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How to prove a file was not tampered?

I have uploaded the assignment on website before and I didn't realize my file is not exist on the website until I found out that my assignment wasn't graded. My lecturer asked me to prove the date of my file which is my assignment is before the deadline and never be tampered. I have the source files that show timestamps that are clearly before the deadline. How to prove the date is reliable?

PS: I sent my assignment file which is as same as my source file to my classmate's laptop by a chat software in order to check the file was correct, and there is still the date of the file in the chat software history list and i can still use the software of his laptop to receive and open the file again in his laptop.

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    Not really possible. Anything is editable, and there's no universal DRM you could use. Thank goodness!
    – Pontus Gagge
    Apr 5, 2011 at 8:52
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    Start using digital signatures in the future and sign all important data. Right now, there's no way to prove that dates are reliable.
    – AndrejaKo
    Apr 5, 2011 at 9:03
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    @AndrejaKo: Don't forget that even digital signatures are based on the signer's timestamp, so a trusted timestamping service is needed.
    – u1686_grawity
    Apr 5, 2011 at 9:06
  • One such trusted timestamping service is the lecturer's memory :)
    – R. Martinho Fernandes
    Apr 5, 2011 at 9:10

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You can't do it now.

But to prove the existence of a document at a given date, you have to use a Timestamping authority that gives you a specific token of time for a specific file.

If you modify the document after that date, the token won't verify the document.

But you can't assess that a document existed before the date you use the service (and that's the point of the timestamping...)

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