We present a content integrity service for long-lived digital documents, especially for objects stored in long-term digital archives. The goal of the service is to demonstrate that information in the archive is authentic and has not been unintentionally or maliciously altered, even after its bit representation in the archive has undergone one or more transformations. We describe our design for an efficient, secure service that achieves this, and our implementation of the first prototype of such a service that we built for HP's Digital Media Platform. Our solution relies on one-way hashing and digital time-stamping procedures. Our service applies not only to transformations to archival content such as format changes, but also to the introduction of new cryptographic primitives, such as new one-way hash functions. This feature is absolutely necessary in the design of an integrity-preserving system that is meant to endure for decades. Notes: Copyright 2006 IS&T. Published in and presented at the IS&T Archiving 2006 Conference, 23-26 May 2006, Ottawa, Canada. 6 Pages