Subj : Crc utility
To   : Tony Langdon
From : mark lewis
Date : Fri Sep 30 2016 08:17:00


30 Sep 16 13:51, you wrote to Alan Ianson:

 AI>> Does anyone know of a utility to get the CRC of a file? A node is
 AI>> creating tic files by hand, or a script I guess and we need the CRC
 AI>> value of the file to put in the tic.

 TL> Seems Mystic doesn't require the CRC.  I'm able to hatch files without it.

there shouldn't really be any TIC software that /requires/ the CRC... it's only 
real use is to ensure that the file is not changed in transit...

depending on your OS, you may have the necessary tools already at hand...

  cksum
  crc32
  md5sum
  sha1sum
  sha224sum
  sha256sum
  sha384sum
  sha512sum

cksum or crc32 is the one you want but you should check with a known file that 
has a known CRC value and see which program outputs the same match... i say 
this because at least crc32 hashes can have pre- and post- conditioning applied 
to them...

i recently ran into a case where i had to be sure i was using the proper one... 
one case was hashing a user's name for use with JAM bases... another was 
hashing a file for integrity checking... in the case of working with JAM 
message bases, to properly match the crc generated and used by legacy software, 
i had to initialize the crc variable to $ffffffff (as a signed longint) and 
then feed each character individually to the crc routine... i could have fed 
the entire blob of data and let the routine walk through it but that gave a 
different and wrong result... at the end, we did not XOR with $ffffffff but we 
do have to invert the result so the bytes are reversed since to match what's in 
the JAM data files... this byte reversal is because the legacy method used is 
for traditional analogue modems and the UART wants everything reversed for 
transmission... granted a UART is not used in this case but it is the way the 
sofware is written and there's a lot of software that uses this same 
formulation method...

[time passes]

i just checked what i was checking for JAM with the cksum and crc32 tools but 
neither returned the desired results... i'll have to go play, now, with 
checking a file and its TIC to see if i can determine which is correct if 
either one is... pre- and post- conditioning and whether to flip the bytes or 
not make a huge difference in the results...

)\/(ark

Always Mount a Scratch Monkey
Do you manage your own servers? If you are not running an IDS/IPS yer doin' it 
wrong...
.... The one question you've always wanted clarified. What did she say?
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 * Origin:  (1:3634/12.73)

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