Trusted timestamps explained: RFC 3161 tokens and EU qualified time stamps

How an RFC 3161 time stamp works

RFC 3161 describes a Time Stamping Authority (TSA) as a trusted third party that gives proof that a datum existed at an instant in time ¹. The RFC gives the example of showing that a signature was applied before a certificate was revoked ¹.

• The requester computes a hash of the data and sends it in a request. The request does not identify the requester ¹.
• The TSA checks only that the hash length matches the algorithm. It does not examine the data ¹.
• The TSA adds a trustworthy time value, a unique serial number and the identifier of its policy, then signs the result with a key used only for time stamping ¹.
• The requester checks the signature, that the hash matches the one sent and that the time is plausible, using a local clock or a nonce ¹.

What the token contains

Limits of a time stamp

A token proves that the hashed data existed no later than the stated time ¹. It says nothing about who made the data or whether its contents are true. The TSA does not even see the data ¹.

Tokens depend on the TSA's key. RFC 3161 says that if the key is compromised, tokens signed with it can no longer be trusted ¹. It says tokens should be time-stamped again or notarized later, since signing keys have finite lifetimes ¹.

RFC 3161 names several optional transports, including HTTP and email using the media types application/timestamp-query and application/timestamp-reply. No transport is mandatory ¹.

EU law: electronic and qualified time stamps

Regulation (EU) No 910/2014, known as eIDAS, defines an electronic time stamp as data in electronic form that binds other data in electronic form to a particular time ². A qualified electronic time stamp is one that meets the requirements of Article 42 ².

Under Article 41, an electronic time stamp cannot be denied legal effect or admissibility as evidence solely on the grounds that it is electronic or not qualified ². A qualified electronic time stamp enjoys a presumption of the accuracy of its date and time and of the integrity of the bound data ². One issued in one Member State is recognised in all Member States ².

• It binds the date and time to data so as to reasonably preclude undetectable change ².
• It is based on an accurate time source linked to Coordinated Universal Time ².
• It is signed with an advanced electronic signature or sealed with an advanced electronic seal of the qualified trust service provider, or an equivalent method ².

Time stamps compared with archive dates

A web archive's capture date is the archive's own statement. Under the Memento protocol, the Memento-Datetime header gives the archival datetime of a capture ³. It is not a signature from an independent authority.

Courts have treated archive dates on their own terms. In a 2019 case the Federal Court of Australia said the dates applied by the Internet Archive were not hearsay, as the archive created them, and that the page content was hearsay ⁴. A party wanting stronger proof of when a file existed can add an RFC 3161 token from a TSA, which binds the file's hash to a signed UTC time ¹.