“Easy Cryptographic Timestamping of Files”, 2015-12-04 (; backlinks):
Scripts for convenient free secure Bitcoin-based dating of large numbers of files/strings
Local archives are useful for personal purposes, but sometimes, in investigations that may be controversial, you want to be able to prove that the copy you downloaded was not modified and you need to timestamp it and prove the exact file existed on or before a certain date. This can be done by creating a cryptographic hash of the file and then publishing that hash to global chains like centralized digital timestampers or the decentralized Bitcoin blockchain. Current timestamping mechanisms tend to be centralized, manual, cumbersome, or cost too much to use routinely. Centralization can be overcome by timestamping to Bitcoin; costing too much can be overcome by batching up an arbitrary number of hashes and creating just 1 hash/timestamp covering them all; manual & cumbersome can be overcome by writing programs to handle all of this and incorporating them into one’s workflow. So using an efficient cryptographic timestamping service (the OriginStamp Internet service), we can write programs to automatically & easily timestamp arbitrary files & strings, timestamp every commit to a Git repository, and webpages downloaded for archival purposes. We can implement the same idea offline, without reliance on OriginStamp, but at the cost of additional software dependencies like a Bitcoin client.