Just like the case of a box that has been sealed. We can detect if the box has been opened or not by looking at the seal. Is there any way in cryptography or any encryption scheme that allows this to happen? If we have a ciphertext, I wonder if there is a mechanism that can detect if someone has decrypted it?
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2$\begingroup$ If you use an encryption scheme that requires interaction with some other party to decrypt, that party will be notified of the decryption (but can't track where the ciphertext came from nor where the plaintext goes afterwards). For non-interactive decryption the answer is no (except if you count the other party learning the key as "decrypting"). $\endgroup$– SEJPMCommented Oct 9, 2018 at 6:53
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$\begingroup$ What it the guy who opens the box has access to the seal and re-sealed it? $\endgroup$– kelalakaCommented Oct 9, 2018 at 6:54
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$\begingroup$ Assume once you open the box, the seal is broken or damaged $\endgroup$– Thành Nguyễn VănCommented Oct 9, 2018 at 8:21
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$\begingroup$ I can think of a possible solution, where the party that wants to decrypt the message first needs to contact and initiate a decryption session with the issuing party. Is such a protocol fit for your requirements? $\endgroup$– zetaprimeCommented Oct 9, 2018 at 14:56
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1 Answer
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Digital data can always be copied. So it is easy to see that such a seal would not work in general: you could just leave the original ciphertext where it is and decrypt the copy.
There are ways around this, but they need some other trusted party or device. For instance you could use a simple secret sharing technique where (information from) a trusted third party is needed to reconstruct the decryption key. Or the trusted third party could do the decryption themselves, but in that case they may learn the plaintext. There are also plenty of techniques where the trusted third party never knows the plaintext.
Obviously you don't know the other party actually learns the plaintext. It could perform the digital equivalent of looking away. You can just learn if they received the plaintext and had the opportunity to see it. It would be easy to require the other party to sign the plaintext to verify that information was received.
You can of course generate the digital variant of a sealed box in hardware. This lets you get around the fact that the ciphertext can be easily copied.
answered Oct 9, 2018 at 12:24