Is my .img file corrupted?

Question

I downloaded the firmware (.img) of my Android device (Boyue T62D) just to have a backup of it. I'm in the Indian countryside with an extremely limited internet connection, so this is a huge file for me. The reasons I'm unsure are:




  1. I can't extract the file (just to look at the contents) in any way on my computer (OS X 10.10.1). Is this to be expected with an Android image? I tried to unpack it using a variety of methods (like converting it to iso first). (none too fancy though).


  2. I was out of the house when the file was downloading, and when I came back it looked like the file was done, but the Internet connection was broken. But the file seems to have the same size as it is supposed to have. So what got me wondering was the fact that I can't unpack it. If you guys could just tell me that this is standard, and that .img files are very hard/impossible to extract on OS X, that would be great.



— Fiksdal | Sources

Answer

Usually when downloading files (very likely for software) you have a checksum provided by the supplier. The checksum is nothing else but the hash value of the file you're going to download. Quite often this is MD5, but due to the collision potential in MD5 it starts to drift towards SHA (SHA1 mostly, somtimes SHA2).



After your download has finished, you compute the hash on the file yourself and compare the value with the one presented by the download page (or other source). If the two values match, this means that your download equals the file supplied on the website.



I started my VM for accessing such sites and had a look if there is a MD5 hash present. Right next to the download link, it was and that's the value I have copied to my comment. Therefore, if you follow the procedure I have written down above and look for such a hash value (check for MD5 or MD5SUM, SHA1 or SHA1SUM) you don't need any third-party assistance.


— Zonk | Sources

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