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I can't get my head around something.


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8 replies to this topic

#1 joeyh

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Posted 23 February 2014 - 06:12 PM

Hello All!

 

Hope someone on here can help. I will try to be concise as possible, if 2 identical images are saved on an a system but with different filenames ie. image1.jpeg and image 2.jpeg. Would the part of the binary string that make up the image (which i think is done at the assembley level) be identical.

 

I am trying to take the following into account whilst trying to understand this:

  • different systems have different macine languages based on CPU so it is fair to say if the binary string was identical they would not be if the same across different CPU's.
  • I remember hearing (sorry that is really vague i know) that on the machine level a word document would not all be located in a concurrent format.
  • Time and date stamp might affect it somehow??????

Thanks

Joe

 

 

 

 



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#2 Chris Cosgrove

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Posted 23 February 2014 - 06:48 PM

As I understand it, if you have two or more copies of a file in different locations, the file data is the same but the metadata may well be different.

 

To use your example of an image, I have thousands of photos on my system which all came out of our cameras with names like IMG_1234.jpg or DSC_4567.jpg. If I then, as I normally do, rename these and save them to a different folder, then the image part of the file will be the same - or it wouldn't be the same image - but that part of the metadata that gets added, like date created, will be different.

 

I am no sort of programmer, but when you are dealing with standardised file formats, such as JPEGs and PDFs, the operating system you are trying to run them on will still interpret a .jpg file as an image and a .pdf file as some sort of document. This is why I can send copies of my photos to people without having to worry about what sort of operating system they are running.

 

If this clarifies your thinking any !

 

Chris Cosgrove



#3 joeyh

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Posted 23 February 2014 - 07:03 PM

Chris, Many thanks for the prompt response. I thought this was the case on the individual system. So i suppose the second part of the question would be if a sent an image from a comuter using cpu brand x to a freind who saved the image on their system using cpu brand y. Assume the person using brand y has changed the filename. How would you go about locating that image using only the binary information which was sent by the person using brand x
?

 

hope that makes sense?



#4 Chris Cosgrove

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Posted 26 February 2014 - 07:22 PM

'hope that makes sense?' - I think I can work it out !

 

So, you have a photo you have called Cat.jpg on your system and you send it to your Mum, who promptly renames the image Dog.jpg. Well, you still have the photo called Cat on your system, so you know where it is and what it is, and likewise, your dear old Mum knows what it is and where she put it (well, we can hope so !). So far no problem.

 

I think what you are asking is if you have access to her computer, can you find this photo you sent her and she saved ?  Only if you are willing to plow your way through all the photos on her computer. You can rank files by date saved, so if you know the date she got it, this would narrow your search considerably but I don't know any other way to do it, but equally, I don't know it all. The easiest way to search for a file is by filename, and if you don't know the filename you are going to have problems searching for an image. Even if you know the EXIF information, I don't think you can search by this. Could try.

 

Chris Cosgrove



#5 myrti

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Posted 26 February 2014 - 08:26 PM

Hi,


you might want to look into checksum algorithm like md5 or sha1. They basically create unique identfiers for dthe content of a file. As long as you don't change the content, but only the 'metadata', the checksum will remain the same and you can positively identify a file by its content rather than its name or change date.

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#6 joeyh

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Posted 27 February 2014 - 07:39 AM

Many thanks for the responses/ I will look into the MD5 and SHA1 to try and gain a better understanding. 

 

thanks

joe



#7 joeyh

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Posted 27 February 2014 - 08:18 AM

Hi 

 

I should also say the reason i wanted to understand how this worked, was to try and understand how difficult it would be to create a piece of software that could be given to datacentres for free. And the software would have the binary data for images which involve child exploitation, obviously this would have to come from the police. When the binary strings were located they could be deleted and the images taken of the net.

 

I appreciate this is a massive undertaking which is more than likely out of my knowledge base. But i wouldn't mind giving it ago with just my home pc's and a picture of my cat :D

 

 

thanks

joe  



#8 Animal

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Posted 27 February 2014 - 06:30 PM

Who said picture of a cat? :P

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#9 Chris Cosgrove

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Posted 27 February 2014 - 07:11 PM

I think even the OP will admit that his #1 was - let's say - loosely worded, and I had no inkling of the purpose of his enquiry. Even addding in Myrti's comments about checksum algorithms, there is her rider 'As long as you don't change the content'. The problem is it is trivial to change an image, after all, I do it all the time. Apart from the frontline applications like Photoshop and PhotoPlus there are plenty of others out there to add a little, trim a little, change a shade and so on, and so on.

 

From immediate personal experience, I have just had to bin my 4:3 aspect ratio Hanns-G monitor and replace it with a 16:9 widescreen job. So, of course, all my home grown wallpapers didn't fit. The most time consuming part of the exercise was tracking down some of my original photos to do the modification work on them. Changing the resolution and cropping to fit a 1920 x 1080 monitor was trivial. Currently a photo of the entrance to Inverness Harbour taken from the Kessock Bridge on a very pleasant sunny and windy Spring morning. If you want a look . .

 

http://imageshack.com/a/img545/195/o7bh.jpg

 

Chris Cosgrove






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