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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