Three European scientists have produced a document which correctly states who will win the 2008 US presidential election.
They don’t need to give you the document (although they do) because they can give you the MD5 checksum. A checksum, which is a kind of fingerprint for the document, accordingly to the conventional wisdom can only come from that one document. In principle, other documents may exist with the same fingerprint, but are technically infeasible to find. Then, after the election is over, they will make the PDF available and invite you to measure its MD5. If it matches, that proves it has to have been the same one they fingerprinted back in Nov 30, 2007.
The fingerprint analogy goes pretty far. Imagine that they had said,
We know who will win the 2008 presidential election. To prove it, we found someone with his name and fingerprinted him. Here’s the fingerprint ….
On the marrow of Election day, the person will visit your house, let you take his fingerprints and show his driver’s license to prove his name is that of the person who won.
The catch, of course if that MD5s aren’t really as good as fingerprints, despite widespread belief, even among experts. Marc Stevens, Arjen Lenstra and Weger have simply found an appealing way to exploit duplicatability of MD5s, constructing 12 documents with the same MD5, each with the name of different candidate (and some non-candidates). In our metaphor, this is coming up with 12 different people named after the candidates and then bioengineering them to have the same fingerprints. No matter who wins in 2008, they are “correct,” simply by choosing which one to reveal.