Applied Cryptography: Protocols, Algorithms, and Source Code in C, Second Edition

CHAPTER 4

Intermediate Protocols

4.1 TIMESTAMPING SERVICES

In many situations, people need to certify that a document existed on a certain date. Think about a copyright or patent dispute: The party that produces the earliest copy of the disputed work wins the case. With paper documents, notaries can sign and lawyers can safeguard copies. If a dispute arises, the notary or the lawyer testifies that the letter existed on a certain date.

In the digital world, it's far more complicated. There is no way to examine a digital document for signs of tampering. It can be copied and modified endlessly without anyone being the wiser. It's trivial to change the date stamp on a computer file. No one can look at a digital document and say: “Yes, this document was created before November 4, 1952.”

Stuart Haber and W. Scott Stornetta at Bellcore thought about the problem [682, 683, 92]. They wanted a digital timestamping protocol with the following properties:

• — The data itself must be timestamped, without any regard to the physical medium on which it resides.
• — It must be impossible to change a single bit of the document without that change being apparent.
• — It must be impossible to timestamp a document with a date and time different from the present one.

Arbitrated Solution

This protocol uses Trent, who has a trusted timestamping service, and Alice, who wishes to timestamp a document.

• (1) Alice transmits a copy of the document to Trent.
• (2) Trent records the date and time he received the document ...