The Esignature Post

A key requirement for an electronically signed document is a trusted time stamp. This is very important for non-repudiation of contracts (was I bound by this at the time?) and for other important documents such as the disclosure of patentable inventions.

Proof of the existence of a signed document at a given time can be demonstrated by having a time stamp of the document signature issued by a trusted timestamp authority (TSA). A TSA is an entity that can demonstrate that it maintains its clock against a recognized time source such as NIST in the US. There are also standards for trusted time stamps such as RFC 3161 and ANSI X9.95, and a number of vendors that provide this service.

The basic process for obtaining a trusted time stamp is this:
The requesting party generates a hash or digital signature (only hashes may be signed in RFC 3161 compliant signatures) of the signed document and sends this value to the TSA. The TSA attaches a time stamp token to the hash, then digitally signs it with the TSA’s certificate. The requesting party validates the time stamp token and then associates this token with the signed document. This establishes that the signed document existed at the time of the time stamp. It does this without the TSA being aware of the conents of the digital document being signed, thus preserving the confidentiality of the signed document.

There are several ways of generating a time stamp token. RFC 3161 requires support of a time stamp token that contains the time as Zulu time, specified to the second, or better. It also requires that the TSA include a unique serial number with every timestamp issued so that the TSA name and token serial number define a unique time stamp.

Some other methods of generating timestamps rely on “document chaining” which includes a piece of the previous timestamp with each certificate thus establishing the validity of the time. Other methods combine this with using a single timestamp for a given interval of time so that many documents receive the same timestamp, and it is calculated as function of all of these documents. Should this timestamp be challenged several other document authors could be used as witnesses! An interesting survey of timestamping methods by Michael de Mare can be found here.

Some other methods proposed use transient keys, meaning the key used for signing the document hash is a function of the time. Generation of “trusted time stamps” is an area of significant activity, and many vendors will emphasize the advantages of their (often proprietary) methods.

Usage of a trusted time stamp is necessary for increasing the reliability of an electronic signature in a document. For many documents an institution may rely on its own time server for a TSA – this is legitimate as long as the institution is able to document its timekeeping policies and demonstrate their reliability in the event that the time stamps are challenged.

With many vendors pushing their own methods for time stamping and the significant complexity that can be attached to the topic it is easy to get confused. However the question that a relying party should ask is “How good do they have to be?” There are many ways of legally establishing the time of the occurence of an event. Just insure that you can justify yours!