A timestamping authority (digital notary) Does there currently exist a free online service that accepts a file, hashes it, takes an authoritatively chosen timestamp (from one or more time services), signs these and sends this signed message back? E.g. a digital notary, but only with regards to the timestamp and not the validity of the content as legal notaries might be obligated to guarantee.

Such a service might have an extremely simple API: Simply wget http://service/&lt;checksum&gt; and receive a signed message containing one or more timestamps and the checksum. The emphasis would be on the type of authority who might make such a service.

Edit: Although the original question did not emphasise the following, the Bitcoin-based timestamping services provided below do not rely on one authority, but rather the entire Bitcoin network. They can be seen as tamper-proof and verifiable without the help of the service that helped you sign it!
If you search on "timestamp", "timestamping", and "notary" on Crypto.SE and Security.SE, you'll find lots of references.  I've collected a number of timestamping services that were mentioned in one of those places; this should provide a number of companies and online services you can check out:


http://www.proofofexistence.com/
https://www.btproof.com/
https://www.universign.eu/en/
http://www.guardtime.com/
http://securo.it/
http://www.cryptostamp.net/
http://www.itconsult.co.uk/stamper.htm
https://www.globalsign.com/timestamp-service/


I cannot vouch for or endorse any of those.

See, for instance, the following questions on this site, and the following other resources:


https://security.stackexchange.com/q/19184/971
https://security.stackexchange.com/q/43995/971
How are timestamps verified?
Signature and Timestamp for Long Term Document Archival Question
Timestamping services authorities, utilizing Linked, hybrid and other than PKI schemes
https://security.stackexchange.com/q/24458/971
https://en.wikipedia.org/wiki/Trusted_timestamping
http://tools.ietf.org/html/rfc3161
https://security.stackexchange.com/q/32493/971
https://security.stackexchange.com/q/47289/971


Happy reading!