Molecular,
Sorry for such a late reply to your response. Your proofofexistence site looks very interesting, but I have to admit I'm struggling making it work.
In the meantime I ran a test. I created a PDF file and generated its SHA256 hash as my private key. I used bitaddress.org's "wallet details" to generate the corresponding public address. I then sent a small fraction of a millibitcoin to that address to time stamp the transaction on the block. Now I can prove that the PDF document existed at the time of the transaction.
Is there something I'm missing?
Thanks.
That site is not mine.
To prove your pdf existed, I would go a similar route as you took: I would hash the doc, convert the private key to an address and verify that the address has received money in the past. If it has, either the document must have existed or the extremely unlikely event that someone chose the hash of the document as a private key by accident must have happened. Since we can rule out the latter with high probability, the documents existence at the time of the transaction is proven.
Please note: you should move the money from that address. Anyone with the document can access it.