Creating a unique hash based on the CONTENTS of an image (PNG) in Ubuntu?

For the purposes of insuring images are not tampered with, I would like to create a unique hash based on the contents of an image file (a PNG specifically). I've googled, and I know it's very possible to create a hash based on a file, but it seems to take into account things other hten the contents of the image?

For instance, to test, I create a very large PNG file with random colors/lines/shapes/etc. Then I saved the file as test1.png. I then created a single pixel black dot in the corner of the image and saved as test2.png.

I ran md5sum on both images, and got different hash values (expected). I then downloaded test2.png, removed the single black pixel, and saved the file as test3.png. test3.png and test1.png contain the EXACT same image.

Now, from what I understand PNG should be a lossless compression, so that shouldn't be th开发者_StackOverflowe issue (?). I'm a bit in the dark (as you can probably tell) about all of this, so if anybody can give me any ideas, I'd much appreciate it!

You didn't say so, but I guess you are getting different hashes for test1.png and test3.png?

PNG files can contain a fair bit of metadata in addition to the image data; it's possible that some of the metadata is different. It's also possible for the same image data to be compressed in different ways. If you really want to know, compare the files to find out what exactly is different.

If you really want to hash just the contents of the files, you'll most likely have to convert them to a raw RGB format and hash that instead.