Proof for a creation date

[Daniel Kahn Gillmor]

On Thu 2016-12-01 21:12:50 -0500, Bertram Scharpf wrote:
> I want to make evidence that I created a document _before_ a certain
> point of time.

One approach i've seen recommended is to create a
cryptographically-strong digest of the signed document in question and
then post it to a public, append-only log somewhere.

For example, take the SHA256 digest of the document, pretend that value
is the address of a bitcoin wallet, and throw a little bit of bitcoin
into it (this value will never be recoverable because no one knows the
corresponding secret key). This puts the digest into the blockchain at a
acertain date for anyone to see.

Your subsequent argument is that one of the two possibilities must hold:

 (a) you have some ability to perform a collision attack against
     SHA-256, or

 (b) the signed document existed at some point before the bitcoin
     transaction was publicly logged.

since most people won't believe (a), (b) looks pretty likely.

You could use any other globally-visible log that allows for injection
of a bitstring long enough for a strong digest (32 octets is probably
sufficient), it doesn't have to be the bitcoin blockchain.  for example,
if you can get something into a public X.509 certificate, you could post
it to one of the certificate transparency logs.

Regards,

        --dkg
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