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I would like to make a digital equivalent of an envelop with a seal that must be broken to open. The idea is that it can be seen that the seal has been broken. I know that it is impossible to make this 100% safe, but for the use case I am imagining, it is sufficient that very difficult to open the seal without "breaking it". I am thinking of storing a text file with some important information.

Any ideas?

Edit: This is as much a thought experiment as something I've actually decided to do. I kind of figured that the data probably have to be stored remotely, like the web server or email auto-reply suggestions, but I was wondering if somebody could come up with a unique solution for storing it locally. I suppose storing just the encryption key remotely or IRL may be as close as we can come.

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Are you talking about encrypting the file? –  Linger Oct 19 '12 at 14:09
    
Even if you found something like this, the problem with files is that, unlike envelopes, digital information is easily reproduced. If your intention is to be able to tell whether someone has read a particular file on your computer, what's to stop them from making a copy and reading it on another computer? The original left on your computer would then still have its "seal" intact. –  Indrek Oct 19 '12 at 14:13
    
If this is an "After I Die" set of instructions or information sort of file, here is the best plan that I have come up with. Encrypt the file and put the encryption key on a piece of paper in and actual envelope. Put the envelope in a safe deposit box or give it to whomever you want to have access at the appropriate time. –  EBGreen Oct 19 '12 at 14:17
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What you want is not possible with digital files. –  Ramhound Oct 19 '12 at 14:38

3 Answers 3

up vote 3 down vote accepted

If you give someone your digital envelop then there is no way to guarantee this. They already have the contents and can manipulate those at will.

Thus to be sure that you know when someone has/can open them you will either need to:

  1. Store them yourself, and have a way for others to fetch them.
  2. Encrypt the file and have a way to log when someone gets the key to decrypt it.

A automatic reply on a email with a certain formatting might be the easiest way do for this. (With in the answer either the file, or the decryption key).

Note that option 1 is the most secure, but option 2 might be best for large files.

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not sure if this is possible but a digitally signed PDF with a embedded Script that is executed upon the pdf being opened and thus changing the validity of the signature could be an option.

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The only reliable way to do this is to store the files on a webserver that you control, and track access to them. You could just serve the files via Apache or similar and track access_log, which would tell you who accessed them when, or, if you want a specific identity attached, protect the files via some authentication mechanism (htpasswd is the simplest, but I'm sure there are some out there that will only force authentication the first time something is accessed, giving you a simulacrum of "seal breaking" functionality).

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