checksum different on transferring the same file.

Discussion in 'Windows, Linux & Others on the Mac' started by bj097, Nov 3, 2015.

  1. bj097 macrumors member

    Joined:
    Dec 27, 2013
    #1
    I happen to find that when I  transfer (with USB cable) from the Apple iPad or iPhone to my computer (PC) , for the same photo / video, it gives out different checksums each time.. If something is identical it should be with same checksum. If checksum is different, the file is also different.

    Why would it be like that? is it the failure of USB cable or is it just normal for Apple devices?
     
  2. PhoneyDeveloper macrumors 68030

    PhoneyDeveloper

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    Sep 2, 2008
    #2
    I'm surprised at that. Can you give some more details? How do you do the transfer? Is the file size the same?
     
  3. apunkrockmonk macrumors 6502a

    apunkrockmonk

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    Rochester, NY
    #3
    I'm no programmer but could it be the file metadata?
     
  4. 1458279 Suspended

    1458279

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    California
    #4
    What are you using to check the checksum?

    Are there different tools that work differently? Maybe a 16bit vs 32bit or written for a different OS?

    Is there a way you can make it read only before the transfer or do some kind of byte compare?

    What are you using to transfer the file?
     
  5. bj097 thread starter macrumors member

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    Dec 27, 2013
    #5

    file size (byte) is the same , I cannot see anything differrent under properties > details. Transferring using USB connected directly to the PC with the windows based
     
  6. bj097 thread starter macrumors member

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    Dec 27, 2013
    #6
    I cannot see the reason of using different checksum tool when they are supposed to give the same result.
    I use ExactFile.
     
  7. 1458279 Suspended

    1458279

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    #7
    If all checksums are the same, then maybe the transfer process adds something to it like an update stamp or ownership rights (read/write permission). Another long shot is maybe a malware type thing that attaches to the file.
     
  8. PhoneyDeveloper macrumors 68030

    PhoneyDeveloper

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    Sep 2, 2008
    #8
    So you are syncing the same image multiple times and checking the checksums on your desktop computer and file sizes are the exact same to the byte? Are these png files?
     
  9. bj097 thread starter macrumors member

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    Dec 27, 2013
    #9
    Both vids and pics that I took with my iPad

    their sizes (and resolution) are the same but I've tested their CRC32

    they are all different. I've got couple of copies
     
  10. mfram macrumors 65816

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    Jan 23, 2010
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    San Diego, CA USA
    #10
    Are you using the same CRC algorithm with the same generator polynomial? There are different standards for CRC32 which would give different answers. Try something like MD5 which doesn't have that issue.
     
  11. bj097 thread starter macrumors member

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    Dec 27, 2013
    #11
    CRC32 is CRC32 . Could it be anything different? Sorry I don't undesytand.
     
  12. 1458279 Suspended

    1458279

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    #12
    What system(s) are the CRC32 routines running on?
    This could be the difference in how the data is treated. There's different character sets for different devices/OSs and so on. One routine could be treating the data differently.
     
  13. mfram macrumors 65816

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    Jan 23, 2010
    Location:
    San Diego, CA USA
    #13
    Wikipedia lists four different common generator polynomials and 3 different representations of those polynomials. That's 12 possibilities right there. CRC is a mathematical description of an operation. There are different ways to compute a CRC. My point is: Did you write the code on the mobile device and your computer such that you know the implementations are the same? Or are you using different code bases which might be computing different values? That is, there's more than one "standard" for CRC32.

    MD5 or one of the SHA functions won't have that issue assuming you're using the same version.
     
  14. bj097, Dec 31, 2015
    Last edited: Dec 31, 2015

    bj097 thread starter macrumors member

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    Dec 27, 2013
    #14
    You guy could just test it out on your own if you doubt. Very easy.

    Connect the same iPad to the same computer through USB cable.
    Import the photos and videos to PC when the autoplay windows pops out.

    Create a list of checksums for the photos and videos.

    Delete the PreviouslyAcquired.db files. Disconnect the iPad and connect it again. Repeat the whole process. Importing all the photos and videos to PC on another folder

    Using the checksum files to test the newly imported files.

    All checksums error.


     
  15. jdelgado macrumors regular

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    Oct 25, 2009
    #15
    Do you use ExactFile on a PC or is there a Mac version?
     
  16. Freyqq macrumors 68040

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    Dec 13, 2004
  17. bj097 thread starter macrumors member

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    Dec 27, 2013
    #17
    PC version
     
  18. bj097 thread starter macrumors member

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    Dec 27, 2013
    #18
    As for photos taken by camera, after removing the date of import, the MD5/ CRC32 is the same. It was likely because of metadata reason

    But for videos, there is no date of import and no difference in metadata, but checksums are always different for the same video
     
  19. bj097 thread starter macrumors member

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    Dec 27, 2013
    #19
    Anyone knows why video (but not photos) on the iPad has different checksums after each time it imports to the PC?
     

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