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I’m willing to create a system of transferable documents (identified by it’s ID) whose author can transfer his ownership of that document to another person (identified by his/her ID).

For example:

  1. Alice; owner of document 1.
  2. Alice transfers his ownership of that document to Bob.
  3. Now: Bob is owner of document 1.
  4. Alice says she is the owner of document 1, but she fails.

(Item 4 is very important)

We can make sure that the system with it’s author remains untouched by using digital signature. But if Alice made a copy of that document signed when she was the owner, there would be no way to prevent her from saying she is not the owner of the document.

So we would need something to make a signature to expire whenever it is transferred.

IF I HAD A DATABASE: I would simply add that signature to a ban list.

Are there any solutions to preserve the uniqueness of this document?

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The problem is actually not to make sure that Alice cannot copy the document (i.e. preserve uniqueness as you asked) but to make sure that it is clear who owns this document currently.

A solution to this problem might be if not only the documents are signed but the transactions (transfer of ownership) are signed too. In this case Alice might claim that she still owns the document but Bob can refute this claim by showing the signed transaction that Alice has transferred the ownership to Bob. These transaction might also be made public so that is is not necessary that Bob explicitly refutes Alice claims but that everybody can see independently who the current owner is.

If the ownership of documents changes multiple times it need to be clear what the latest transaction on the document was in order to determine the current owner. This can be achieved by having a clear order of transactions, for example by using trusted timestamps or by putting all transactions on a common blockchain. A search for blockchain document ownership provides lots of discussions, PoC and even products about this topic for further reading.

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  • Thank you very much for your answer! But “to make sure that it is clear who owns this document currently”; if Alice suddenly comes and says she made no transfers (with the copy I said), we would need to check all the existing documents to see if there was a transfer made before? Otherwise she would pass without problem – J. Francis Jan 3 at 16:29
  • @J.Francis: The ownership is not part of the document but the ownership is defined by the signed transactions - thus the transactions associated with the specific document would need to be checked. Blockchain or other kind of hash chains actually allow for a list of transactions associated with each document which can also be quickly shared and looked up. – Steffen Ullrich Jan 3 at 16:51

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