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checksum different on transferring the same file.

B

bj097

macrumors 6502
Original poster
Dec 27, 2013
347
23
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?
 
I'm surprised at that. Can you give some more details? How do you do the transfer? Is the file size the same?
 
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?
 
PhoneyDeveloper said:
I'm surprised at that. Can you give some more details? How do you do the transfer? Is the file size the same?
Click to expand...


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
 
KarlJay said:
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?
Click to expand...

I cannot see the reason of using different checksum tool when they are supposed to give the same result.
I use ExactFile.
 
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.
 
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?
 
PhoneyDeveloper said:
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?
Click to expand...

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
 
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.
 
mfram said:
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.
Click to expand...

CRC32 is CRC32 . Could it be anything different? Sorry I don't undesytand.
 
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.
 
bj097 said:
CRC32 is CRC32 . Could it be anything different? Sorry I don't undesytand.
Click to expand...

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.
 
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.


 
Last edited:
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
 
bj097 said:
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
Click to expand...

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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