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Jan Waclawek
08/12/07 23:27
Read: 650 times
Bratislava
Slovakia


 
#143143 - and what
Responding to: Russ Cooper's previous message
Russ Cooper said:
Too bad it's necessary to start over from the very beginning for every different polynomial. (c:

And what. How many are actually in use for CRC-16? I believe if I make the same for the right-shifting variant, I'll cover 90% of the true CRC16s (excluding those which their authors like to call "CRC" but only to sound sophisticated, and is in fact a plain checksum or similar).

Russ said:
Have you run into articles like this one that raise some doubt about how a correct implementation of CCITT CRC-16 is really supposed to work? It points out that many websites (including the online CRC calculator that you have mentioned) give 29B1h as the CRC for the test string "123456789". However, the author of the site believes for one reason or another that the correct result is actually E5CCh.

Sure. This is just the example of poor definition of things I was talking about in the previous thread (for example, you'll obtain this result from the online calculator if you tick the "indirect algorithm"). And this is why my methodology is rather empirical and as a definition requirement I don't want example of the algo - which is pretty much the same all the time - but also example inputs and outputs.

Russ said:
I don't suppose this would matter too much if your use of the algorithm was limited only to your own programs. Of course it would matter a lot if you were exchanging data with somebody else's program that calculated the CRCs differently.

This is exactly what I'm talking about... Although, if I think more, it should also cover cases when the checksum calculation has a flaw (and the "counterpart" you are trying to create needs to have implemented the same flaw of course).

Russ, I seem to recall such a case... ;-)

JW



List of 32 messages in thread
TopicAuthorDate
CRC16 CCITT - What a challenge!      Jan Waclawek      08/06/07 13:08      
   Have you seen this?      Richard Erlacher      08/06/07 17:40      
      Of course!      Jan Waclawek      08/07/07 02:36      
         Balance      Russ Cooper      08/07/07 07:16      
            I know, Russ,      Jan Waclawek      08/07/07 07:53      
         Code? What code?      Russ Cooper      08/07/07 07:24      
            in the code library - look to the left column      Jan Waclawek      08/07/07 07:37      
               Thanks! I missed that note in the first post.      Russ Cooper      08/07/07 08:37      
   I need a hint      Russ Cooper      08/11/07 00:56      
      write down the bits      Jan Waclawek      08/11/07 01:22      
         Thanks for the hint      Russ Cooper      08/12/07 11:12      
            Your observations plus a comparison in C      Dan Henry      08/12/07 11:58      
               The rest of the story ...      Russ Cooper      08/12/07 16:43      
                  Compilers, but Keil C51 8.09 for the stated 28~      Dan Henry      08/12/07 17:04      
                  and this processes only a nibble, isn't it...      Jan Waclawek      08/12/07 23:27      
                     No, it process the entire byte.      Dan Henry      08/13/07 12:59      
                        now it\'s my pencil time...      Jan Waclawek      08/13/07 13:40      
                           FYFI      Dan Henry      08/13/07 14:40      
                              Clarification re: 8-bit optimized term      Dan Henry      08/13/07 17:46      
                              on optimisation      Jan Waclawek      08/14/07 02:56      
                                 Jan's optimization      Dan Henry      08/14/07 07:16      
                                    Re: on optimization      Dan Henry      08/19/07 09:18      
                                       Can you please be more specific?      Jan Waclawek      08/19/07 13:56      
                                          More specifically ...      Dan Henry      08/25/07 15:10      
            and what      Jan Waclawek      08/12/07 23:27      
               More on checksums        Russ Cooper      08/18/07 09:59      
   No reason to hide the excreted code      Dan Henry      08/12/07 17:41      
      Excreted code needs context like any other code      Dan Henry      08/12/07 19:01      
   One More Comment      Michael Karas      08/13/07 02:45      
      ... and a marginal remark ...      Jan Waclawek      08/13/07 07:02      
         Init all ones frequently specified      Dan Henry      08/13/07 13:06      
   18 cycles      Jan Waclawek      08/16/07 08:16      

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