The startup GuardTime offers exactly this service: http://www.guardtime.com/signatures/ http://www.guardtime.com/signatures/ You hash your data, and send the h

The startup GuardTime offers exactly this service: http://www.guardtime.com/signatures/ http://www.guardtime.com/signatures/ You hash your data, and send the hash to them. They embed the hash in a long hash-chain, and publish the current head of the hash chain in Financial Times every month. Once the hash chain head has been published, it is impossible to forge the signed content without destroying all copies of the newspaper in existence. It seems that the service is free for basic use, only availability SLAs and customer support is paid.