Overview

“How to Time-Stamp a Digital Document” is a seminal cryptography paper by Stuart A. Haber and W. Scott Stornetta, published in the Journal of Cryptology (Vol. 3, No. 2, 1991) and first presented at CRYPTO ‘90. It defines the problem of proving when a digital document existed or was last modified, given that digital data is trivially copied and altered and its storage medium carries no trustworthy date. The authors propose computationally practical time-stamping methods that certify the data itself rather than the medium, preserve the privacy of the document’s contents, and resist tampering even when the time-stamping service colludes with a client. The work is widely regarded as an intellectual precursor to hash-linked ledgers and blockchain.

Key points

• Frames the core requirements of a digital time-stamp: it must bind a time to the data (not the physical medium), keep the document content private, and make both back-dating and forward-dating infeasible.
• Uses one-way cryptographic hash functions so a client submits only a fixed-length digest of the document to the time-stamping service (TSS), which never sees the plaintext, preserving confidentiality.
• Digital signatures let the TSS issue a signed certificate binding the submitted hash to a time value.
• Linking scheme: each request is cryptographically chained to the previous and following requests (a hash chain), fixing relative temporal order so that altering any timestamp would require forging the entire subsequent chain.
• Distributed-trust scheme: the document hash pseudo-randomly selects a set of other clients as witnesses who each sign the timestamp, so no single party (including a corrupt TSS) can unilaterally forge a valid time-stamp.
• Establishes the foundational framework for later systems in secure timestamping and proof-of-existence, and is cited as prior art in the Bitcoin whitepaper.

Relevance to Truestamp

This paper is the origin of the ideas Truestamp operationalizes: hashing a document to a digest, timestamping that digest without exposing content, and linking records so temporal order cannot be forged. Truestamp’s submission window proofs and its RFC 6962 Merkle tree construction extend the hash-linking approach introduced here, while the paper’s privacy property (timestamp the hash, never the data) mirrors how Truestamp establishes data integrity and a verifiable timestamp.

Citations

1. How to Time-Stamp a Digital Document. Stuart A. Haber and W. Scott Stornetta, Journal of Cryptology, Vol. 3, No. 2, pp. 99-111, 1991 (presented at CRYPTO ’90).