRC with C001H
> A, # C0 XOR
> D, A LD
>
> A, E LD
> A, # 01 XOR
> E, A LD
>
>L: L1
>
> SCF \ Right shift the CRC and then XOR it with work area
> CCF
>
> H RR
> L RR
>
> A, D LD
> A, H XOR
> D, A LD
>
> A, E LD
> A, L XOR
> E, A LD
>
> SCF \ Right shift the CRC and then XOR it with work area
> CCF \ again
>
> H RR
> L RR
>
> A, D LD
> A, H XOR
> D, A LD
>
> A, E LD
> A, L XOR
> E, A LD
>
> DE PUSH \ Shove the new CRC back on the stack
>
> EXX
> NEXT,
>
>End-code
>
Received on Wed May 08 17:54:23 2002