ANSI ASC X9.95 Standard

The ANSI X9.95 standard for trusted timestamps expands on the widely used RFC 3161 - Internet X.509 Public Key Infrastructure Time-Stamp Protocol by adding data-level security requirements that can ensure data integrity against a reliable time source that is provable to any third party. Applicable to both unsigned and digitally signed data, this newer standard has been used by financial institutions and regulatory bodies to create trustworthy timestamps that cannot be altered without detection and to sustain an evidentiary trail of authenticity. Timestamps based on the X9.95 standard can be used to provide:

A superset of the IETF's RFC 3161 protocol, the X9.95 standard includes definitions for specific data objects, message , and trusted timestamp methods, such as digital signature, MAC, linked token, linked-and-signature and transient-key methods. X9.95 compliance can be achieved via several technological approaches, such as transient-key cryptography. Several vendors market X9.95-compliant systems.

In an X9.95 trusted timestamp scheme, there are five entities: the time source entity, the Time Stamp Authority, the requestor, the verifier, and a relying party.

Before a timestamp-service commences operations, the Time Stamp Authority calibrates its clock(s) with an upstream time source entity, such as a legally defined master clock for the jurisdiction the TSA is time-stamping evidence for. When trusted time has been acquired, the TSA can issue timestamps for unsigned and digitally signed data based on all of the jurisdictions it maintains timing solutions for.

Applications using timestamps on unsigned data can provide evidence to a verifier that the underlying digital data has existed since the timestamp was generated.

When a requestor requires a trusted timestamp for a piece of data, it creates a hash of the data using a cryptographic hash function and sends it to the TSA (through a network connection). The TSA then signs the hash and the time of signature to create a trusted timestamp. This trusted timestamp is finally returned to the requestor, who can store it along with the data.

...
Wikipedia