Enabling the Archival Storage of Signed Documents

Documents in digital formats are increasingly becoming a common form of expression for anything from rants and opinions to transaction records and contracts. Archiving such documents for the long term, particularly when their only form is digital, can be very important. Sadly, the principal digital expression of an author's intent, the digital signature, is not fit for long-term archives of documents; signing keys can expire or become compromised, rendering the documents they signed indistinguishable from illicit forgeries. We propose KASTS, an extension of traditional archival storage systems that enables the long-term storage of signed documents. KASTS combines time stamping of signed documents with storage of past signature verification keys. We argue that such an extended archival storage system is feasible and describe one possible design for it.